Hort. Environ. Biotechnol. 52(2):133-139. 2011. DOI 10.1007/s13580-011-0200-y
Research Report
Antioxidant Phytochemicals in Lettuce Grown in High Tunnels and Open Field Myung-Min Oh1, Edward E. Carey2, and C. B. Rajashekar3* 1
Department of Horticultural Science, Chungbuk National University, Cheongju 361-763, Korea 2 K-State Horticulture Research and Extension Center, Olathe, Kansas 66061, USA 3 Division of Horticulture, Kansas State University, Manhattan, Kansas 66506, USA *Corresponding author:
[email protected]
Received October 4, 2010 / Accepted December 8, 2010 GKorean Society for Horticultural Science and Springer 2011
Abstract. Genotype along with growing and management conditions can affect the content and the composition of phytochemicals in plants. Two lettuce (Lactuca sativa L.) cultivars, ‘Baronet’ and ‘Red Sails,’ were grown in an open field and high tunnels to examine the effect of growing conditions on their phytochemical content. The total phenolic concentration and antioxidant capacity of lettuce increased in response to transplanting from greenhouse to both open field and high tunnels. However, the increase was much greater when seedlings were transplanted to the open field and was more than 4 fold over the pre-transplant stage. The concentrations of two major phenolic compounds, chicoric acid and chlorogenic acid, were about 2.5-5.5 times higher in both cultivars when grown in open field than in high tunnels. Also, growing lettuce in open field resulted in a greater activation of key genes (phenylalanine ammonia-lyase, L-galactose dehydrogenase and 쩂-tocopherol methyl transferase) involved in the biosynthesis of phenolic compounds, ascorbic acid and 쩀-tocopherol. ‘Red Sails’ accumulated caffeic acid 4 times as much in open field as it did in high tunnels and overall contained higher amount of phenolic compounds, especially in open field, than did Baronet. Although lettuce plants grown in open field were richer in phytochemicals, a significant reduction in biomass accumulation occurred when the lettuce plants were grown in open field compared to high tunnels regardless of cultivar. These results show that growing conditions, in addition to genotype, can significantly affect the content of many phenolic compounds in lettuce and that growing lettuce under open field can have a positive impact on its health-promoting qualities. Additional key words: antioxidant capacity, gene expression, genotype, growing conditions, health, phenolic compounds
=bhfcXiWh]cb Lettuce has been one of the major leafy vegetables in the world since ancient times (Harlan, 1986). It contains a number of nutritive and health-promoting compounds such as phenolic compounds, vitamin A, C, and E, calcium, and iron that prevent many chronic diseases (Caldwell, 2003; Llorach et al., 2004; Nicolle et al., 2004; Romani et al., 2002). Many of these compounds found in lettuce have an effective antioxidant and other health-promoting properties (Caldwell, 2003; Llorach et al., 2008; Marchand, 2002; Wu et al., 2004) and of the various types of lettuce commonly grown, leaf-type lettuce is most abundant in health-promoting phytochemicals (Liu et al., 2007). Many phenolics including flavonoids present in lettuce have been extensively studied with regard to their antioxidant properties (Altunkaya et al., 2009; Llorach et al., 2008). A number of factors including plant genotype and growing
conditions can have a large impact on the quality of vegetable crops, especially in relation to their health-promoting phytochemicals (Jin et al., 2009; Schreiner, 2005). A variety of abiotic and biotic factors such as light, temperature, nutrient status, and pest infection can directly affect the biosynthesis of phytochemicals in plants (Dixon and Paiva, 1995). In addition, previous studies have suggested that harsher environmental conditions could lead to improved phytochemical content in plants (Demmig-Adams and Adams, 2002; Oh et al., 2009a, 2009b). Thus, growing conditions are likely to influence the phytochemical content and composition in plants. In this study, lettuce plants were grown in open field and high tunnels to provide contrasting growing conditions. High tunnels, which are unheated and passively ventilated production structures, have been widely used in Asia and Europe and are increasingly being used in the US (Lamont et al., 2003; Spaw and Williams, 2004; Wells, 1996). High
Myung-Min Oh, Edward E. Carey, and C. B. Rajashekar
AUhYf]U`g UbX AYh\cXg D`Ubh AUhYf]U`g The study was conducted at K-State Horticulture Research and Extension Center, Olathe, Kansas State University in a Haygrove high tunnel, 24.4 m × 7.3 m (L × W), covered with Luminance THB polythene (Visqueen Building Products, London, UK) and an adjacent open field, 24.4 m × 7.3 m (L × W). Seeds of two lettuce cultivars (Johnny’s Selected Seeds, Winslow, ME, USA), ‘Baronet’, a green loose leaf type, and ‘Red Sails’, a red loose leaf type were germinated in plastic pots, 8 cm × 8 cm × 7 cm (L × W × H), containing a commercial growing medium (Metro-Mix 350, Sun Gro, Canada). Fifty seedlings per cultivar were grown at 25/18G (day/night) in a greenhouse for 3 weeks before transplanting to both the high tunnel and open field with Kennebec silt loam. Based on previous soil analyses at Kansas State University soils laboratory (Manhattan, KS, USA), chicken manure compost (3N-4P-2K; Early bird compost, CMPP, Inc., High Point, MO, USA) was applied at a rate of 6.7 kg Nha-1 for plots in high tunnels and open field 1 week before transplanting. Drip irrigation system was used in this study. Both lettuce cultivars (36 plants per cultivar) were planted on either side of the drip tape and irrigated as needed. The length of each row and spacing between rows were 3.6 m and 1.5 m, respectively. The seedlings were transplanted at 0.3 m spacing within the row. Three plots per cultivar were randomly laid out under both open field and high tunnels. Air temperature inside and outside was monitored by HOBO sensor (Onset Computer Corp., Pocasset, MA, USA). Outside air temperature and the difference between day and night temperatures (DIF) during whole
experiment period, and continuous changes of air temperature in open field and high tunnel for 8 d before harvest are shown in Fig. 1. HchU` D\Ybc`]W 7cbWYbhfUh]cb To measure the total phenolic concentration of the lettuce leaves, three leaf samples were collected from three randomly selected lettuce plants in each treatment. Leaf samples were immediately frozen in liquid N2 and stored at -20Guntil use. A modified Folin-Ciocalteu reagent method (Ainsworth and Gillespie, 2007) was used to analyze the total phenolic concentration of lettuce leaves as described elsewhere (Oh et al., 2009b). A freshly made 1 mgmL-1 gallic acid (Acros Organics, Geel, Belgium) solution was used as a standard. 5bh]cl]XUbh 7UdUW]hm Leaf samples for measuring antioxidant capacity were collected as described before. The antioxidant capacity of lettuce leaves was measured by a modified ABTS [2,2'azino-bis (3-ethylbenzthiazoline-6-sulphonic acid)] method (Awika et al., 2003; Miller and Rice-Evans, 1996; Pennycooke et al., 2005) as outlined elsewhere (Oh et al., 2009b).
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tunnels are known to produce higher yields and better quality of crops by extending their growth period and by providing protection from various adverse factors (Hodges and Brandle, 1996; Rader and Karlsson, 2006). Although studies on green leafy vegetables have indicated higher antioxidant capacity and phenolics in response to open field cultivation relative to high tunnels, the results have been variable in regards to species, growth stage and specific phenolic compounds (Zhao et al., 2007, 2009). In addition, there is limited information on crop quality in relation to health-promoting phytochemicals in crops grown in high tunnels relative to those grown in open fields. Thus, in this study, we examined whether growing conditions involving open field and high tunnels and genotype affect the quality of lettuce crop in relation to its antioxidant properties and phytochemical content. We also determined if these factors can influence the activation of key genes involved in the biosynthesis of major antioxidants in lettuce plants.
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Fig. 1. Field air temperature data. Outside air temperature and the difference between day and night temperatures (DIF) during the whole experimental period (A) and continuous changes of air temperature in open field and high tunnel for 8 days before harvest (B) at K-State Horticulture Research and Extension Center, Olathe, KS are presented.
Hort. Environ. Biotechnol. 52(2):133-139. 2011.
