Photosynthetic acclimatization to cyclical water deficit in rubber tree seedlings

Plants subjected to successive water deficit cycles may be able to rapidly develop acclimatization responses. Thus, the objective of this study was to analyze the acclimatization capacity of the photosynthetic machinery of rubber tree seedlings submitted to water deficit cycles (WDC). To do so, we designed four treatments: control plants (fully irrigated, CT); 0P (not previously submitted to any WDC); 1P (previously submitted to one WDC); and 2P (previously submitted to two WDC). The water deficit cycle was characterized by suspending irrigation until rubber tree net assimilation rate (A) reached zero (A≤0). Then the plants were rehydrated until substrate moisture reached its maximum water retention capacity, and the irrigation was maintained until the rate A of the plants recovering from water deficit reached 90% of the rate of the control plant, and then a new cycle of water deficit started again. In the third and last cycle (WDC3), treatments 0P, 1P and 2P were simultaneously subjected to water deficit and comparative eco-physiological evaluations between treatments when each group of plants reached A≤0. After WDC3, 2P plants achieved greater A, stomatal conductance, transpiration and intrinsic efficiency of water use after 21 days, compared to 1P and 0P plants. The leaves of treatments 2P and 0P showed the same relative water content (RWC), higher photosynthetic pigment concentration, and 2P showed greater water absorption capacity in the substrate under low water availability, maintaining high A values in comparison with the other treatments 36 days after the start of WDC3. The results indicate that the 2P plants showed acclimatization of the photosynthetic machinery in maintaining a positive carbon balance when subjected to WDC, indicating that leaf turgor was maintained through cellular and osmotic adjustment, accumulation of photosynthetic pigments and greater soil water absorption capacity.