Effects of supplemental UV‐B radiation on primary photosynthetic carboxylating enzymes and soluble proteins in leaves of C₃ and C₄ crop plants

Abstract

Effects of increased UV‐B radiation on activities of primary photosynthetic carboxylating enzymes and on contents of soluble proteins were studied in soybean (Glycine max [L.] Merr. cv. Bragg), pea (Pisum sativum L. cv. Little Marvel), tomato (Lycopersicon esculentum L. cv. Rutgers), and sweet corn (Zea mays L. cv. Golden Cross Bantam). The purpose was to evaluate the responses of agronomic crops to increases in solar UV‐B radiation. Plants were grown and exposed under greenhouse conditions for 6 h daily to supplemental UV‐B radiation which was provided by Westinghouse FS‐40 fluorescent sun lamps filtered with 0.127‐mm film of cellulose acetate (UV‐B treated) or Mylar S (Mylar control). Three UV‐B levels were tested: 1.09 (treatment T1), 1.36 (treatment T2), and 1.83 (treatment T3) UV‐Bseu where 1 UV‐Bseu equals 16.0 mW‐m2 weighted by EXP‐[(λ‐265)/21]2. These UV‐B levels corresponded to 6%,21%, and 36%, respectively, of decrease in stratospheric ozone content, based on the interpolations of UV‐B irradiances at a solar elevation angle of 60°. Leaves of plants of soybean, pea, and tomato exposed to UV‐B radiation were generally low in RuBP carboxylase activity. On a fresh weight basis, all three UV‐B radiation levels significantly reduced the enzyme activity in soybean and pea, whereas tomato plants showed significant reduction in RuBP carboxylase activity only when exposed to 1.83 and 1.36 UV‐Bseu. An apparent decrease in soluble proteins was observed in leaf extracts of soybean and pea plants exposed to 1.36 and 1.83 UV‐Bseu whereas higher amounts of proteins were detected in leaves of tomato plants grown under UV‐B radiation. Leaves of sweet corn plants grown under Mylar control were low in PEP carboxylase activity and proteins as compared with those of control plants receiving no supplemental UV and UV‐B treatment. Activities of PEP carboxylase in crode extracts from leaves of sweet corn were significantly suppressed under 1.36 and 1.83 UV‐Bseu as compared with the no UV control. Some stimulation of PEP carboxylase activity was observed in corn plants exposed to 1.09 UV‐Bseu.

Biological weighting function for xanthophyll de‐epoxidation induced by ultraviolet radiation

“…Excitation pressure could be increased by UVR inhibition through decreased dissipation of reductant and adenosine 5 0 -triphosphate (Neale et al 1993). The latter may occur as UVR damages 'downstream' components of the photosynthetic pathway, for example, through lower activity of Rubisco (Bischof et al 2002, Lesser et al 1996, Nogués and Baker 1995, Vu et al 1982. In any case, the inhibition of photosynthesis by UVR exposure is relatively greater than the increase in de-epoxidation.…”

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

“…This decrease in activity was due to a reduction in the amount of Rubisco present (Table 2) and not simply to a deactivation of the enzyme. UV-B-induced reductions in both Rubisco activity and content have been previously reported in both higher plants (Vu et al 1982;Vu et al 1984;Strid et al 1990;Jordan£'ra/. 1992;He etal 1993;Huang e/a/.…”

“…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).…”

Yields and protein content of two cowpea varieties grown under different production practices in Limpopo province, South Africa

“…According to Thornley (2002), the factor influencing leaf nutrient content also includes the position of leaf tissue. According to Vu et al (2006), UV-B radiation of 1.36 and 1.83 UV-BSEU can lead to decrease in soluble protein in leaf extracts of legumes when exposed to such amount of radiation. This possibly affected photosynthesis, quality of photosynthates and protein partitioning.…”