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 subjecte d to a slow-developing drought by withholding water for 7 days after w hich watering was resumed and measurements were made for an additional 48 h. In general, during the first 4-5 days of stress, only minor cha nges in any parameter were detected. Drastic changes, however, occurre d 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 af ter rewatering. Leaf photosynthetic rates declined 60% by day 5 and we re undetectable on day 7, continued to be undetectable 16 h after rewa tering but recovered to near control levels 48 h after rewatering. Rub isco 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 maj or changes were observed in the amount of the Rubisco large subunit po lypeptide and its transcript. The level of the small subunit polypepti de remained relatively stable during the first 6 days of stress, decli ned slightly on day 7, but recovered within 8 h after rewatering. The transcript of small subunit declined gradually and consistently to les s than 10% of control at the end of stress period, but recovered to ne ar control levels 24 h after rewatering. While non-stomatal/biochemica l factors appeared to have contributed to photosynthesis inhibition, e specially under severe stress conditions (e.g. recovery in leaf photos ynthesis began 24 h prior to that of Rubisco activity), collectively, the data supported the hypothesis that inhibition of photosynthesis un der moderate drought conditions is most likely due to stomatal limitat ions. This conclusion was based on two observations: declining interna l CO2 concentrations during the initial days of stress and presence of substantial Rubisco activity (80%) at the end of stress (other bioche mical inhibitions not related to Rubisco cannot be ruled out).