Photosynthesis, fluorescence and mesophyll conductance responses to increasing salinity levels inJatropha curcasat early vegetative stages

Abstract

AbstractSalinity is often a great limitation in marginal environments with potential for developing alternative non‐edible crops for biodiesel, and the physiological responses involved in the recovery of plants subjected to high salinity are poorly studied. The aim of this study onJatropha curcasis to identify salinity tolerance responses of net photosynthesis rate under saturating irradiances (Amax), its recovery capacity and the role of mesophyll conductance (gm) overAmax. Two experiments were performed with seedlings in pots under outdoor conditions and hydroponic conditions, respectively, with salinity intensities ranging from 3 to 12 dS/m, their isosmotic treatments with polyethylene glycol (PEG) and controls without abiotic stress.Amaxand growth rate were mainly affected by salinity effects in all the ranges, with a drastic 60% drop in dry biomass under 6 dS/m, revealing a significant sensitivity of this species. However, a surprising increase inAmaxwas promoted by the presence of NaCl, with respect to their respective isosmotic treatments with PEG, although it was still lower than the unstressed plants. This advantage disappeared from 9 dS/m, but negative effects of NaCl toxicities were never detected. The photochemical apparatus resulted extremely resistant in this species, since Fv/Fm and leaf greenness were affected only at 12 dS/m. So,gmwas strongly and linearly associated withAmax.This association derives from the overall range of stress intensities tested, thus appearing as the main useful trait for enhancing photosynthesis depletion under salinity stress, without losses of the water use efficiency. A drastic 75% drop was also detected in the electron use for photosynthesis, revealing thatAmaxwould also be modulated by metabolic impairments under salinity. Moreover, full recovery after only 8 days was observed, confirming the high resistance of the species to NaCl stress even under very high salinities. This study contributes to a better understanding of the physiological processes involved in the response ofJ. curcasto salinity during early vegetative stage, generating possibilities for improving tolerance of this species under environments exposed to salinity.