Photosynthesis of plants of shade-tolerant ecotype of Paubrasilia echinata are more tolerance to drought than the sun-tolerant ecotype

Frequent and prolonged droughts are predicted for the Northeast region of Brazil until the end of this century. How the sun-tolerant and shade-tolerant tree species of the tropical forests in this region of the planet will respond to this climate scenario is a matter of global interest. To clarify this issue, we used plants of the shade -tolerant ecotype and the sun-tolerant ecotype of Paubrasilia echinata as a biological model. One batch of each ecotype was subjected to water suspension (drought simulation) for 17 days followed by water restitution (rainfall simulation) for 36 days. In the initial, intermediate, and final time of drought (7, 12, and 17 days, respectively) and rain (5, 12, and 36 days, respectively), we analyzed the water potential (psi wp), electrolyte leakage (EL), net assimilation of CO2 (A), stomatal conductance (gs), transpiration (E), water use efficiency (A/ E), the internal concentration of CO2 (Ci), instantaneous carboxylation efficiency (A/Ci), and fluorescence pa-rameters of the chlorophyll a, among which are the photochemical performance indices (PIABS and PITOTAL). The photosynthetic activity and psi wp of the shade-tolerant ecotype were more tolerance to the short dry period than the sun-tolerant one. Even so, the two ecotypes withstood water scarcity with 100% survival. The restitution of water, after a period of drought, allowed some of the gas exchange variables (E, A/E, and Ci), the psi wp, and membrane damage (EL), from both ecotypes, to completely recover their values to those at time zero, reaching the values of their respective controls. On the other hand, the variables most closely associated with CO2 assimilation (A, gs, and A/Ci) and ATP and NADPH production (PITotal and PIABS) did not fully recover from drought damage. We conclude that, if the droughts intensify in the northeast region of Brazil by the end of this century, the CO2 assimilation, and the photochemical efficiency of photosynthesis of the shade-tolerant species will possibly suffer greater impacts than the sun-tolerant ones. Predictably, this can affect the plant production of shade-tolerant species.