Leaf gas exchange, chloroplastic pigments and dry matter accumulation in castor bean (Ricinus communis L) seedlings subjected to salt stress conditions

Brazilian castor bean (Ricinus communis L) crops have expanded towards semiarid lands, in which soil salinity is an important factor limiting plant development. In order to evaluate the effects of salt stress on leaf gas exchange, pigments, and dry matter accumulation of seedlings, seeds of castor bean var. BRS Nordestina were planted in 15-L pots filled with 13 kg of soil (control) or with soil plus 2 g NaCl kg−1 soil corresponding to 30 mM NaCl (salt treatment). Seedlings were grown under greenhouse conditions and the effects of salinity were assessed by measuring physiological parameters at 38 and 59 days after germination. Salt treatment induced decreases in leaf water potential at pre-dawn (ψpd, 42%), stomatal conductance to water vapor (gs, 36%), and net carbon assimilation rate (A, 24%) only at 38 days after germination. At the same time, the values for transpiration rate were unchanged and the hydraulic conductance was increased (34%). After 59 days under hypersalinity, leaf ψpd increased to −0.44 MPa, although gs, A, transpiration rate, and hydraulic conductance were not different between treatments. Salt stress effects on chlorophyll a, chlorophyll b and chlorophyll a + b contents were evident only on experimental day 59. The dry matter accumulation of leaves, roots and stems, as well as the total dry matter, and the root to above ground ratio increased with plant growth in both treatments, however, these parameters were lowered in salt-stressed seedlings. In short, all physiological variables were more drastically affected after 38 days under high salinity, and despite acclimatization of A after 59 days exposure to salt, no recovery of the dry matter accumulation was observed in the seedlings, suggesting that this species does not tolerate salt stress at its initial growth stages.