In the present investigation, changes of photosynthetic parameters including net photosynthetic rate (PN); stomatal conductance (gs), transpiration rate (E), and water-use efficiency (WUE) were studied in Jatropha (Jatropha curcas cv. DARL-2) under different concentrations of cobalt (Co) (0, 100, 200, 400, 600, 800, and 1000 mg kg-1) toxicity. The PN, gs, and E trends were shown in increasing trends up to 200 mg kg-1 Co in comparison to controlled plants. However, trends of WUE and photosynthetic pigments [chlorophyll a, b, (a+b), and carotenoids] were found in decreasing order in all concentrations of cobalt stress as compared to controlled plants. The PN value was ~1.33, and ~2.44 fold higher in 100 and 200 mg kg-1 but decreased ~2.61, ~6.86, and ~15.40 fold in 400, 600, and 800 mg kg-1 cobalt treatment, respectively. Similarly, the gs rate was ~1.33, and ~1.69 fold higher in 100 and 200 mg kg-1 but decreased ~1.89, ~3.38, and ~8.71 fold in 400, 600, and 800 mg kg-1 cobalt treatment. The E was significantly ~1.44, ~2.74, and ~1.35 fold higher in 100, 200, and 400 mg kg-1 decreased ~1.03 and ~1.43 fold in 600 and 800 mg kg-1. The WUE was ~0.93, ~0.89, ~3.50, ~6.51, and ~10.76 fold decreased in all Co treated plants. In addition, the content of photosynthetic pigments [chlorophyll a, b, (a+b), and carotenoids] was varying in all Co treated plants. All studied plants were survived morphologically up to 800 mg kg-1 but seedlings were not survived due to severe cobalt toxicity stress in 1000 mg kg-1. Among studied plants, Jatropha seedlings showed the best survival potential under 200 mg kg-1 Co stress.
