Photosynthesis, fluorescence, shoot biomass and seed weight responses of three cowpea (Vigna unguiculata (L.) Walp.) cultivars with contrasting sensitivity to UV-B radiation

Current and projected increases in ultraviolet-B (UV-B) radiation (280–320 nm) may alter crop production. An experiment was conducted to study the influence of UV-B radiation on photosynthetic CO2 assimilation, photosystem II (PSII) photochemistry, pigments, UV-B absorbing compounds, vegetative and reproductive growth responses of three cowpea cultivars, California Blackeye (CB)-5, CB-27, and Mississippi Pinkeye (MPE). Cowpea cultivars were subjected to four levels of biologically effective UV-B radiation of 0 (control), 5, 10 and 15 kJ m−2 d−1 from 8 days after emergence (DAE) to maturity of the crop in sunlit, controlled environment chambers. Increase in UV-B radiation caused a decrease in maximum light-saturated photosynthesis (Amax) in all the three cultivars, but the magnitude of decrease varied, 12% in MPE, 37% in CB-27, and 43% in CB-5 at 15 kJ of UV-B. The lower damaging effect of UV-B on Amax in MPE was supported by a smaller decrease in maximum rate of Rubisco activity (Vcmax) coupled with higher levels of light-saturated rate of electron transport (Jmax) compared to other sensitive cultivars, CB-5 and CB-27. The tolerance nature of MPE was further evidenced by less or no effect of UV-B on photosynthetic electron transport rate (ETR), photochemical quenching (qP), and quantum yield of PSII photochemistry (ΦPSII) compared to CB-5 and CB-27 across the UV-B treatments. Further, maximal electron transport rates (ETRmax) estimated at saturated light substantially decreased with increase in UV-B in CB-5 and CB-27, while there was no or less influence on MPE. All cultivars showed linear relationship between ΦPSII and quantum yield of CO2 fixation (ΦCO2)(ΦCO2) derived from light–response curves and internal-CO2-response curves of chlorophyll a (Chla) fluorescence. MPE maintained significantly higher concentration of UV-B absorbing compounds compared to the CB-5 and CB-27 across UV-B levels. Plants grown at elevated UV-B radiation produced significantly lower plant and seed dry weights in both the sensitive cultivars (CB-5 and CB-27) while the effect on MPE was minimal. This study revealed that the current and projected UV-B radiation could significantly decrease the growth and yield of sensitive cowpea cultivars, possibly caused by reduction in the photosynthetic efficiency of plants. In general, MPE showed more tolerance to UV-B radiation based on most of the parameters studied as compared to the CB-5 and CB-27.