Photosynthetic behaviour of Hedysarum carnosum and Hedysarum coronarium under drought stress

Drought stress is a factor affecting plant growth in the arid regions of Tunisia. The experiment was performed on two species: Hedysarum coronarium (H. coronarium; Sulla coronaria) and Hedysarum carnosum (H. carnosum; Sulla carnosa) which were subjected to two water regimes (60 and 100% of field capacity). The aim was to study the photochemical stability of PSII (using the fluorescence transient OJIP in light- and dark-adapted states) and gas exchange under drought stress. Results revealed that drought stress affected the majority of the studied physiological parameters: a sharp decrease in relative water content, total chlorophyll content, and gas exchange in both species during the first month of treatment followed by a more or less constant phase (2 months) and a decrease to the lowest levels of the studied parameters. Nevertheless, H. coronarium plants maintained relatively higher stomatal conductance, photosynthesis activity, transpiration rate, and total chlorophyll content under drought stress conditions. In addition, a change in fluorescence intensity as a function of time was noted in both species. The effect of drought stress was also invested on the different fluorescence intensities of the transitions J, I, and P and consequently on the different oxidation–reduction states of PSII electron acceptors. Drought stress had no effect on the fluorescence level O, whereas the effect which was observed at the J transition level was much higher at the I and P levels. In addition, H. coronarium was more able to protect its PSII from water-deficit stress as consequently was more drought-tolerant compared to the endemic species of H. carnosum in arid climate. Overall, our study contributed to the understanding of the mechanisms of adaptation and stability of PSII under drought stress in H. carnosum and H. coronarium.