Increased heat sensitivity of photosynthesis in tobacco plants with reduced Rubisco activase

High temperature inhibits photosynthesis by several mechanisms including de activation of Rubisco. The inhibition of photosynthesis by high temperature and its relationship to Rubisco deactivation was studied using tobacco (Ni cotiana tabaccum L. cv W38) transformed with a Rubisco activase gene insert ed in the antisense orientation and untransformed controls. High temperatur e (42 degreesC) reduced photosynthesis in both lines of plants. However, ph otosynthesis recovered nearly completely in wild-type plants and very littl e in plants lacking Rubisco activase. The F-o' level of chlorophyll fluores cence decreased and q(N) increased in the control plants during heating. In the antisense plants, q(N) was always high and F-o' increased slightly dur ing heat stress. NADP-malate dehydrogenase activation was unaffected by hea t stress in control plants but was increased in the transgenic plants, cons istent with a high redox status in the chloroplast. In wild-type plants, th e inhibition of photosynthesis could be explained by a reversible decarbamy lation of Rubisco and an acceptor-side limitation imposed on photosynthetic electron transport. However, in the anti-activase plants, carbamylation wa s low and constant and could not explain how photosynthesis was reduced at high temperature. Because ribulose bisphosphate was saturating at high temp erature, the reduction in photosynthesis must have been caused by some impa irment of Rubisco function not reflected in measurements of activation stat e or carbamylation status. This in vivo Rubisco impairment was not relieved upon return to lower temperature. We speculate that the reversible decarba mylation of Rubisco at moderately high temperature may be a protective mech anism by which the plant avoids more serious effects on Rubisco and the res t of the photosynthetic apparatus.