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-13-1(Tol) was selected as drought-tolerant and NM-54(Sens) 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-13-1(Tol) and NM-54(sens), respectively. The greater reduction in quantum yield of PSII in cv.NM-54(sens) due to drought stress was due to PSII photodamage. Raw OJIP induction curves and F-o and F-m normalised curves showed that significant changes in fluorescence occurred at the O, J, I and P steps only in cv. NM-54(sens). Double normalised differential kinetics indicated adverse effects at the antennae, oxygen-evolving complex and intersystem electron acceptors in cv.NM54(sens). Moreover, JIP-test analysis showed that drought stress caused a greater decrease in performance index (PIABS) in cv.NM-54(sens) as compared to that in cv. NM-13-1(Tol), which is associated with an increase in V-j, rate of accumulation of closed reaction centres (M-o), 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.NM54(sens), which resulted in damages at the donor and acceptor sides of PSII. However, cv.NM-13-1(Tol) somehow maintained the RWC around the threshold and thus protected PSII. Of various JIP-test parameters, PIABS, F-v/F-m, V-j and M-o are key indicators of drought stress tolerance in mung bean cultivars.