RuizZealand et al.—Shading paclobutrazol influenceScience, on apricot fruitfulness New Journal and of Crop and Horticultural 2005, Vol. 33: 399–406 0014–0671/05/3304–0399 © The Royal Society of New Zealand 2005
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Effect of shading and paclobutrazol during dormancy on apricot (Prunus armeniaca ) productivity
D. RUIZ J. EGEA P. MARTÍNEZ-GÓMEZ Departamento de Mejora Vegetal CEBAS-CSIC P.O. Box 164 E-30100 Espinardo, Murcia, Spain email:
[email protected] Abstract Shading all or parts of trees and the application of paclobutrazol during the dormant season, were studied as strategies to improve apricot (Prunus armeniaca) fruitfulness. Shading the whole tree reduced flower bud abscission, probably because of a decreased level of gibberellins resulting from a reduction of temperature in the shaded trees. Partial shading of individual branches did not reduce percentage of flower bud abscission compared to unshaded branches. Shading also accelerated ovule maturity at anthesis. These results show the importance of solar radiation on the basic structure of the tree (the trunk and primary branches) for the productive behaviour of apricot. Paclobutrazol treatments did not significantly influence flower bud drop and fruit set on apricot during the 2 years of the study. However, these treatments slightly accelerated ovule maturity at anthesis, probably because of the reduction in the level of gibberellins. Keywords Prunus; gibberellins; fruitfulness; bud drop; ovule development
H05055; Online publication date 9 November 2005 Received 18 May 2005; accepted 29 July 2005
INTRODUCTION Some apricot (Prunus armeniaca L.) cultivars growing in Spain, known by the generic name of “Pepitos”, i.e., ‘Guillermo’ or ‘Pepito del Rubio’, are of good quality but show erratic productivity. Studies have indicated that low yields are the result of a combination of factors, including low floral density, high flower bud abscission, and low floral fertility (Austin et al. 1996; Egea & Burgos 1998). A small reduction of this high level of flower bud abscission, for example from 90% to 80%, could be very important in practice, for final yield. Several authors have indicated that pollination compatibility is an important factor affecting fruit set (Austin et al. 1996; McLaren & Fraser 1996; McLaren et al. 1996). Lack of winter chilling is also an important factor increasing flower bud abscission (Brown 1958; Legave 1978). Brown (1953) and Uriu (1964) have shown a relationship between irrigation level and flower bud abscission. However, more recently, Alburquerque et al. (2003) did not find any clear correlation between irrigation or winter temperatures and percentage flower bud abscission. In Mediterranean conditions, during the dormant season, the whole tree structure is frequently exposed to direct solar radiation for many hours each day and thereby temperatures likely higher than air temperature. McLaren et al. (1996) described significant correlations between percentage fruit set and maximum temperature during the blooming period in the apricot cultivar ‘Sundrop’. Beppu et al. (2001a), working with ‘Satohnishiki’ sweet cherry, found that increasing maximum temperatures up to 25°C for 1 month before anthesis, markedly affected fruit set because of accelerated degeneration of the nucelli and embryo sacs. In addition, Martínez-Gómez et al. (2002) found that an increase in the maximum temperature of sunny days by 6–7°C increased flower bud abscission. Therefore, a decrease of direct incidence of solar radiation on the whole tree or parts of trees during the dormant season could be a good strategy to improve apricot productivity. On the other hand, high levels of gibberellins have also been described as causing flower bud mortality
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(Painter & Stembridge 1972; Southwick et al. 1997) and regulate embryo sac development sometimes inducing early degeneration (Beppu et al. 2001a). Paclobutrazol (beta-[(4-chlorophenyl)methyl1]-alpha-(1,1-dimethyl1)-1-H-1,2,4-triazole-1-etanol) is a well known plant bioregulator (PBR) (Lever 1986) used in a wide range of fruit crops as an inhibitor of gibberellin biosynthesis (Dalziel & Lawrance 1984). This PBR causes a reduction of cell division rates and a reduction in vegetative growth resulting in trees with a more compact structure (Webster & Quinlan 1984; Quinlan & Richardson 1986; Granger & Traeger 1993; Jacyna & Dodds 1995; Grochowska et al. 2004). It also causes greater partitioning of assimilate to reproductive growth, flower bud formation, and fruit growth (Lever 1986; Quinlan & Richardson 1986). Paclobutrazol has been used in several Prunus species for improving productivity and fruit quality (Knowles & Dossier 1986; Blanco 1987; Facteau & Chestnut 1991; George & Nissen 1992; Lurie et al. 1997; Grochowska et al. 2004). However, studies of paclobutrazol application in apricot are limited and not conclusive about reproductive behaviour and increase in yield (Proebsting & Mills 1985; Jacyna & Dodds 1995). The objective of this work is to determine the consequences of avoiding the direct incidence of solar radiation on the whole tree or parts of the tree and the application of different paclobutrazol treatments during the dormant season for flower bud drop, fruit set, and ovule development of apricot cultivars ‘Guillermo’ and ‘Pepito del Rubio’.
MATERIAL AND METHODS Plant material The experiments were conducted on 15-year-old trees of the Spanish apricot cultivars ‘Pepito del Rubio’ and ‘Guillermo’ grafted onto seedlings of apricot cultivar ‘Real Fino’, grown in an experimental orchard in Cieza (Murcia, south-east Spain). ‘Pepito del Rubio’ and ‘Guillermo’ are self-compatible local cultivars from Murcia, characterised by high fruit quality with a high degree of flower bud drop and very irregular yields. Artificial shading of the whole tree During 2002, three whole ‘Guillermo’ apricot trees were shaded with a shading net made of black polyethylene (shading rate of 80%), thus avoiding the direct incidence of solar radiation (Fig. 1A).
Another three unshaded ‘Guillermo’ apricot trees were used as controls. The shading net was set up during winter (20 January) and removed just before flowering, 11⁄2 months later. Artificial shading of individual branches During the second year (2003), only two branches of each of the ‘Guillermo’ apricot trees were shaded with the shading net (Fig. 1B). The tested trees were different to those of the first experiment. Another three unshaded ‘Guillermo’ apricot trees were used as controls. The shading net was set up on the same date (20 January) and removed just before flowering, 11⁄2 months later. Effect of artificial shading on radiation and temperature of branches Radiation was evaluated using a Megatron Type E (Megatron Ltd, London, United Kingdom); this parameter was measured in the shaded and the unshaded branches in both experiments. The temperatures of the different branches were measured in the shaded and the unshaded branches in both experiments, using an infrared thermometer (Everest Interscience Inc, Tucson, United States). Radiation and temperature were evaluated at in the early afternoon (1400 h) in both experiments during the period of artificial shading of the branches. Measurement were made in the primary branches, which had diameters of 18–20 cm during the first experiment. Radiation and temperature during the second year were measured in 2-year-old branches (secondary branches), with diameters 2.5–3 cm. Application of paclobutrazol treatments Application of paclobutrazol was performed in winter (end January) during 2002 and 2003 using the commercial product Cultar® (paclobutrazol 25%) (Syngenta, Surrey, United Kingdom). Three different doses of paclobutrazol were applied directly in the soil near the trunk, including 0.75, 1, and 1.25 g/tree (commercial doses recommended in Spain for adult apricot trees), studying eight ‘Pepito del Rubio’ trees (replications) per treatment during the 2 years of the experiment. Eight Pepito del Rubio’ trees were kept untreated as controls. Evaluation of flower bud drop and fruit set In all the experiments, two different vigorous branches per tree, with lengths of c. 145 cm and 3 cm diam. were randomly chosen for each tree. Flower bud abscission percentage was determined by comparing the initial number of flower buds (stage A;
Ruiz et al.—Shading and paclobutrazol influence on apricot fruitfulness
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Fig. 1 A, Whole ‘Guillermo’ apricot (Prunus armeniaca) tree shaded with a shading net made of black polyethylene during 2002. B, Branch shaded with a shading net made of black polyethylene spread over a wooden framework on a ‘Guillermo’ apricot tree during 2003.
Baggiolini 1952) and the final number of open flowers (stage F; Baggiolini 1952). Flowers were handpollinated with self-compatible pollen. After 6 weeks, percentage fruit set was calculated from the number of fruits divided by the number of pollinated flowers. Developmental stage of primary ovules at anthesis Ovules of just-opened flowers were examined in all the treatments during 2002 and 2003, to determine the stage of development of the embryo sac at anthesis as well as the presence of malformations. Twenty flowers per tree were picked in the field and fixed in FAA (90% ethanol at 70%, 5% formaldehyde at 40%, 5% glacial acetic acid at 99.5%). The ovaries were scraped to remove the velvety layers that covered them and placed in ethanol 70% to remove the fixative. Flowers were dehydrated using tertiary butyl alcohol series (TBA) and then embedded in paraplast. Serial sections at
10 µm were mounted on slides impregnated with an adhesive of gelatin, glycerin, and 3% formaldehyde. Samples were stained as previously described by Gerlach (1969), and observed under an Olympus BH2 microscope (Tokyo, Japan). Although 20 pistils per treatment were examined, only the results of the primary ovules that are the only functional ovules are presented (Alburquerque et al. 2002). Statistical analysis Each of the treatments was performed in a completely randomised experimental design. Percentage of flower bud drop, fruit set, and ovules with the embryo sac at different stages of development were recorded in ‘Guillermo’ and ‘Pepito del Rubio’ apricot cultivars in each treatment during the 2 years of the study calculating mean values and standard errors. A c2 test was used to identify significant differences among means of the different treatments.
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RESULTS AND DISCUSSION ‘Guillermo’ apricot Effect of artificial shading on radiation and temperature of branches Radiation and temperature in the shaded and unshaded branches in both experiments are shown in Table 1. There was a decrease in radiation intensity in shaded branches to c. 85% of that of the unshaded branches. In the early afternoon, radiation reached 84 000 (in 2002) and 75 000 lux (in 2003) in the unshaded part of the tree and 12 500 and 11 500 lux, respectively, in the shaded branches. Higher incidence of solar radiation in unshaded trees significantly increased temperature in the basic structure of the tree (trunk and primary branches) by 6°C in the early afternoon in 2002 (Table 1). In 2003, the temperature in the basic structure of the tree with direct solar radiation was 28.2°C, higher than the temperature observed in the secondary branches, with mean values of 18.9°C (shaded) and 21.7°C (unshaded). Covering of branches with the shading net greatly reduced the radiation intensity and the solar radiation, with a consequent reduction of temperature mainly in the basic structure of the tree (primary branches and trunk). Beppu & Kataoka (2000) found that artificial shading reduced daily maximum air temperature by 3.2°C and the occurrence of double pistils in ‘Satohnishiki’ sweet cherry. In our study, shading of branches reduced daily maximum temperature by 2.7°C. However, in the basic structure of the tree (primary branches and trunk) the reduction of temperature as a result of shading the whole tree was between 6 and 9°C. Effect of artificial shading on flower bud abscission and fruit set Flower bud abscission was significantly lower, from 92.6 to 77.3, when the whole tree was shaded by the shading net. However, the percentage fruit set was similar in shaded and unshaded trees (9.6 and 11.1 respectively). Artificial shading of individual branches did not affect flower bud abscission in shaded and unshaded branches (92.9 and 89.7 respectively), however, fruit set was reduced in unshaded branches (16.7% and 6.2% respectively) (Table 2). In both experiments, a delay in the flowering date of 5–6 days was observed in flowers from shaded parts of the tree, probably because of the reduction of diurnal temperatures.
Flower bud abscission was reduced in the whole shaded trees whereas fruit set was not affected. In this situation, reduction of flower bud drop with maintenance of fruit set, final yield was higher. These results seem to indicate that the reason of the increase of the yield could be the temperature increase. In previous studies, Martínez-Gómez et al. (2002) reported that an increase of 6–7°C achieved by bagging small branches with plastic bags during the same period as the shading was carried out in this study, produced an important increase in flower bud abscission. On the other hand, McLaren et al. (1996) described reduction in fruit set in the apricot cultivar ‘Sundrop’ when the maximum temperature during bloom increased significantly. When artificial shading was only applied to the individual secondary branches, the reduction of flower bud abscission was not significant. Beppu et al. (2001a) observed an increase of endogenous gibberellins in apricot flowers with the increase of temperature in the tree. Effect of artificial shading on the developmental stage of primary ovules at anthesis Ovules from flowers developed on the shaded trees were slightly more advanced in anthesis than the ovules from unshaded trees (Table 3). In flowers from shaded trees, embryo sacs with four or even eight nuclei were observed, whereas in flowers from trees exposed to direct isolation, the sac of 80% of ovules did not open and the rest were in the two-nuclei stage. Ovules from both treatments (shaded and unshaded branches) in 2003 were more retarded, in comparison to those from of the first year. The acceleration of ovule maturity observed in the whole covered trees was not observed in the second experiment, where only individual branches were shaded. On unshaded branches, small percentages of malformed and degenerated (without megaspore) ovules were found. The results are similar to Beppu & Kataoka (2000) where high temperatures in the branches had an effect on the occurrence of flower malformations (ovules without megaspore). The slight differences in ovule maturity at anthesis between shaded and unshaded trees were probably because of differences in air temperature as described previously by Egea & Burgos (1998) and possibly an increase in the level of gibberellins as a result of this increase of temperature (Beppu et al. 2001a). The negative influence of these hormones in the development of flower buds is well known (Painter & Stembridge 1972).
Ruiz et al.—Shading and paclobutrazol influence on apricot fruitfulness ‘Pepito del Rubio’ apricot Effect of paclobutrazol treatments on flower bud drop and fruit set Percentages of flower bud drop during the 2 years of the study in the control trees (between 42.8% and 63.2%) (Table 4) were less than the high percentages observed in other apricot cultivars (Martínez-Gómez et al. 2002). However, fruit set values (between 31.0 in 2002, and 10.6 in 2003) were very low in comparison with the percentages observed in other apricot cultivars (Alburquerque et al. 2002).
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Seasonal variation in flower bud drop, and to a lesser degree fruit set, have been reported by McLaren et al. (1996), Egea & Burgos (1998), and Alburquerque et al. (2002). Flower bud drop was significantly higher during the first year of the study (62.8% on average in all treatments) in comparison with the second year (42.4%). Fruit set was much lower during the second year of the study. No statistically significant differences were found for the percentages of flower bud drop and fruit set in treated and untreated trees during the 2 years of the study.
Table 1 Mean values of radiation and temperature in the early afternoon (1400 h) in shaded trees, shaded branches and unshaded (control) ‘Guillermo’ apricot (Prunus armeniaca) trees. Standard error is indicated in parentheses. Values with different letters indicate statistically significant differences at the 5% level, according to the c2 test.
Treatment
Radiation (lux) Primary branches Secondary branches (18–20 cm) (2.5–3 cm)
Artificial shading of whole tree (2002) Shaded trees 12 500 (408) b – Control 84 000 (2828) a – Artificial shading of individual branches (2003) Shaded branches – 11 500 (2073) b Control – 75 000 (3000) a
Temperature (°C) Primary branches Secondary branches (18–20 cm) (2.5–3 cm) 27.6 (0.3) b 33.5 (2.8) a
– –
– 28.2 (2.3)
18.9 (2.8) a 21.7 (1.9) a
Table 2 Percentage of flower bud abscission and fruit set in shaded trees, shaded branches, and unshaded (control) ‘Guillermo’ apricot (Prunus armeniaca) trees. Standard error is indicated in parentheses. Values with different letters indicate statistically significant differences at the 5% level, according to the c2 test. Treatment
Flower bud abscission (%)
Fruit set (%)
77.3 (4.2) b 92.7 (3.8) a
9.6 (2.6) a 11.1 (5.2) a
92.9 (3.7) a 89.7 (6.5) a
16.7 (6.7) a 6.2 (4.6) b
Artificial shading of the whole tree (2002) Shaded trees Control Artificial shading of individual branches (2003) Shaded branches Control
Table 3 Percentages of ovules with the embryo sac at different stages of development, in flowers at anthesis from in shaded trees, shaded branches, and unshaded (control) Guillermo’ apricot (Prunus armeniaca) trees. Values with different letters indicate statistically significant differences at the 5% level, according to the c2 test. Treatment
No megaspore
Megaspore to tetrad
Artificial shading of the whole tree (2002) Control 0a 80.0 a Shaded trees 0a 56.2 b Artificial shading of individual branches (2003) Control 8.3 a 58.3 a Shaded branches 0b 75.0 a
Two nuclei
Four nuclei
Eight nuclei
Eight organised nuclei
20.0 a 25.0 a
0b 12.6 a
0b 6.2 a
0a 0a
16.6 a 16.6 a
8.3 a 8.4 a
0a 0a
8.5a 0b
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In agreement with our results, Klinac et al. (1991) and Khurshid et al. (1997) found a slight effect of paclobutrazol application in the reproductive behaviour of pear and apple during the first year but continued application did not produce any increase in yield. Facteau & Chestnut (1991) indicated a slight reduction of fruit set in cherry after application of paclobutrazol. Grochowska et al. (2004) observed increase of fruit set (productivity) after the application of paclobutrazol in plum, with less effect on cherry and apple. Klinac et al. (1991) also described an increase of even distribution of flower after the application of paclobutrazol in pear and Beppu et al. (2001b) indicated an slight increase of fruit set in cherry after the application of paclobutrazol. Effect of paclobutrazol treatments on the developmental stage of primary ovules at anthesis During the first year of the study (2002) ovules from all treatments were slightly more advanced than in the second year (Table 5). In this first year it is possible to observe a delay in the development of the ovule of the untreated trees (control) in comparison
with the treated trees. This delay was also observed in the second year of study. Significant differences were found for the percentages of ovules at different stages in treated and untreated trees during the 2 years of the study. Ovules from flowers developed in treated trees with 1 and 1.25 g/tree of paclobutrazol were slightly more advanced at anthesis than the ovules from untreated trees or treated with 0.75 g/tree during the 2 years of the study. Application of paclobutrazol accelerated the ovule maturity at anthesis in flowers from treated trees, and higher doses of application of this PBR increased the acceleration of the ovule maturity. A decrease in the level of gibberellins as a result of the treatment with paclobutrazol could explain these small differences with respect to retardation of ovule maturity at anthesis observed in the untreated trees as previously indicated Painter & Stembridge (1972) in peach. George & Nissen (1991) also showed an acceleration in the fruit development period in the peach cultivar ‘Flordaprince’ and a considerably advance of fruit maturity after the application of paclobutrazol. Costa et al. (1995)
Table 4 Percentage of flower bud drop and fruit set in ‘Pepito del Rubio’ apricot (Prunus armeniaca) trees in each paclobutrazol treatment during the 2 years of the study. Standard error is indicated in parentheses. Values with different letters indicate statistically significant differences at the 5% level, according to the c2 test. Flower bud drop (%) 2002 2003
Treatment Control 0.75 g paclobutrazol/tree 1 g paclobutrazol/tree 1.25 g paclobutrazol/tree
63.2 (16.2) a 60.0 (26.1) a 56.5 (23.8) a 62.7 (28.8) a
42.8 (15.4) a 44.9 (19.3) a 42.8 (20.2) a 39.0 (23.3) a
Fruit set (%) 2002
2003
31.0 (5.9) a 40.3 (9.1) a 35.7 (9.19 a 38.4 (15.3) a
10.9 (3.8) b 15.1 (4.3) ab 18.7 (6.8) a 10.6 (4.7) b
Table 5 Percentage of ovules with the embryo sac at different stages of development, in flowers at anthesis in ‘Pepito del Rubio’ apricot (Prunus armeniaca) trees with each paclobutrazol treatment during the 2 years of the study. Values with different letters indicate statistically significant differences at the 5% level, according to the c2 test. Treatment
No megaspore
Development stage during 2002 Control 0a 0.75 g paclobutrazol/tree 0a 1 g paclobutrazol/tree 0a 1.25 g paclobutrazol/tree 0a Development stage during 2003 Control 0b 0.75 g paclobutrazol/tree 8.3 a 1 g paclobutrazol/tree 0b 1.25 g paclobutrazol/tree 8.3 a
Megaspore to tetrad
Two nuclei
Four nuclei
Eight nuclei
Eight organised nuclei
38.5 a 41.7 a 15.4 b 16.7 b
15.4 b 25.0 ab 30.8 a 41.7 a
30.8 ab 16.7 c 46.2 a 33.3 ab
15.4 a 16.7 a 7.7 b 8.3 b
0a 0a 0a 0a
75.0 a 33.3 b 63.6 a 41.7 b
25.0 ab 33.3 a 18.2 b 25.0 ab
0c 8.3 b 18.2 a 16.7 a
0b 16.7 a 0b 8.3 a
0a 0a 0a 0a
Ruiz et al.—Shading and paclobutrazol influence on apricot fruitfulness described a delay in the maturation of embryo sac in pear after the application of this PBR. Our results are in accordance with the findings of Egea & Burgos (1998) who indicated that slight differences in the ovule development could not explain the differences in fruitfulness of apricot cultivars between different environmental conditions. CONCLUSIONS Artificial shading of the whole tree during dormancy improved the productivity of the apricot in Mediterranean conditions. This may be because of a decreased level of gibberellins in shaded trees as a resulting of reduced temperature of the trees. Our results show the important influence of the incidence of solar radiation on the structure of the tree (the trunk and primary branches), on flower bud abscission, and development of the ovule at anthesis. However, results showed the lack of influence of paclobutrazol application on the flower bud drop and fruit set in apricot. Exogenous applications of paclobutrazol in the soil did not affect the reproductive behavior of apricot, although, slightly accelerated the ovule maturity at anthesis.
ACKNOWLEDGMENTS This study was financed by the project “Mejora Genética del Albaricoquero” (AGL2001-112-C02-01) of the Spanish Ministry of Science and Technology.
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