Dc. Dreesmann et al., EXPRESSION OF GENES ENCODING RUBISCO IN SUGAR-BEET (BETA-VULGARIS L) PLANTS SUBJECTED TO GRADUAL DESICCATION, Plant and Cell Physiology, 35(4), 1994, pp. 645-653
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).