Sulfur and Calcium Simultaneously Regulate Photosynthetic Performance and Nitrogen Metabolism Status in As-Challenged...

In the present study, the role of sulfur (K₂SO₄: S; 60 mg S kg^-1 sand) and/or calcium (CaCl₂: Ca; 250 mg Ca kg^-1 sand) applied alone as well as in combination on growth, photosynthetic performance, indices of chlorophyll a fluorescence, nitrogen metabolism, and protein and carbohydrate contents of Indian mustard (Brassica juncea L.) seedlings in the absence and presence of arsenic (Na₂HAsO₄.7H₂O: As₁; 15 mg As kg^-1 sand and As₂; 30 mg As kg^-1 sand) stress was analyzed. Arsenic with its rising concentration negatively affected the fresh weight, root/shoot ratio, leaf area, photosynthetic pigments content, photosynthetic oxygen yield, and chlorophyll a fluorescence parameters: the O-J, J-I and I-P rise, Q_A^- kinetic parameters, i.e., ΦP₀, Ψ₀, ΦE₀, and PI_ABS, along with F_v/F₀ and Area while increased the energy flux parameters, i.e., ABS/RC, TR₀/RC, ET₀/RC, and DI₀/RC along with F₀/F_v and S_m due to higher As/S and As/Ca ratio in test seedlings; however, exogenous application of S and Ca and their combined effect notably counteracted on As induced toxicity on growth and other important growth regulating processes. Moreover, inorganic nitrogen contents, i.e., nitrate (NO₃^-) and nitrite (NO₂^-) and the activities of nitrate assimilating enzymes, viz., nitrate reductase (NR) and nitrite reductase (NiR) and ammonia assimilating enzymes, viz., glutamine synthetase (GS) and glutamate synthase (GOGAT) along with protein and carbohydrate contents were severely affected with As toxicity; while under similar condition, ammonium (NH₄⁺) content and glutamate dehydrogenase (GDH) activity in both root and leaves showed reverse trend. Furthermore, S and Ca supplementation alone and also in combination to As stressed seedlings ameliorated these parameters except NH₄⁺ content and GDH activity, which showed an obvious reduction under similar conditions. These findings point out that exogenous application of S and/or Ca particularly S+Ca more favorably regulated the photosynthesis, contents of protein, carbohydrate and inorganic nitrogen, and the activities of nitrate and ammonia assimilating enzymes, which might be linked with the mitigation of As stress. Our results suggest that exogenous application of S+Ca more efficiently defends Brassica seedlings by declining As accumulation in root and shoot tissues and by maintaining the photosynthesis and nitrogen metabolism as well.