Differential Inhibition of Photosynthesis ( in vivo and in vitro) and Changes in Chlorophyll a Fluorescence Induction Kinetics of four Tobacco Cultivars under Drought Stress

Abstract

Summary This study was conducted in an attempt to explain the observed difference in drought tolerance of four cultivars of Nicotiana tabacum L., by monitoring plant water status and several aspects of photosynthesis, including chlorophyll a fluorescence, under controlled environmental conditions. Drought stress of increasing intensity, ranging from light to severe, was induced over a twelve-day period by withholding water. Photosynthetic oxygen evolution was monitored in vivo at a CO 2 concentration exceeding 5 % to exclude any stomatal limitation and was expressed as quantum efficiency (Φ). Partial photochemical reactions were determined polarographically with isolated thylakoids of the drought-stressed tissue. From the Kautsky curves the I/P-ratio and ΔF-values were determined at various decreasing leaf water potentials (Ψ L ). All four cultivars exhibited a drought stress-induced decline in Φ, PSI-, PSII- and PSI + PSII-activity, but the extent and nature differed among the different cultivars. The Φ-values of the drought-tolerant cultivars were characterized by a fast initial decline that stabilized as the Ψ L declined, whereas the Φ-values of drought-sensitive cultivars did not show a similar response, but declined at a slow continuous rate as the drought stress intensified. The PSII-activity and PSI- + PSII-activity were influenced in much the same way. PSII-activity was found to be more sensitive than PSI-activity and even more so in the case of the drought-sensitive than in the drought-tolerant cultivars. It was concluded that the observed stress-induced decline in Φ and PSI- + PSII-activity could largely be ascribed to an inhibition of PSII-activity. This conclusion is corroborated by both the fluorescence data regarding the I/P-ratio and ΔF-values, of which the former was found to increase earlier and reach higher end-values in the drought-sensitive cultivars, and the latter, which indicated that the drought-tolerant cultivars were characterized by a higher photosynthetic efficiency at low Ψ L . A mechanism is proposed which might retard or reverse drought stress induced stromal acidification and might explain the apparently higher photosynthetic dehydration tolerance of the drought-tolerant cultivars. The possible involvement of ABA is discussed.