Comparative Analysis of Differential drought Stress-induced Suppression of and Recovery in Carbon Dioxide Fixation: Stomatal and Non-stomatal Limitation in Nicotiana tabacum L.

Abstract

Summary Observed differences in drought tolerance in C 3 plants have in the past been incorrectly ascribed to differences in water use efficiency (WUE) and low CO 2 compensation points (Γ). When these parameters thus were applied as screening procedures for cultivars with higher net photosynthetic rates (A) and/or lower photorespiration rates, they have proved to be invalid. To clarify this discrepancy, the stomatal and non-stomatal limitations to photosynthesis were evaluated in four tobacco cultivars of different, but known, drought tolerance. Experiments were carried out under controlled environmental conditions at increasing drought stress and upon rewatering. A drought stress-induced decrease in the carboxylation efficiency (∂A/a∂c i ) ;which was less pronounced in the drought-tolerant cultivars, was observed in all four cultivars. This coincided with a slower stress-induced increase in the Γ and intercellular CO 2 concentration (c i ) in the drought-tolerant cultivars, which was due to the maintenance of higher A rates. All cultivars showed a decrease in WUE and hence an increase in the marginal cost in terms of water used to carbon gained (∂E/∂A). These changes occurred 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 relative degree of stomatal limitation (l) did not increase by much, (ca. 35 %) nor differ significantly among the cultivars, the increase in c i and Γ was interpreted as indicating that mesophyllic rather than stomatal factors were responsible for the drought stress-induced decrease in A, because of the decrease in the ∂A/∂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-sensitive cultivars were much longer. We conclude that in all cultivars decreasing leaf water potential (Ψ L ) caused simultaneous reductions in g and the biochemical capacity for photosynthesis. The ∂A/∂c i component of the biochemical capacity, however, proved to be the most sensitive to drought stress. Thus, in tobacco, 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 ∂A/∂c i , which in turn could result in the maintenance of higher A rates, which is vital for fast recovery upon rewatering.