Comparative effects of salinity and drought on seed germination, seedling growth, photosynthetic productivity, pigments content and antioxidant enzymes of castor bean (Ricinus communis)

Salinity and drought are major abiotic stresses limiting plant growth and crop productivity. To evaluate the comparative effects of salt and drought stress on castor bean (Ricinus communis L.) germination and early seedling growth, a germination experiment and a seedling experiment were conducted to determine the responses of castor bean (cvs Zibi 5 and Zibi 9) to polyethylene glycol (PEG-6000) and sodium chloride (NaCl) with a range of gradient water potential levels, including 0 (control), -0.13 MPa, -0.2 MPa and -0.6 MPa. Results showed that both stresses caused by NaCl and PEG-6000 decreased germination percentage (GP), germination index (GI), fresh weight (FW), leaf gas exchange parameters (Pn, Ci and Tr) and chlorophyll pigments (Chl a and Chl b). The PEG-treated plants suffered higher restrictions in seed germination and seedlings growth than the NaCl-treated plants. Moreover, the PEG treatments caused more significant reductions in GP, Chl a, FW and Tr, as compared with NaCl treatment. On average, the GP under PEG stress was decreased by 22.3% and 22.0% for Zibi 5 and Zibi 9, respectively, in comparison to NaCl stress. Our study showed that both salt and water stress caused similar reductions in germination percentage, biomass accumulation, leaf gas exchange and chlorophyll pigments, while severer detrimental effects were induced by water stress as compared with salt stress.