USE OF TRANSGENIC PLANTS WITH RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE ANTISENSE DNA TO EVALUATE THE RATE LIMITATION OF PHOTOSYNTHESIS UNDER WATER-STRESS

The biochemical lesion that causes impaired chloroplast metabolism (and, hence, photosynthetic capacity) in plants exposed to water deficits is still a subject of controversy. In this study we used tobacco (Nicotiana tabacum L.) transformed with ''antisense'' ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whether Rubisco or some other enzymic step in the photosynthetic carbon reduction cycle pathway rate limits photosynthesis at low leaf water potential (PSI(W)). These transformants, along with the wild-type material, provided a novel model system allowing for an evaluation of photosynthetic response to water stress in near-isogenic plants with widely varying levels of functional Rubisco. It was determined that impaired chloroplast metabolism (rather than decreased leaf conductance to CO,) was the major cause of photosynthetic inhibition as leaf PSI(W) declined. Significantly, the extent of photosynthetic inhibition at low PHI(W) was identical in wild-type and transformed plants. Decreasing Rubisco activity by 68% did not sensitize photosynthetic capacity to water stress. It was hypothesized that, if water stress effects on Rubisco caused photosynthetic inhibition under stress, an increase in the steady-state level of the substrate for this enzyme, ribulose 1,5-bisphosphate RuBP), would be associated with stress-induced photosynthetic inhibition. Steady-state levels of RuBP were reduced as leaf PSI(W) declined, even in transformed plants with low levels of Rubisco. Based on the similarity in photosynthetic response to water stress in wild-type and transformed plants, the reduction in RuBP as stress developed, and studies that demonstrated that ATP supply did not rate limit photosynthesis under stress, we concluded that stress effects on an enzymic step involved in RuBP regeneration caused impaired chloroplast metabolism and photosynthetic inhibition in plants exposed to water deficits.