UV irradiance as a major influence on growth, development and secondary products of commercial importance in Lollo Rosso lettuce ‘Revolution’ grown under polyethylene films
Introduction
One of the most exciting developments in commercial protected cropping in recent years has been the introduction of spectral filters incorporated into horticultural polythene films, which block specific wavebands. Reduction in plant height has been observed in a range of species under polyethylene films with low transmission of FR light, offering an alternative method of producing compact high quality plants without using chemical growth regulators (Haeringen et al., 1998, Rajapakse et al., 1999, Runkle and Heins, 2001, Rajapakse and Li, 2004, Fletcher et al., 2005). A new generation of UV transparent and UV blocking plastic films has also been developed which have a number of potential applications for protected cropping. Plastic films which are opaque to UV offer an environmentally friendly way of controlling pests and diseases (Doukas and Payne, 2007). Recent research has shown that the incidence of whitefly was significantly reduced under UV blocking films compared to standard films (Doukas, 2002, Doukas and Payne, 2007). Currently, commercial production under plastic film structures is based on standard horticultural films (or glass) that block some of the UV radiation. In contrast, plastic films with high UV transmission may offer a way of increasing secondary products and therefore potential health benefits in response to increased UV radiation. Little research has been carried out, however, to quantify the effects of UV radiation on crop quality.
UV radiation can be regarded as a stress factor which is capable of significantly affecting plant growth characteristics. Plant height, leaf area, leaf length have been showed to decrease, whereas leaf thickness was increased in response to UVB radiation (Teramura, 1983, Tevini and Teramura, 1989, Rozema et al., 1997). In addition, photosystem II can be adversely affected by UVB radiation (Teramura, 1983, Caldwell et al., 1989, Tevini et al., 1991, Rozema et al., 1997). Chlorophyll fluorescence has been shown to be a simple and reliable technique for measuring the performance of photosystem II (Maxwell and Johnson, 2000) and the extent to which environmental stress can impair photosynthetic efficiency (Maxwell and Johnson, 2000), e.g. temperature (Lu and Zhang, 2000, Daymond and Hadley, 2004), UV light (Kolb et al., 2001, Reddy et al., 2004), salinity (Chen et al., 2004, Jiang et al., 2006) and water (Saccardy et al., 1998).
Plants produce a wide range of flavonoids and related phenolic compounds which tend to accumulate in leaves of higher plants in response to UV radiation (Tevini and Teramura, 1989, Rozema et al., 1997). It has been suggested that plants have developed UV-absorbing compounds to protect them from damage to DNA or to physiological processes caused by UV radiation (Stapleton, 1992). These UV-absorbing compounds accumulate in the epidermis, preventing UV radiation from reaching the photosynthetic mesophyll (Stapleton, 1992, Braun and Tevini, 1993). In the red ‘Lollo Rosso’ lettuce, studied here, the main phenolic metabolites are phenolic acids (cafeoyltartaric, chlorogenic, dicaffeoylquinic), flavonoids (quercetin-3-glucuronide, quercetin-3-glucoside, quercetin 3-6-malonyglucoside, quercetin-3-6-malonyglucoside 7-glucoside) and the anthocyanin cyanidin 3-malonyglucoside (Ferreres et al., 1997).
Plants may produce secondary products to protect them against UV light damage, but these metabolites also play an important role in human health. Phenolics, flavonoids and anthocyanins are responsible for antioxidant activity in fruits and vegetables (Wang et al., 1996, Cao et al., 1997). Epidemiological studies have shown a positive relationship between fruit and vegetable consumption and reduced incidence of chronic and degenerative diseases (Heinonen et al., 1998, Prior et al., 1998). These natural antioxidants have scavenging properties against oxygen free radicals, offering protection to lipids, proteins and nucleic acids (Heinonen et al., 1998).
Most published results on the effects of UV radiation on plants are based on the effect of UV light added to either natural or artificial light (using UV lamps) (Rozema et al., 1997, Krizek, 2004). Published studies based on UV exclusion from natural radiation are limited (Krizek et al., 1997, Krizek et al., 1998, Krause et al., 1999, Kolb et al., 2001, Kolb et al., 2003). Krizek (2004) stated that results of experiments using supplementary UV radiation are difficult to interpret due to unrealistically high UV radiation levels and inappropriate levels of UVA (320–400 nm) irradiance. It seems that ambient levels of PAR and UVA mitigate damage caused specifically by UVB radiation. According to Caldwell et al. (1994), UVB radiation caused biomass reduction only when PAR and UVA levels were reduced to less than half that of sunlight levels. In addition, UVA proved to be effective in ameliorating UVB damage when PAR was low.
Here we describe the effect of UV radiation on quality and on growth and development of ‘Lollo Rosso’ lettuce under field conditions, using films that sequentially block specific bands within UV region of the spectrum. The potential for increasing secondary product levels by using a highly UV transparent film compared to standard horticultural UVI/EVA film is assessed.
Section snippets
Growth conditions and plant husbandry
Two experiments were conducted during the summers of 2005 and 2006 at the School of Biological Sciences Laboratories Field Unit, Shinfield, Reading, UK. Each experiment was carried out in a suite of 10 tunnels, measuring 3 m × 2.7 m × 6.8 m (W × H × L), specifically designed for studying experimental cladding materials. Six plastics that block progressively at 20-nm intervals across the UV region were supplied by British Polyethylene Industries Agri, Stockton-on-Tees, UK. The spectral transmission of each
Effect of UV on growth
Lettuce showed a strong response to UV radiation. In reduced UV radiation vegetative growth was increased compared to plants in high UV radiation levels (Fig. 2). No significant interaction between vegetative growth and planting month was observed in 2005 and in both years total above ground dry weight in lettuce plants under UV350, UV370, UV380 and UV400 was higher than in plants under the UV280 and UV320 (P < 0.001). Plants under a complete UV blocking film (UV400) produced 40% and 122% more
Discussion
The Lolo Rosso lettuce ‘Revolution’ used in this study showed a clear response to an increase in the proportion of UV light by increasing secondary metabolites and reducing growth. Total anthocyanins were greatest when both UVA and UVB were present (UV280 film). Where UVB was excluded and UVA was transmitted, as in the UV320 film, anthocyanin content was reduced. This means that a highly UV transparent film (UV280) is capable of increasing anthocyanin content compared to a standard
Conclusion
The key findings of this study are the high levels of secondary products under UV transparent films. Plants in the presence of UVB and UVA (UV280) appeared not to be stressed and this may be because they accumulate secondary products which effectively protect the photosynthetic apparatus. Our results indicate that the quality of Lollo Rosso (in terms of phenolic compounds) can be enhanced under a highly UV transparent film (UV280) compared to the standard horticultural UVI/EVA film (UV320).
Acknowledgements
The authors wish to thank the RETF (Research Endowment Trust Fund) of The University of Reading and British Polyethylene Industries for co-funding this study and Mr. David McLay for his technical support during the experimental period.
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