Photosynthetic limitations in leaves of young Brazilian Green Dwarf coconut (Cocos nucifera L. ‘nana’) palm under well-watered conditions or recovering from drought stress

Abstract

Young plants of two Brazilian Green Dwarf coconut ecotypes from contrasting regions in relation to climate (UGD, from a hot and humid climate and JGD, from a hot and dry climate) were submitted to three consecutive drying/recovery cycles, under greenhouse conditions to determine the photosynthetic limitations encountered during the recovery phase of water deficiency. Three hypotheses were tested: (1) non-stomatal factors contribute to the incomplete recovery of net photosynthesis rate (A), (2) photochemical impairment is an important component of the non-stomatal limitations to A and (3) the two coconut ecotypes respond differently to the drought recovery. In each drying/recovery cycle, irrigation was suppressed until A reached zero. Then the plants were rewatered and the recovery was accompanied for 8 days. After 4 days of rewatering, CO2 and light response curve parameters were used to discriminate the stomatal and non-stomatal factors contributing to the incomplete recovery of A. Upon rewatering for 4 days, the pre-dawn leaf water potential of the stressed plants increased from −1.20 to −0.15 MPa on average. Only small differences were detected in the intercellular to atmospheric CO2 concentration ratio but A did not recover completely to the control values. In both ecotypes, light and CO2-saturated A, carboxylation efficiency, maximum quantum yield of PSII and maximum electron transport rate were significantly lower in stressed plants in comparison with the well-watered ones. Relative stomatal limitation (LS) ranged from 16 to 20% without significant differences between treatments and between ecotypes. The relative mesophyll limitation (Lm) was significantly higher in JGD in cycle 1 but this changed after repeated drought stress, decreasing to values significantly lower than those in UGD in the other two cycles. The results showed that after 4 days of rewatering stressed plant stomatal limitations were comparable to that of well-watered control plants (∼20%). However A did not recover completely after drought stress due to non-stomatal factors. Important differences between the ecotypes were identified. The ecotype JGD presented smaller Lm, faster recovery of the apparent quantum efficiency of CO2 assimilation, adjustment of some carbon balance components and signs of photosynthetic capacity acclimation after repeated drying/recovery cycles.