Linking changes in chlorophyll a fluorescence with drought stress susceptibility in mung bean [Vigna radiata (L.) Wilczek].

In the present study, the mung bean cv. NM-13-1Tol was selected as drought-tolerant and NM-54Sens as drought-sensitive. The effects of progressive drought (16 days) on the photosystem II (PSII) activity was assessed using OJIP and JIP-test in the selected two mung bean cultivars differing in drought tolerance. Drought stress reduced the relative water content to 70% (at threshold) and 62% (below the threshold) in cv.NM-13-1Tol and NM-54sens , respectively. The greater reduction in quantum yield of PSII in cv.NM-54sens due to drought stress was due to PSII photodamage. Raw OJIP induction curves and Fo and Fm normalised curves showed that significant changes in fluorescence occurred at the O, J, I and P steps only in cv. NM-54sens . Double normalised differential kinetics indicated adverse effects at the antennae, oxygen-evolving complex and intersystem electron acceptors in cv.NM54sens . Moreover, JIP-test analysis showed that drought stress caused a greater decrease in performance index (PIABS ) in cv.NM-54sens as compared to that in cv. NM-13-1Tol , which is associated with an increase in Vj , rate of accumulation of closed reaction centres (Mo ), energy fluxes for absorption (ABS/RC), trapping (TRo /RC), electron transport (ETo /RC), and dissipation of absorbed energy as heat (DIo /RC). In conclusion, two-week drought stress reduced the RWC below the threshold in cv.NM54sens , which resulted in damages at the donor and acceptor sides of PSII. However, cv.NM-13-1Tol somehow maintained the RWC around the threshold and thus protected PSII. Of various JIP-test parameters, PIABS , Fv /Fm , Vj and Mo are key indicators of drought stress tolerance in mung bean cultivars.