DECREASED RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE/OXYGENASE IN TRANSGENIC TOBACCO TRANSFORMED WITH ANTISENSE RBCS .8. IMPACT ON PHOTOSYNTHESIS AND GROWTH IN TOBACCO GROWING UNDER EXTREME HIGH IRRADIANCE AND HIGH-TEMPERATURE

Wild-type and antisense rbcS tobacco (Nicotiana tabacum) plants were grown in a glasshouse in midsummer in Portugal with an irradiance of 1500-2000 mu mol m(-2)s(-1) and daytime temperatures of 30-35 degrees C. The Rubisco content of the transformants was lower by 35, 80 and over 90% than that of the wild-type. Gas exchange was measured over three separate days. There was a near-linear relation between Rubisco content and photosynthetic rate during the period of high irradiance, allowing a flux control coefficient of 0.83-0.89 to be estimated. The relation deviated slightly from linearity, because the internal CO2 concentration (c(i)) was higher in the transformants than in the wild-type (190 and 275 mu mol mol(-1) in plants with 35 and 80% less Rubisco, respectively, compared with 175 mu mol mol(-1) for wild-type), compensating to some extent for the decreased Rubisco content. This increase in c(i) occurred because the stomatal conductance (g) remained unaltered or was even higher in plants with decreased Rubisco, despite the lower rate of CO2 assimilation. As a consequence, water use efficiency declined. The decreased rate of photosynthesis was not accompanied by a stoichiometric decrease in apparent growth rate. These results are discussed in relation to earlier studies of the plant set in growth cabinets. It is concluded that tobacco can adjust over a wide range of growth conditions to avoid a one-sided limitation by Rubisco, but that in extreme environmental conditions this capacity to adapt is exhausted.