EXPRESSION OF GENES ENCODING RUBISCO IN SUGAR-BEET (BETA-VULGARIS L) PLANTS SUBJECTED TO GRADUAL DESICCATION

Growth chamber-grown sugarbeet (Beta vulgaris L.) plants were subjected to a slow-developing drought by withholding water for 7 days after which watering was resumed and measurements were made for an additional 48 h. In general, during the first 4-5 days of stress, only minor changes in any parameter were detected. Drastic changes, however, occurred 5 days after the onset of treatment. During the stress period, leaf water potential declined from -1.0 to -3.7 MPa and stomatal resistance increased sharply by 10 fold. Both parameters fully recovered 24 h after rewatering. Leaf photosynthetic rates declined 60% by day 5 and were undetectable on day 7, continued to be undetectable 16 h after rewatering but recovered to near control levels 48 h after rewatering. Rubisco activity declined slowly (20%) during 7 days of stress, declined further 24 h after rewatering (65% of control), but recovered to near control levels 48 h after rewatering. Throughout stress period, no major changes were observed in the amount of the Rubisco large subunit polypeptide and its transcript. The level of the small subunit polypeptide remained relatively stable during the first 6 days of stress, declined slightly on day 7, but recovered within 8 h after rewatering. The transcript of small subunit declined gradually and consistently to less than 10% of control at the end of stress period, but recovered to near control levels 24 h after rewatering. While non-stomatal/biochemical factors appeared to have contributed to photosynthesis inhibition, especially under severe stress conditions (e.g. recovery in leaf photosynthesis began 24 h prior to that of Rubisco activity), collectively, the data supported the hypothesis that inhibition of photosynthesis under moderate drought conditions is most likely due to stomatal limitations. This conclusion was based on two observations: declining internal CO2 concentrations during the initial days of stress and presence of substantial Rubisco activity (80%) at the end of stress (other biochemical inhibitions not related to Rubisco cannot be ruled out).