The interplay between leaf water potential and osmotic adjustment on photosynthetic and growth parameters of tropical dry forest trees

Mimosa tenuiflora and Piptadenia stipulacea are commonly accepted as drought-tolerant species but little is known about their response to drought followed by rehydration. Therefore, the interplay between leaf water potential and osmotic adjustment on photosynthetic and growth parameters of these species was examined. A greenhouse study was conducted in a split-plot design with two water conditions in the main plots (control; drought followed by rehydration), and eight sampling times in the subplots (1, 4 and 7 days of drought, and 1, 3, 6, 12, and 17 days of rehydration). Plant water status and biochemical changes were assessed as well as leaf gas exchange and subsequent growth. Under drought stress, both species maintained a low leaf water potential throughout the day by accumulating compatible solutes, thus allowing a rapid and full recovery of water status when rehydrated. Although these plants minimized water loss by closing their stomata, neither showed stomatal limitations to photosynthesis. The inhibition of this process during drought was possibly related to mesophyll limitations as well as to a reversible downregulation of photosystems, along with adjustments of their stoichiometry. Water deficits also triggered morphological adaptations at the whole plant level, leading to reduced growth, mainly of the shoots in M. tenuiflora and the roots in P. stipulacea.