Nursery management practices influence the quality ...

Review n. 33 – Italus Hortus 24 (3), 2017: 39-52

doi: 10.26353/j.itahort/2017.3.3952

Nursery management practices influence the quality of vegetable seedlings Astrit Balliu*, Glenda Sallaku, Thoma Nasto Agricultural University of Tirana, Albania Ricezione: 26 gennaio 2018; Accettazione: 23 marzo 2018

La gestione del vivaio influenza la qualità delle plantule Riassunto. Produrre trapianti vegetali di "buona qualità" richiede una serie di pratiche e interventi orticoli che, nel complesso, consentono la crescita delle piante dall'inizio alla fine in maniera costante, ininterrotta e con uno stress minimo, garantendo un tasso di crescita adeguato, un rapporto ottimale tra radice e germoglio, una buona capacità di insediamento e un potenziale alto rendimento dell’impianto che ne deriva. Poiché la crescita del trapianto vegetale è una sintesi di molti fattori che si moderano a vicenda, piuttosto che seguire raccomandazioni standardizzate, piantine di ortaggi di buona qualità possono essere prodotte solo attraverso un'attenta selezione del substrato e delle miscele di substrato, una gestione appropriata del regime termico e della fertilizzazione, della dimensione del modulo e dell'età del trapianto e , infine della corretta combinazione di promotori e delle tecniche di controllo della crescita delle piante. Parole chiave: substrato, sostituti della torba, dimensioni del modulo, età del trapianto, fertilizzazione, biostimolanti vegetali, indurimento.

Introduction Vegetable transplants have been used for decades and the methods used to produce them, gradually, have tremendously changed (Balliu et al. 2017). Fifty years ago, the standard method for producing vegetable seedlings was to sow the seed in a nursery bed and then dig up the seedlings when they were large enough to be successfully transplanted, starting from the 1970s the use of cell trays for growing vegetable seedlings was largely used (Nichols, 2013). That is a system, where each transplant grows in an individual *

cell and so there is less competition among plants and greater uniformity, the water and nutrients are delivered through overhead booms and roots are air pruned (Nichols, 2013). Since the quality of transplant affect plant establishment, initial growth and consequently yield (Prunty et al., 2015), the production method of vegetable seedlings is extremely important. Although the term “good quality” leave room for some subjective interpretation, and there are not well distinct standards, high quality vegetable transplants are generally defined as those with 1) no infections of diseases or pests, 2) ability to survive in unfavorable environments after transplanting, 3) well developed root system, and 4) well developed leaf area without visual defects of leaves such as chlorosis or necrosis (Balliu et al. 2017). The quality of vegetable transplants is affected by many factors and therefore the management of transplant production is complicated and requires broad knowledge’s and experience (Mcavoy and Ozoreshampton, 2015). Practically, it is a chain of several horticultural practices and interventions which overall determines the growth rate (Oagile et al., 2016), the root to shoot ratio (Sallaku et al., 2009b), the stand establishment capabilities (Babaj et al., 2012) and yield potentials (Ali, 2016) of new plantlets. Good quality vegetable seedlings in very small module sizes can only be produced through proper use and appropriate manipulations of water (Liptay et al., 1998) and fertilization regimes (Liu et al., 2012) which overall aiming to raise the plant from start to finish by slow, steady, uninterrupted growth and with minimal stress (Mcavoy and Ozores-hampton, 2015). Furthermore, the quality of vegetable seedlings is affected by the physical and chemical qualities of substrate (Ceglie et al., 2015), module size (Oagile et al., 2016) and transplants age (Ibrahim et al., 2013; Jellani et al., 2016), plant growth promoters (Baum, 2015; De Pascale et

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al., 2018) and growth control and hardening techniques (Garner and Björkman, 1999; Agehara and Leskovar, 2015) . Since the number of factors affecting the quality of vegetable transplants is very large, and therefore hard to have all included in a single review, this one deals with the recent findings and current trends of only some of nursery management practices regarding vegetable transplant production; substrate and peat substitutes, cell size, transplant age, fertilization, plant biostimulants use and growth control and hardening techniques, acknowledging that despite voluminous research done, often contradictory results are published, and yet, no clear answers are provided regarding many specific issues with practical importance for vegetable growers. Substrate; peat and its substitution alternatives Various growing media may be used in commercial seedling production, but still, the most common material used for vegetable seedling production is peat (Gruda and Schnitzler, 2004; Díaz-pérez et al., 2011). Its long-time success is certainly due to the physical properties (slow degradation rate, low bulk density, high porosity, high water holding capacity) and the chemical characteristics (relatively high cation exchange capacity) that makes peat particularly suitable as growing media for a large number of vegetables and ornamentals (Fascella 2015). However, the frequent irrigation deteriorates the physical characteristics (porosity and water and air volume) of peat. Therefore, commercial nurseries often mix peat with perlite or vermiculite to increase the water-holding capacity of growing substrate and avoid water content volume fluctuations of solely peat substrate (Balliu et al. 2017). Nowadays, the search for alternative high-quality and low-cost materials as growing media in horticulture is a necessity due to the increasing demand and rising costs for peat, and it’s uncertain availability in the near future owed to environmental constraints (Fascella 2015). Therefore, extensive research has been carried out regarding the use of different farm, industrial and consumer waste by-products as components of nursery substrates. Different residual biomasses, such as coir (coconut (Cocos nucifera L.) husk fibre), rice (Oryza sativa L.) hulls, switchgrass (Panicum virgatum L.), spent mushroom compost (Agaricus bisporus (J.E. Lange) Imbach, and Pleurotus ostreatus (Jacq.) P. Kumm), beached Posidonia residues (Posidonia oceanica L.), extracted sweet corn tassel (Zea mays L.), and giant reed 40

(Arundo donax L.) wastes have been studied as partial or total substrate components (Ceglie et al., 2015). Numerous studies have meantime reported the use of organic residues, after proper composting, as peat substitutes in potting media, such as municipal solid waste compost, animal manure compost, green waste compost, and agro-industrial compost (Ceglie et al., 2015). Recently, dewatered aquaculture effluent are also investigated as potential partial substitutes in vegetable seedling production (Danaher et al., 2016). However, although some by-products obtained from waste recycling of human activities, agricultural and food industry, and/or energy production processes represent valid alternative to peat, their use as potential substitutes of conventional materials in vegetable transplants production needs further clarifications (Russo 2005). Currently, industrial growing mixes are widely used in the vegetable transplant industry, composed by growing media constituents which include combinations of peat and other organic or inorganic materials, and growing media additives include fertilizers, liming materials, and biocontrol or wetting agents (Fascella 2015). While their cost is considerably lower and they also may suppress many soilborne diseases, various composts act as well as peat moss does in container media (Raviv, 2005). Assessing the viability of using compost from vine pomace, agricultural solid wastes and urban solid wastes Díaz-pérez et al., (2011) have found that that the partial substitution of peat in the mixture of substrates in watermelon seedling production is possible and the dose shall depend on compost salinity and the type of salts that it has. Evaluating bovine manure compost (BMC) and a green compost (GC) as components of substrates in partial substitution of peat for organic melon seedlings production, Tittareli et al., (2009) have also found that seedling growth in treatments containing 30% and 50% of composts was significantly higher than control. Anyway, plant response to different substrates is strictly related to the tested species and also depends on the materials used and on the proportions in the mixtures and it is very complex to establish the most suitable materials and especially, the best proportions to obtain good results concerning plant growth and productivity (Ceglie et al., 2015). The use of compost into substrates in different ratios may cause some problems as a consequence of its high salt content, unsuitable physical properties (water holding capacity, aeration etc.), heavy metal toxicity and variable quality and composition (Chrysargyris et al. 2013; Fascella 2015). Thus, examining the impact of munic-

Nursery management influence the quality of seedling

ipal solid waste compost (MSWC) mixed with conventional peat substrates at various ratios, as a growth medium in the nursery production of melon (Chrysargyris et al. 2013) found that seed emergence enhanced when low MSWC content (< 30%) was used while increased MSWC content (>60%) reduced (up to 80%) emergence and delayed up to 6 days the emergence time. Under nursery conditions, addition of MSWC (especially in content greater than 30%) reduced leaf numbers produced, seedling fresh weight (and dry matter content), chlorophyll and total carotenoids content. In similar experiments, (Chrysargyris, 2017) found that addition of > 30% MSWC reduced pepper seedling height, leaf number and fresh weight, while leaf chlorophyll and total carotenoids content decreased in > 60% MSWC into the peat, and (Papamichalaki et al., 2014) found that low content (15% to 30%) of MSWC may act as an alternative substitute for peat in watermelon seedling production. Analyzing the influence of MSWC in cucumber seedlings production (Mami and Peyvast, 2010) recommends that MSWC in mixtures with peat should not exceed the level of 5%. Gruda and Schnitzler (1997) did also report that wood fibers could be used as organic substrates, but they warn that N-immobilization can be a cause of nutritional imbalance on young vegetable seedlings. There are reports that olive-mills waste composts could be used as replacement for peat in growing media. According to Urlic et al. (2015), they should be used in a rate not higher than 50% for Brassicas and not higher than 25% for lettuce and endive transplant production. Meantime, Ceglie et al., (2011) reports that treatments with 20% olive pomace compost showed the best performances of tomato seedlings, while 45% could be considered as the upper limits for producing satisfactory tomato seedlings. Similarly, grape-vinecompost can also be used in tomato transplant production in a rate of up to 40% (Kritsotakis and Kabourakis, 2011). One promising method of processing organic wastes into a value-added product is vermicomposting. In this process, organic material is stabilized into an odorless peat-like substance in the digestive tracts of various species (e.g., Eisenia fetida and E. andrei) of earthworms (Paul and Metzger, 2015). The process of vermicomposting tends to result in higher levels of plant availability of most nutrients than does the conventional composting process. Therefore, its presence in compost mixtures stimulates plant growth. Thus, a higher dry matter per plant and a higher relative growth rate was found by Babaj et al. (2009) and Arouiee et al., (2009) in tomato seedlings growing in

a mixture of vermicompost with commercial peat (up to 50%, vol:vol). Improved transplant quality in peppers and eggplants (up to 20%, vol:vol), but not in tomato transplant was also reported by (Paul and Metzger, 2015). Parallel to increased shoot length, fresh weight and leaf area of tomato seedlings due to addition of vermicompost from 10 to 50 % of total pot volume, Dintcheva and Tringovska (2012) reports significant contribution of vermicompost to N and K supply of tomato transplants. In addition, Sallaku et al. (2009a) and Kaciu et al. (2011) reports enhanced salinity stress alleviation capabilities of respectively cucumber and pepper seedlings grew in a mixture of vermicompost with commercial peat (50%:50%, vol:vol) after transplanting in a saline environment. However, due to variability of organic wastes, vermicomposts might have varying nutrient content levels (Dintcheva and Tringovska, 2012), which subsequently might yield different results under varying potting mixtures. Module size and shape According to (Nesmith and Duval, 1998), the trend among many commercial transplant producers is toward more cells per tray (smaller containers), which reduces propagation costs per plant. Yet, it is unclear how plants grown in smaller root volumes will perform under post transplant field conditions. It is largely admitted that reducing transplant container size generally increases the probability of root restriction, but the length of time a plant remains in the container is also a major factor to be considered (Nesmith and Duval, 1998). However, analyzing the influence of pot size, (Poorter et al., 2012) have found that reduced growth in smaller pots is caused mainly by a reduction in photosynthesis per unit leaf area, rather than by changes in leaf morphology or biomass allocation. In general, larger cells will enable production of a larger transplant, the plant has more room to grow, so it is possible to produce an older, more mature transplant without it becoming spindly or root bound (Balliu et al. 2017). That is an well known fact and according to Poorter et al., (2012) a doubling of the pot size increased biomass production by 43%. Thus, tomato transplants grown commercially in a container cell size of 4.4 cm had greater dry matter accumulation at planting and 30 days after planting than plants grown in 2.5 cm cells (Vavrina and Arenas, 1997). Similarly, higher plant height, leaf number and leaf area, and higher shoot fresh and dry weight of tomato seedlings due to a larger module size were also reported by Shopova and Cholakov (2014) and Oagile 41

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et al., (2016). An early report from Maynard et al. (1996) did also confirms that seedling leaf area, and shoot and root weights of muskmelon seedlings before transplanting increased linearly with increasing cell volume. The cell size influences the field performance of the transplant (Balliu et al. 2017). Obviously transplants with relatively large root systems generally suffer less post-plant shock and thus come into production earlier than plants with small root systems (Weston and Zandstra, 1986). Maynard et al. (1996) have also reported that shoot dry weight 20 days after transplanting and vine length of muskmelon seedlings 30 days after transplanting increased linearly with increasing cell volume. Plant development can also be influenced by module size. Evidences brought by (Nesmith and Duval, 1998) confirms that as rooting volume increased, the time from sowing to anthesis was shortened, but meanwhile a delay in fruit maturation was shown for root restricted tomatoes. In contrast, root restriction resulting from small containers did not have any influence on duration of flowering or time to anthesis in summer squash. Varying transplant container size has shown mixed and even contradictory results on harvested yield and differing observations between yields of species and cultivars in response to transplant container size have not been thoroughly explained (Nesmith and Duval, 1998). According to Vavrina and Arenas, (1997), tomato transplants grown commercially in a container cell size of 4.4 cm produced earlier and yielded greater than transplants grown in 2.5 cm cells. Higher transplant biomass and higher early and total yield of tomato seedlings due to increased module size was also reported by Bouzo and Favaro, (2015). However, Nicola and Cantliffe (1996) indicated that yield and earliness of lettuce (Lactuca sativa L.) was more related to growing season and soil type than to transplant quality resulting from various container sizes. The same fact is admitted from Vavrina and Arenas, (1997) whose found that in case of tomato seedlings the cell size impact was more dramatic in the spring than in the fall, perhaps due to environmental complications experienced in the fall. In an earlier report (Maynard et al., 1996) have also found that early yields increased linearly as transplant cell volume increased, but cell volume did not consistently affect total yield of muskmelon. Attention was also paid to the effect of cell shape on the plug transplant growth in different vegetable crops. In study published by (Chen et al. 2002), was noticed that in lettuce and cucumber plug transplants, 42

round cells promoted root circling while triangular cells retarded root growth compared to pentagonal and square cells. Surprisingly, before root ball formation, round cells promoted plant growth, but on the contrary, they inhibited plant growth after root ball formation compared with square and pentagonal cells. In tomato plug transplants, the rate of root circling was increased in order of round, pentagonal, square, and triangular cells. However, cell shape did not affect the plant growth before root ball formation, while the growth of plants in triangular cell was inhibited compared with all other cells after root ball formation. In conclusion, square or pentagonal cells are more suitable for lettuce and cucumber plug transplants than round and triangular cells. Except of triangular cells, the cells with other shapes are suitable for the production of tomato plug transplant. A deeper-celled tray has a larger cell volume so more water and fertilizer are available to the plant. Deeper cells tend to promote faster growth and also they will not need watering as frequently as shallow cells (Kubota et al. 2013). Transplant age The definition of ‘ideal’ transplant age is a long time questions, which has not yet received a decisive answer. Lack of standardized methods, multiple cultivars, processing versus fresh market types, bare-roots vs. containerized, various container cell sizes, superimposed studies, postharvest storage, and other aspects all led to varying results (Vavrina, 1998). Young plants that were tender and capable of quick growth resumption were often associated with large yields, whereas it was believed that hardening reduced early yields in proportion to the degree of hardening (Casseres 1947 after Vavrina and Orzolek, 1993). Over mature transplants generally have spindly stems and excessive leaf growth, whereas their root growth is limited because of the small rooting volume of high-density plug trays (Agehara and Leskovar, 2015). In addition, following transplanting older plants have only a limited time for the readjustment of their vegetative development before the initiation of reproductive growth or the maturation of the vegetative phase (Dullforce, 1954 after Vavrina 1998). However, young plants had to be carried longer in the field to reach optimal yields, and that could reduce market window opportunities (Vavrina and Orzolek, 1993). In fact, numerous studies have already confirmed that the transplant age window for certain crops might be wider than previously thought and current modern cultivars, improved production systems, and technical


expertise enable us to produce high yields regardless of transplant age (Vavrina, 1998). As an example, in a review of 1993, (Vavrina and Orzolek, 1993) admits that after more than 60 years of transplant age research, it appears that comparable production can be obtained from tomato transplants of various ages. Since 1993, (NeSmith, 1993) has reported that 28 to 35 day-old greenhouse-grown summer squash transplants grew more slowly after transplanting than plants that were 10,14, or 21 days old. Older transplants flowered earlier; however, earlier flowering did not result in higher early yield. In support of these conclusions, a number of recently published reports underline the benefits of using younger transplants. A higher leaf gas exchange and photosynthetic productivity were found for 20-25 day old compared to 3035 and 40-45 day old tomato seedlings (Shopova and Cholakov, 2014). Similarly, Ibrahim et al. (2013), reports that pepper seedlings transplanted at 8 and 10 week after sowing (WAS) produced larger leaves, greater number of leaves, more branches per plant, and also grew taller than the seedlings transplanted at 12 or 14 WAS. The variation in the seedling-transplanting age had no statistical significant difference on the fresh fruit yield, but however, pepper seedlings transplanted at 8 and 10 weeks produced more fruits and higher fresh fruit yield per hectare than those seedlings transplanted at 12 and 14 WAS (Ibrahim et al., 2013). Analyzing the effects of 21, 28 and 35 old day cucumber seedlings Tanweer et al. (2005) found that 28 day old seedlings provided the maximum survival rate after transplanting, the maximum fruit number and the highest yield per plant, while the least values of each of above parameters were obtained in 35 day old seedlings. Similarly, Grabowska et al., (2007) have found that cauliflower plants obtained from 4week old transplants gained a significantly higher marketable yield than plants obtained from 10-week old transplants, and in an experiment aimed to determine the effects of 40, 50 and 60 days old seedlings to growth and yield of bitter gourd Jellani et al., (2016) have found that the minimum days to 1st picking, and the maximum availability period, yield and net profit were observed in 40 days old seedlings. Contrary to the abovementioned evidences, in an experiment conducted by Jankauskien et al., (2013), older tomato transplant (9–10 leaves) started to flower before 5–6 leaves transplant, which also did also have the least early yield, while the higher total yield was produced by 7–8 leaves transplants. In addition, Essilfie et al., (2017) showed that in chili pepper 44 day old transplants had the highest yield, tallest plant, and highest number of branches and canopy width

compared to 30 and 37 old seedlings, and similarly, comparing 30, 45 and 60 days seedlings, Ali, (2016) have found that transplantation of 45 and 60 days old cucumber seedlings produced the higher early and total yield with higher gross return and gross margin. Obviously, the definition of best transplanting age remains an open question. It seems that the optimum age for vegetable transplants depends on crop, the cell size used and the growing conditions. Generally, relatively young vegetable transplants produce the best stand and fastest crop development. On the contrary, overgrown plants face difficulties regarding the reestablishment of their root system and commonly experience strong growth retardation after transplanting. Older transplants might result in earlier yields; however younger transplants will produce comparable yields, but it take longer to do so. As a conclusion, there is no unique definition of the best seedlings age or the most appropriate phenological stage of transplant age. As an example, the best transplanting age for tomato in Canada is considered when first flowers are showing, while seedlings over 5 weeks old are less desirable in Mediterranean countries (Zeidan, 2005). Fertilization Since the nutrient content of growing medium is generally quite low, vegetable transplants nutrition depends on the frequency of fertigation and the concentration of the feeding solution (Balliu et al., 2009). Overhead irrigation is the most used method growers use to apply water and dissolved fertilizer nutrients to vegetable transplants during greenhouse production (Leskovar, 1998; Liu et al., 2012). Under this irrigation method, it is advised that the watering should be to cell capacity and with minimum drainage, in order to prevent nutrients leaching from the growing medium. However, in specific cases the grower may apply water in excess of container requirements, so that some water and dissolved nutrients can leach out the bottom of the container and reduce the buildup of soluble salts in the container (Liu et al., 2012). Sub irrigation, commonly referred to as ebb-and-flow or flotation systems is also using in some cases (Leskovar, 1998). However, sub irrigation tends to result in elevated EC values in the upper regions of container medium compared to overhead irrigation (EC values are high toward the top of containers), which is a consequence of water evaporation from the medium surface while leaving behind soluble salts. Therefore, nurseries having irrigation water with high dissolved salts should be cautious with sub irrigation 43

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and consider the potential risk of elevated EC (Schmal et al., 2011). The differential provision of water to root systems influences the morphology, dry-matter partitioning, and physiology of transplants (Leskovar, 1998). Analyzing the effects of different irrigation methods Ahmed et al. (2000) found that sub irrigated lettuce and tomato transplants were taller, had longer leaves, a larger leaf area and more leaves than overhead irrigated transplants. Significant effects of irrigation methods (overhead versus sub irrigation) on fresh and dry weight, height and stem diameter of collards, kale, kale, pepper and tomato transplants were also reported by (Liu et al., 2012). When averaged across all fertilizer treatments, the fresh weight, dry weight, as well as height of sub irrigated plants were generally greater than overhead irrigated plants. In addition they emphasized that in most cases the optimum growth of sub irrigated vegetable transplants was obtained in significantly lower nutrient concentration than overhead irrigated transplants. The necessity of scheduling the delivery of transplants to commercial growers requires that the growth rate of transplants to be strictly controlled. Practically, the most effective way of controlling transplant growth is to moderate their nutritional regime (Dufault, 1998). Unfortunately, despite numerous publications regarding vegetable transplant nutrition it is not possible to summarize the one best way to grow any vegetable transplant, simply because of many interacting and confounding factors that moderate the effects of nutritional treatments (Dufault, 1998; Liu et al., 2012). In a review, (Zandstra and Liptay, 1999) does also underlines that in different climates, and for different crops, there is a great disparity in feeding requirements for plants. As an example they brings the case of very diverse recommendations regarding ‘ideal’ N concentration for tomato feeding; 400 mg L1 in northern climate versus only 20 mg L-1 in a warm southerly climate. Nitrogen is the most powerful nutrient elements to control the growth of young plants. It has been the target of most nutritional research on transplants and has long been recognized to have the greatest effect on transplant growth (Zandstra and Liptay, 1999). Significant growth effects of increased fresh and dry matter biomass, and increased growth rate due to increased N concentration in nutrient solutions were reported in pepper and eggplants (0-300 mg L -1) (Balliu et al., 2007b; De Grazia et al. 2008 a), pepper (0-1200 mg L-1) (De Grazia et al. 2008b) and tomato (25-100 mg L-1) ( Balliu et al., 2007a). In fact, the optimum N concentration in a nutrient solution is 44

highly crop depended. In a recent publication which analyses the effect of N concentration (50 mg L-1 to 500 mg L-1) on the transplant quality parameters of five different vegetable crops (Liu et al., 2012), reports that the maximum shoot dry weight were received by the application of nutrient solutions contained 100 mg L-1 N for pepper, 200 mg L-1 N for tomato, and 350 mg L-1N for collards, kale, and lettuce. Internal plant tissue N status is important for improving post-transplanting seedling growth (Widers 1989 after Dufault 1998), because a higher N in seedling tissues at transplanting may be used immediately for growth than that available in the soil (Liptay and Nicholls, 1993). In addition there are reports that earliness in tomato is improved in transplants grown with 200 ppm versus 50 to 100 ppm (Melton and Dufault, 1991a). Masson et al., (1991) has also reported that tomato transplants grown with 300 to 400 ppm N yielded earlier, but however increased N rate did not affect the total yield. Quite similar results were also reported from (Edelstein and Nerson, 2001) in muskmelon. The way the nutrient composition influences the photosynthetic products partitioning between the different plant parts, namely leaves, stems and roots, is of an utmost importance for the quality parameters of vegetable seedlings. Basically this is a question of the trade-off between biomass investment in leaves, which would increase the supply of photosynthates, and in roots, which would stimulate the uptake of nutrients and water (Poorter and Nagel, 2000). Indeed, while N content in transplants dry matter was increased with increasing N concentration in the nutrient solution, a steady decrease of root dry weight versus total plant dry weight and smaller NRE (N recovery efficiency) values was found in tomato (Balliu et al., 2007a), eggplant (Balliu et al., 2008) and pepper seedlings (Balliu et al., 2009). Commonly, the increase of N concentration enhances the relative growth rate (RGR) of vegetable seedlings (De Grazia et al. 2008 a). However, while at low concentration (25-100 mg L-1) any increase of N concentration, has increased the seedling growth rate by both; faster expansion of plant leaf area (LAR) and higher physiological plant productivity (NAR), at higher N concentration (100-400 mg L-1), increased RGR values were achieved only due to higher values of LAR (Balliu et al., 2007a; Balliu et al., 2007b) which might cost misbalances between root and shoot growth in fast growing transplants. Although N is commonly applied to the containerized vegetable transplants through the irrigation (either overhead or floating), other methods are available. A


simpler method is the addition of nutrients to growing media prior to seeding. According to (Grazia et al., 2008b), transplants growing on substrate premixed with 600 mg L-1 N produced greater individual biomass than those receiving N by fertirrigation at both 600 and 1200 mg L-1 rates. Even though there are reports that such a practice could improve transplant quality (Dufault, 1998), the risk of high salinity in case of high doses of inorganic fertilizers and respective salt toxicity effects on young transplants should be considered. Thus, Zhang et al. (2017) warns that the addition of high levels of N (5.0 g·kg-¹) with high (7.0 g·kg-¹) or low (0.7 g·kg-¹) levels of K not only significantly decreased the root dry weight but also decreased leaf area of cabbage (B. oleracea) seedlings. Despite of delivery method, too high nitrogen may cause overgrowth or even toxicity to the plants. Seedlings grown on high rates of nitrogen fertilization are succulent, less resistant to dry weather and solar radiation, and this leads to a low rate of plant survival after transplanting in the open field (Balliu et al. 2017). In an experiment that evaluated the effect of N concentration in pepper seedlings in the range of 0 to 1200 mg L-1 N was found that the application of 1200 mg N L-1 led to transplants that had thinner leaves and a very low shoot:root biomass ratio (De Grazia et al., 2008b). Deleterious effects (reduced shoot DW) were reported in kale, lettuce, and pepper at concentration greater than 350–440 mg L–1N under sub irrigation, but not with overhead irrigation (Liu et al., 2012). Phosphorus (P) and potassium (K) are also important to guarantee steady and balanced growth of vegetable seedlings. Therefore, combined nutrient solutions with the presence of the three most used elements; N (50-200 ppm), P (10-40 ppm) and K (100300 ppm) are common in nursery application (Balliu et al. 2017). The effect of phosphorus on the vegetative growth of vegetable transplants is not as dramatic as N, and often no effects on growth are noted (Zandstra and Liptay, 1999). In a two year experiment (Melton and Dufault, 1991a), found that stem diameter, leaf number, leaf area, fresh shoot weight, and dry shoot and root weights of tomato seedlings were higher with 40 than with 10 mg L-1 P, with no further effect above 40 mg· L-1. Rare evidences are found to report an effect of P concentration on yield of subsequently field transplanted seedlings. In the above mentioned experiment of Melton and Dufault (1991a), P concentration did not affected tomato yields after field transplanting in both years of experiment. The same authors reports that production of quality transplants requires at least 225 mg· L-1 N, 45 mg· L-1 P and 25 mg· L -1 K (Melton and Dufault, 1991b).

Weston and Zandstra (1989) cited by Dufault, (1998) reports that tomato transplants fertilized with 400 mg· L-1 N and 30 mg· L-1 P produced the greatest early and total yields. No significant effects were found due to N:P ratio in the nutrient solutions used during nursery stage of seedling growth. In an experiments conducted with muskmelon seedlings, (Edelstein & Nerson 2001) did not found any effect of N:P ratio (in the range of 10:1; 5:1, 1:1) on root and shoot dry matter of seedlings, nor in their yield after field transplanting. The ratio of N with P and K did not, also, have any significant effect on the relative growth rate (RGR) of pepper and eggplant seedlings (Balliu et al., 2007b). Very low P concentration in nutrient solutions could however lead to P deficiencies in vegetable seedlings. Deficiencies of P resulted in chloroplasts filled with starch, and dark green and rich in anthocyanin leaves. Overall this may in fact reflect a type of increased hardening not necessary desirable because of reduced rate of transplant root initial growth (Zandstra and Liptay 1999). The symptoms of P deficiency could be deteriorated under low temperature growing conditions. It is accepted that K level in the nutrient solutions can be varied to a great extend with little effect on transplant performance (Zandstra and Liptay 1999). The common response of vegetable seedlings to increased K concentration in the nutrient solution from 50 to 250 mg· L -1 is the reduction of root growth, while no effects was noticed regarding the shoot growth (Melton and Dufault, 1991b). There are nurseries imposing to plants a constant mild salt stress by treating transplants with concentrated nutrient solution, compounded by high concentration of P and K. The salt stress reduces the seedlings growth rate and also stimulates a generative process which favors the earliness (Balliu et al. 2017). However, high levels of nutrient concentration decreases stem diameter, plant height, leaf number, root dry weight, stem dry weight, leaf dry weight, photosynthesis and stomata conductance (Hossain et al., 2010). Plant biostimulants Plant biostimulants contain substance(s) (humic acids, seaweed extracts and protein hydrolysate; Colla et al., 2017) and/or microorganisms (mycorrhiza and PGPR) whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance nutrient uptake, nutrient efficiency, tolerance to abiotic stress, yield and crop quality (Abdel Latef 45

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and Chaoxing, 2011; Calvo et al., 2014; Rouphael et al. 2015; Kapoor and Singh, 2017). Among them, the most known categories are: • microbial inoculants, • humic acids, • fulvic acids, • protein hydrolysates and amino acids • seaweed extracts (Calvo et al., 2014). Although their effects are crop depended, the most interesting biostimulants for vegetable nursery industry are microbial inoculants which include free-living bacteria, fungi, and arbuscular mycorrhizal fungi (AMF) (Calvo et al., 2014). These are also named as biofertilizers and when applied to seed, plant surfaces, or soil, promote growth by several mechanisms such as increasing the supply of nutrients, increasing root biomass or root area, and increasing nutrient uptake capacity of the plant (Cavagnaro et al., 2006; Rouphael et al. 2015). In addition, improved plant tolerance to salinity (Dodd and Perez Alfocea, 2012; Porcel et al., 2012; Meça et al., 2016), the biological control of plant pathogens (Heydari and Pessarakli, 2010) and improvement of the chemical quality of vegetable species (Baum, 2015) are also largely acknowledged. Due to their beneficial effects on terrestrial ecosystems, AM fungi are widely used in plant nurseries to improve the growth of economically important species (Corradi and Bonfante, 2012), by enabling stronger growth of seedlings in nursery and improved performance after planting in the field (Raviv et al. 1998; Al-Karaki 2017). Thus, it is reported that AM fungi have significantly increased shoot and root dry matter and plant height of AMF inoculated pepper plants (Al-Karaki 2017), improved the stand establishment rate of tomato seedlings and enhanced their resistance to soil salinity immediately after transplanting (Vuksani et al., 2015; Balliu et al., 2015) and improve the yield of AMF pre inoculated tomato plants (Al-Karaki, 2006; Abdel Latef and Chaoxing, 2011; Vuksani et al., 2015). Increased yield due to AM inoculation of transplants were also reported in cabbage (Raviv et al. 1998), onion (Makus, 2004), and green pepper (Al-Karaki 2017). Despite many encouraging reports on profits of using AMF, one should mind that the efficiency of inoculation with AMF on their host plants is controlled by the genotype combinations (host plant × AMF), soil properties and by the inoculation method (Baum, 2015). As an example, different from above authors Makus, (2004) reported only improved earliness due to mycorrhizal inoculation of tomato transplants, but not of the final yield, fruit number and average fruit weight. 46

The use of free living bacteria, collectively known as plant growth-promoting bacteria (PGPB) is also increasing in vegetable seedling production. They typically promote plant growth in two ways: direct stimulation and biocontrol (Gamalero and Glick, 2014) and includes activities such as; nitrogen fixation, phosphate solubilization, iron sequestration, synthesis of phytohormones, modulation of plant ethylene levels, and control of phytopathogenic microorganisms (Gamalero and Glick, 2014, Picta and Pastucha 2008). Enhanced growth effects due to inoculation of a certain strain of Burkholderia sp. were reported from (Nowak et al. 2004) in tomato, cucumber and sweet pepper. Similarly, (Ekinci et al., 2014) have showed that the use of strains of Bacillus megaterium, Pantoea agglomerans and Bacillus subtilis was associated with increased fresh shoot weight, dry shoot weight, root diameter, root length, fresh root weight, dry root weight, plant height, stem diameter, leaf area and chlorophyll contents of cauliflower transplants. However, it is noticed that the degree of growth stimulation varied with species and method of inoculation, and that the early growth of transplants and developmental stimulation caused by the bacterium did not always translate into significant effects on yield (Nowak et al. 2004). One of the most used microorganisms in agriculture is Trichoderma sp., which is commonly used during the composting process of organic wastes. Composts inoculated with Trichoderma sp. have been effective for disease control of different fungal pathogens, such as Choanephora cucurbitarum, Rhozoctonia solani, Sclerotinia cepivorum, Botrytis cinerea, Phytophthora sp. and Fusarium oxysporum (López-Mondéjar et al 2011). According to them, the synergistic effect of compost and Trichoderma has favored a higher melon seedling growth in compostbased growing media production. Recent papers clearly demonstrated that fungi belonging to Trichoderma genus can also alleviate plants from abiotic stresses (salinity, drought, extreme temperature) (Balestrini et al., 2017). Recently, plant-derived protein hydrolysates (PHs) have gained prominence as plant biostimulants because of their potential to increase the germination, productivity and quality of a wide range of horticultural and agronomic crops. PHs can improve soil respiration, microbial biomass and activity, since microorganisms can easily use amino acids and peptides as C and N source. They also impacts the plant nutrition by forming complexes and chelates between peptides/amino acids and soil micronutrients (i.e.,Cu, Fe, Mn and Zn), therefore contributing to nutrients


availability and acquisition by the root system (De Pascale et al., 2018). The application of PHs, either by foliar or root (substrate drench) application can alleviate the negative effects of abiotic plant stress due to salinity, drought and heavy metals (Colla et al., 2017). Recently, there are interesting experiments indicating that PHs could be applied as a component of a seed coating blend, too. Amirkhani and co authors (2016) have reported that a 10% suspension of soy flour mixed with cellulose, and diatomaceous earth, applied with rotary pan seed coating equipment at 25, 30, 40 and 50% of the seed weight in broccoli seeds, although have reduced the percentage germination and the germination rate compared with the non treated control, have shown significant improvements on seedling root and shoot growth compared with controls, at the tenth and the thirty day after application. Yet, despite several encouraging evidences, the application of PHs in the nursery industry is still rare. Growth control and hardening techniques Vegetable transplants can quickly outgrow the optimal size for shipping and transplanting, and thus causing a major concern for commercial nurseries, especially when transplanting is delayed because of inclement weather at the time of field establishment (Agehara and Leskovar, 2015). Therefore, transplant conditioning alternatives were commonly applied in order to obtain shorter stem, stronger and more elastic transplants to facilitate long distance transport and better resist potential adverse post transplant conditions. Effective growth control and transplant hardening can be achieved through proper combination of individual practices such as; day-night temperature difference, less frequent watering, increased ventilation, restricted fertilization, mechanical hardening and use of plant growth retardants. In any case, transplant hardening needs to be done without totally stopping the growing process. Over-hardening will require too much time for growth to resume and earliness will be delayed (Prunty et al., 2015). Day-night temperature difference (DIF) Plant stem growth rates of some vegetable species are positively correlated to the difference in day temperature (DT) and night temperature (NT), which is termed DIF (DIF=DT-NT). Using DIF helps to keep the seedlings compact in size without using growth regulators (Erwin and Heins, 1995). Since high temperatures during the first three to four hours after sunrise can cause considerable elongation in vegetable seedlings, it can be mitigated by keeping the green-

house temperature the few hours early in the morning, 4-5 ºC cooler than night temperature (Wien, 1997). Irrigation deficit / vapor pressure deficit Water deficit stress can be applied in varying degrees to seedlings. Its main effect at moderate levels is the reduction of plant leaf area, before photosynthesis or respiration is affected, with the result of a higher rate of dry matter accumulation per unit leaf area (Liptay et al., 1998). A desirable level of water restriction results in stocky, stress resistant seedlings able to withstand environmental stresses after transplanting outdoors. However, as plant transpiration rate is largely affected by environmental conditions, determination of irrigation timing before imposing too much water stress requires a great deal of experience (Balliu et al. 2017). Severe water deficits results in overstressed seedlings with stunted growth and poor establishment when transplanted into the field (Liptay et al., 1998). There are evidences that hardening with a high vapor-pressure-deficit (VPD) can decrease water conductance and thereby enhance drought tolerance of cucumber seedlings by reducing transpiration (Shibuya et al., 2017). This is particularly useful during transplant establishment. The high-VPD hardening could enhance the tolerance to short-term drought without stomatal or non-stomatal limitation of photosynthesis (Shibuya et al., 2017). Someone should mind that after plants are hardened, they will require a full watering before outplanting and good soil moisture availability at the outplanting site will needed during their early establishment in the field (Jacobs et al., 2014). Low temperature conditioning Low temperature represents one of the most harmful abiotic stresses affecting temperate plants (Janská et al., 2010). Chilling stress causes many changes in the metabolism of plants which lead to biochemical, physiological and morphological modification and promote plant’s defense systems against stress factors (Kalisz et al., 2014). In fact, most temperate plant species have evolved a degree of cold tolerance, the extent of which is typically dependent on a combination of the minimum temperature experienced and the length of exposure to cold stress (Janská et al., 2010). Temperate plant species can acclimatize to low nonfreezing temperature through synthesis of cryoprotectans, increasing activity of antioxidative enzymes and launching the synthesis of non-enzymatic antioxidants (Kalisz et al., 2014). Therefore, acclimated plants should better withstand 47

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field conditions and they should be characterized by more intensive growth. Chilling treatment, applied at juvenile stage, can also induce transition to the generative phase. Positive effects of low temperature conditioning were reported in broccoli Grabowska et al., (2013) and Kalisz et al., (2014). According to Grabowska et al., (2013), dark-chilling (2°C/2 weeks), 4-week-old broccoli seedlings were more advanced in the generative development the non-chilled control plants. However, chilling of oldest, 8- and 10-week-old seedlings has negatively affected the inflorescence quality, through planed and spread shape of heads and non-uniform shape of flower buds. Similarly, Kalisz et al., (2014) have found that chilling of broccoli seedlings for 1 week or 2 weeks at 6, 10 and 14°C have positively influenced the weight of broccoli heads, and the heaviest heads were obtained as a result of transplant chilling at 6°C. However, the cold stress has negatively affected the relative growth rate (RGR) of cauliflower seedlings during the cold treatment period. Due to that, end of nursery period they have recorded a smaller dry weight (W) and leaf area (LA) compared to common non treated seedlings (Kuçi et al., 2012). The exposure of plants of a tropical or subtropical nature, such as members of the Cucurbitaceae and Solanaceae, to chilling temperatures may stunt plant growth, induce wilting and leaf necrosis, and increase disease susceptibility (Korkmaz and Dufault, 2001). Experiments conducted with watermelon and melon have evidenced that exposure of watermelon and melon seedlings to cold stress in the range from 9 to 81 hours, was followed by decreased seedling’s shoot and root fresh and dry weights, leaf area, chlorophyll and carbohydrate contents. Plants cold stressed for up to 81 hours transpired more for first week after transplanting than those exposed to shorter periods of cold stress, and vining (date first runner touched the ground), flowering, and fruit set were delayed significantly as cold stress hours increased. Although early yields were unaffected, total yields decreased linearly with increasing hours of cold stress (Korkmaz and Dufault, 2001). Mechanical conditioning Stem elongation can be reduced by mechanical stimulation such as brushing the upper canopy, shaking, and vibration by wind or forced aeration. These mechanical conditioning methods inhibit stem elongation by stimulating ethylene production, which in turn inhibits cell elongation and promotes stem thickening (Agehara & Leskovar 2014). Brushing has proved successful in Solanaceous 48

crops, (tomato, pepper, and eggplant), but care should be taken with cucurbits, which are more fragile and can be damaged by brushing. Mechanically conditioned transplants of processing tomatoes resumed growth after transplant shock as quickly as did untreated plants (Garner and Björkman, 1999), and subsequent canopy development and yield from mechanically hardened tomato (Garner and Björkman, 1999) and melon (Ayastuy et al., 2011) plants was similar to those from control plants. However, mechanical stress by vibration during transportation is known to negatively impact the transplants. It could physically damage the transplants or promote ethylene production which in high concentration induces adverse physiological impacts such as flower abortion or leaf yellowing especially during long distance transportation (Kubota et al., 2013). Plant growth retardants and plant hormones Plant growth retardants, such as daminozide, paclobutrazol, and uniconazole, are used in ornamental plug production to improve plant compactness, marketable value, and shelf life. There are also reports showing that melon seedlings hardened with low paclobutrazol concentration (10 mg L -1) showed a good post-transplanting growth which was reflected in a higher crop yield compared to the control (Ayastuy et al., 2011). However, regulations for their use are rather restrictive for vegetable crops, and at present, uniconazole is the only registered chemical for use in solanaceous crops (Agehara and Leskovar 2014, 2015). In fact, the growth modulating effect of uniconazole appears to be limited to height control, which is beneficial for producing compact transplants, rather than as a growth holding strategy (Agehara and Leskovar, 2015). While growth retardants acts mostly as gibberellin biosynthesis inhibitors (Ayastuy et al., 2011), abscisic acid (ABA) is a plant hormone, which triggers adaptive growth responses to water stress (Agehara and Leskovar, 2015). Its immediate physiological response is stomatal closure, which in turn inhibits photosynthesis and transpiration-driven mass flow of nutrients, whereas the morphological response is inhibition of leaf expansion. Thus, the overall effect of ABA is shoot growth suppression (Agehara and Leskovar, 2015). It is proved that treatment of tomato seedlings with an ABA analog prior to transplanting can help prevent the desiccation that is an important part of transplant shock without compromising longterm growth and yield potential of crops (Sharma et al., 2005; Agehara and Leskovar, 2014). Earlier, Berkowitz and Rabin (1988) reports that ABA appli-


cation can reduce transplant shock and increase yield of bell pepper. Different from long lasting effect of uniconazole, application of ABA at a specific rate and length of exposure (5-day ABA-exposure at 1.0 mM) in pepper seedlings could eliminate the risk of stressing young seedlings to the point of physiological injury as previously reported for some hardening techniques (Agehara and Leskovar, 2015; Ban et al., 2017). However, the efficiency of ABA application is crop and time depended. Multiple dose applications initiated at the cotyledon plant growth stage were proved to be more effective to control transplant height of bell pepper seedlings (Biai et al., 2011). Abstract Producing ‘good quality’ vegetable transplants demands a series of horticultural practices and interventions which overall enable the raise of plant from start to finish by slow, steady, uninterrupted growth and with minimal stress and guarantee appropriate growth rate, optimum root to shoot ratio, good stand establishment capabilities and high yield potentials of new plantlets. Because growth of vegetable transplant is a summary of many interacting and confounding factors which moderate each other, rather than following standardized recommendations, good quality vegetable seedlings can only be produced through careful selection of substrate, or substrate mixtures, appropriate manipulations of temperature regime, fertilization regime, the module size and transplanting age, and proper combination of plant growth promoters and growth control and hardening techniques. Key words; substrate, peat substitutes, module size, transplant age, fertilization, plant biostimulants, hardening. Bibliography ABDEL LATEF, A.A.H., CHAOxING, H., 2011. Effect of arbuscular mycorrhizal fungi on growth, mineral nutrition, antioxidant enzymes activity and fruit yield of tomato grown under salinity stress. Sci. Hort. 127: 228-233. https://doi.org/10.1016/j.scienta.2010.09.020 A GEHARA , S., L ESKOVAR , D.I., 2014. Growth Reductions by Exogenous Abscisic Acid Limit the Benefit of Height Control in Diploid and Triploid Watermelon Transplants. HortSci. 49: 465–471. A GEHARA , S., L ESKOVAR , D.I., 2015. Growth suppression by exogenous abscisic acid and uniconazole for prolonged marketability of bell pepper transplants in commercial conditions. Sci. Hort. 194: 118–125. https://doi.org/10.1016/j.scienta.2015.08.010 AHMED, K., CRESSWELL, G. C., HAIGH, A. M., 2000. Comparison of sub-irrigation and overhead irrigation of tomato and lettuce seedlings. J. Hort. Sci. Biotechn. 75: 350-354.

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