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 (an d, hence, photosynthetic capacity) in plants exposed to water deficits is still a subject of controversy. In this study we used tobacco (Nic otiana tabacum L.) transformed with ''antisense'' ribulose-1,5-bisphos phate carboxylase/oxygenase (Rubisco) DNA sequences to evaluate whethe r Rubisco or some other enzymic step in the photosynthetic carbon redu ction cycle pathway rate limits photosynthesis at low leaf water poten tial (PSI(W)). These transformants, along with the wild-type material, provided a novel model system allowing for an evaluation of photosynt hetic response to water stress in near-isogenic plants with widely var ying levels of functional Rubisco. It was determined that impaired chl oroplast metabolism (rather than decreased leaf conductance to CO,) wa s 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 st ress. It was hypothesized that, if water stress effects on Rubisco cau sed photosynthetic inhibition under stress, an increase in the steady- state level of the substrate for this enzyme, ribulose 1,5-bisphosphat e RuBP), would be associated with stress-induced photosynthetic inhibi tion. 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 st udies that demonstrated that ATP supply did not rate limit photosynthe sis under stress, we concluded that stress effects on an enzymic step involved in RuBP regeneration caused impaired chloroplast metabolism a nd photosynthetic inhibition in plants exposed to water deficits.