Modulation of photosynthesis by drought in two desert scrub species exhibiting C3-mode CO2 assimilation

The mechanisms involved in the modulation of photosynthesis by drought in non-CAM desert plants have received little attention and were investigated in this study through measurement of chlorophyll a fluorescence (OJIP) transients. We focused on two species, Augea capensis Thunb. and Zygophyllum prismatocarpum E. Meyer ex Sond., from the Namib desert. Measuring sites were selected near Skorpion Zinc Mine, Namibia for field studies. Controlled experiments, where the relationship between changes in fluorescence rise kinetics and inhibition of CO2 assimilation were determined, were also conducted. Analysis of OJIP transients revealed that drought induced a quicker fluorescence rise in both species under all experimental conditions. The appearance of a K and I peak in the fluorescence transient suggested inhibition of the oxygen evolving complex and limitations in electron transport respectively. Down-regulation of photochemical activity occurred predominantly through photosystem II (PSII) reaction center deactivation. The performance index (PIABS), a parameter derived from the OJIP transient by means of the JIP-test (translation of original fluorescence measurements into biophysical expressions quantifying the stepwise flow of energy through PSII), was a very sensitive indicator of the physiological status of plants under both field and laboratory conditions. A very good correlation existed between decline in CO2 assimilation capacity and decline in PIABS values, which provides evidence linking changes in OJIP fluorescence rise kinetics during drought under both field and laboratory conditions with changes in overall photosynthetic capacity. Moreover, an integrated down-regulation of the whole photosynthetic process occurred in such a fashion that the intrinsic balance between electron transfer reactions and reductive carbon metabolism was maintained. The OJIP transient appears to hold considerable promise as a tool for dissecting the drought response of photosynthesis in desert ecosystems.