Effects of Supplementary Ultraviolet-B Radiation on Rice and Pea Plants

Abstract

To compare the effects of supplementary ultraviolet-B (UV-B) radiation on a tropical/subtropical and a temperate plant, two indica rice cultivars (Er Bai Ai and Lemont) and peas were exposed to supplementary UV-B radiation for 8 days (biologically effective irradiance of 0.68 W m-2, 12 h per day). Marked decreases occurred in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution over the 8 day treatment period. The greatest decline always occurred in pea leaves, while in rice, cv. Er Bai Ai was more susceptible to UV-B radiation than cv. Lemont. Compared with control plants, the content of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein decreased significantly after the UV-B treatment; the greatest decrease occurred in pea leaves, while the rate of decrease in rice was greater in cv. Er Bai Ai than cv. Lemont. Over the 8 day UV-B treatment period, the increase of UV-B-absorbing compounds was greater in rice leaves than in pea leaves, and greater in cv. Er Bai Ai than in cv. Lemont, although cv. Lemont was more UV-B tolerant. We suggest that the increase in these compounds is not the only indicator of resistance to UV-B damage; other factors, yet to be identified, may also confer UV-B tolerance. While leaf orientation may be related to the severity of UV-B damage, naturally near-horizontal pea leaves were still much more susceptible to UV-B damage compared with rice leaves which were held horizontally over the 8 day treatment period. Brown spots appeared on the upper surface of leaves of cv. Er Bai Ai after 2 days of UV-B treatment and showed a cumulative increase with the duration of exposure. Our study confirms the multiplicity of photosynthetic responses and of different protective strategies that may account for the differential sensitivity of plants to supplementary UV-B radiation.

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV‐B

“…The few studies which have examined the potential sites of limitation to photosynthesis concurrently in UV-Btreated leaves have suggested that there is a simultaneous decline in many potentially limiting factors (Strid et al 1990;Huang et al 1993;He et al 1993;Lingakumar & Kulandaivelu 1993;He et al 1994a;Day & Vogelmann 1995), and as yet no consensus has emerged for the primary site of UV-B-induced inhibition of photosynthesis. Recent field trials of oilseed rape revealed large differences in the effect of enhanced exposure to UV-B on two cultivars.…”

“…Reductions in Rubisco activity and stomatal conductance have been implicated as factors limiting CO2 assimilation in leaves exposed to elevated levels of UV-B. Prolonged exposure to UV-B has been demonstrated to result in decreases in both Rubisco activity and content (Vu, Allen & Garrard 1982;Vu, Allen & Garrard 1984;Strid et al 1990;Jordan, Chow & Anderson 1992;He et al 1993;Huang et al 1993;Kulandaivelu & Nedunchezhian 1993;Lingakumar & Kulandaivelu 1993), which may be accompanied by decreases in the mRNA transcripts of both the large and small subunits of Rubisco (Jordan et al 1992;Zhang et al 1994). Exposure to UV-B can also modify the rate of stomatal opening and closing, and reduce the rate of leaf transpiration (Negash 1987;Middleton & Teramura 1993;Day & Vogelmann 1995), although other studies have not implicated stomatal effects as limiting for photosynthesis (Murali & Teramura 1986;Sullivan & Teramura 1989;Teramura etal 1991;Ziska & Teramura 1992).…”

“…Photosynthesis and photosynthetic productivity in many species can be inhibited by exposure to increased UV-B radiation (Murali & Teramura 1986;Strid, Chow & Anderson 1990;Sullivan & Teramura 1989;Teramura, Ziska & Sztein 1991;Ziska & Teramura 1992;He et al 1993;Middleton & Teramura 1993;He etal. 1994a;Day & Vogelmann 1995).…”

Changes in chlorophyll content and fluorescence and fruit yield contributing traits in different genotypes of strawberry (Fragaria x ananassa DUCH.)

“…Strong inbreeding caused a decrease in chlorophyll a and b content in all offspring. 3. The following characters: fruit yield per plant, weight of single fruit, number of fruit per plant and weight of leaves per plant, were lower in the S 3 generation as compared with parental forms.…”

“…Chlorophyll fluorescence measurements are completely non-invasive, allowing photosynthesis to be studied in vivo. This method is particularly useful when the effects of different environmental factors [3,4,5,6], and biotic stress [7,8] on plants are studied. Inbreeding generally increases homozygosity in plants and more homozygous genotypes may be susceptible to environmental harshness. In contrast, outcrossing usually increases or maintains heterozygosity that tends to produce genotypes better able to buffer themselves against environmental variability [9].…”