THE EFFECT OF ULTRAVIOLET LIGHT ON PLANT DEVELOPMENT AND FRUIT PRODUCTION

Thesis Overview

INTRODUCTIONThe amount of ozone in the stratosphere over middle latitudes in the northern hemisphere has declined since 1978 (Stolarski et al. 1992). Assuming full compliance with the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer and its 1992 amendments, it is predicted that ozone depletion at northern mid-latitudes will peak at the turn of the century and be 12–13% less in winter}spring and 6–7% less in summer}autumn relative to 1960 (Madronich et al. 1994). Stratospheric ozone will return to pre-depletion concentrations by the end of the 21st century (Madronich et al. 1994). However, continued implementation of the Montreal Protocol remains uncertain (Greene 1995; Jordan 1995) and factors other than the concentration of ozone depleting substances may place the ozone layer at risk. For example, spring-time ozone destruction in thearcticstratosphere,whichhassignificantlyreduced ozone concentration in north temperate latitudes in several recent years, is predicted to increase by global warming models (Austin et al. 1992). Other factors remaining constant, ozone reductions are accompanied by predictable increases in surface ultravioletB radiation (UV-B, 280–315 nm). In recent springs, episodic ozone losses have been associated with significantincreasesinground-levelUV-Binanumber ofEuropeancountries,includingtheUnitedKingdom (UK) (DoE 1996).Many experiments have shown that plants can be damaged or otherwise affected by UV-B radiation (Tevini 1993). However, 80% of the studies of plant responses to UV-B published up to 1994 were conducted in controlled environment (CE) chambers or glasshouses (Caldwell & Flint 1994). In these experiments, plants usually received daily integrated doses of photosynthetically active radiation (PAR: 400–700 nm) and UV-A (315–400 nm) much less than summer sunlight: conditions which tend to exaggerate plant responses to UV-B (Caldwell & Flint 1994; Teramura & Sullivan 1994; Fiscus & Booker 1995). Of the 20% of studies conducted in the field, many involved pot-grown plants, ” 90% were of ! 4 months duration during a single growing season and, in the two which considered crop responses over a number of growing seasons, the magnitude of responses varied from year to year (Teramura et al. (1990) with soyabeans; Barnes et al. (1988), with a wheat}wild oat mixture). In field experiments, ambient solar UV-B was either reduced using wavelength-selective filters or supplemented by radiation from UV-B-emitting lamps. When artificial UV-B sources were used, incident solar UV-B was supplemented either by a fixed dose-rate (‘squarewave’) addition or a variable (‘modulated’) addition that was a constant proportion of the ambient dose (or of the dose incident on a control plot). A typical square-wave system provides supplementary UV-B radiation at one or two fixed dose-rates for a certain number of hours centred around solar noon. It takes no account of variation in cloud cover and provides an addition based on the modelled consequences of ozone depletion under clear sky conditions. Squarewave systems thus tend to provide UV-B additions greater than those expected from any realistic prediction of future ozone losses, and so are likely to over-estimate the effects of stratospheric ozone loss on crop production. Fiscus & Booker (1995) pointed out that a proper understanding of crop responses could only be obtained from field experiments that simulate the consequences of likely ozone loss and, in particular, from those using ‘modulated’ lamp systems, which provide a more realistic increased UVB environment. Unfortunately, modulated systems are technically more complex and expensive than square-wave systems and have not been widely used (Caldwell & Flint 1994). Statement of problemIrrespective of UV-B delivery system, most agricultural UV-B field experiments have been performed underconditionsunlikethoseintheUK.Anexception is the study of four cultivars of pea (Pisum sativum L.) in which increases in UV-B equivalent to a 15% ozone depletion reduced yield by an average of c. 10% (Mepsted et al. 1996). However, the experiment of Mepsted et al. (1996) was limited to one season and results may have been influenced by particular conditions during that season, for example late sowing, the unusually dry conditions and pathogen attack. The present study aimed to extend the observations of Mepsted et al. (1996) to a second year and to consider barley (Hordeum vulgare L.), a crop of greater economic significance in the UK (cultivated areas in 1994: pea 124000 ha, barley 1106000 ha; FAO 1996).Purpose of studyIt is against this backdrop the study is aimed at investigating the Effect Of Ultraviolet Light On Plant Development And Fruit Production. The study will specifically present an assessment and statistical power of the experiments. SIGNIFICANCE OF STUDYThis study will be of great importance to the society as well as the nation at large because of its scientific, social, economic, and technological value particularly the agriculture.