COMPARATIVE-ANALYSIS OF DIFFERENTIAL DROUGHT STRESS-INDUCED SUPPRESSION OF AND RECOVERY IN CARBON-DIOXIDE FIXATION - STOMATAL AND NONSTOMATAL LIMITATION IN NICOTIANA-TABACUM-L

Observed differences in drought tolerance in C3 plants have in the pas t been incorrectly ascribed to differences in water use efficiency (WU E) and low CO2 compensation points (GAMMA). When these parameters thus were applied as screening procedures for cultivars with higher net ph otosynthetic rates (A) and/or lower photorespiration rates, they have proved to be invalid. To clarify this discrepancy, the stomatal and no n-stomatal limitations to photosynthesis were evaluated in four tobacc o cultivars of different, but known, drought tolerance. Experiments we re carried out under controlled environmental conditions at increasing drought stress and upon rewatering. A drought stress-induced decrease in the carboxylation efficiency (partial derivative A/partial derivat ive c(i)) which was less pronounced in the drought-tolerant cultivars, was observed in all four cultivars. This coincided with a slower stre ss-induced increase in the GAMMA and intercellular CO2 concentration ( c(i)) in the drought-tolerant cultivars, which was due to the maintena nce of higher A rates. All cultivars showed a decrease in WUE and henc e an increase in the marginal cost in terms of water used to carbon ga ined (partial derivative E/partial derivative A). These changes occurr ed more slowly in the drought-tolerant cultivars, which was due to the maintenance of higher A values, as stomatal conductance (g) decreased more slowly in spite of higher transpiration rates (E). As the relati ve degree of stomatal limitation (1) did not increase by much, (ca. 35 %) nor differ significantly among the cultivars, the increase in c(i) and GAMMA was interpreted as indicating that mesophyllic rather than stomatal factors were responsible for the drought stress-induced decre ase in A, because of the decrease in the partial derivative A/partial derivative c(i) component of the mesophyllic photosynthetic capacity. Upon rewatering all the parameters monitored recovered to a greater or lesser extent, though the recovery time needed by the drought-sensiti ve cultivars were much longer. We conclude that in all cultivars decre asing leaf water potential (psi(L)) caused simultaneous reductions in g and the biochemical capacity for photosynthesis. The partial derivat ive A/partial derivative c(i) component of the biochemical capacity, h owever, proved to be the most sensitive to drought stress. Thus, in to bacco, during drought stress diffusion itself only marginally limits A , a fact not widely appreciated. Drought tolerance in the four tobacco cultivars investigated may therefore be resistance to stress-induced decreases in partial derivative A/partial derivative c(i), which in tu rn could result in the maintenance of higher A rates, which is vital f or fast recovery upon rewatering.