Foliar application of iron, zinc, and manganese nano-chelates improves physiological indicators and soybean yield under water deficit stress

To investigate the effect of foliar application of nano-chelates of iron, zinc, and manganese subjected to different irrigation conditions on physiological traits, and yield of soybean (cultivar M9), a split plot experiment was conducted in a completely randomized block design with three replications in two crop years (2016–2017). The main plot included four levels of irrigation (I): full irrigation (I1), irrigation withhold at flowering stage (I2), irrigation withhold at podding stage (I3), and irrigation withhold during the grain filling period (I4). Also, the subplot included eight levels of foliar application with Fe, Zn, Mn, Fe + Zn, Fe + Mn, Zn + Mn, Fe + Zn + Mn nano-chelates, and distilled water (control). The results of combined analysis of variance suggested that the effect of irrigation and foliar application of nano-chelate was significant on all traits. Water deficit stress significantly reduced the grain yield. The minimum numbers of pods per plant, number of grains per plant, 100-seed weight per plant, leaf area index, leaf chlorophyll concentration, total dry weight of plant, and the grain yield were obtained by irrigation withhold at podding stage. Foliar application of combined nano-chelates increased the soybean resistance against water shortage more considerably than the separate consumption of these elements. Under drought stress in podding stage, the application of Fe + Zn led to the highest yield with a mean of 2613.84 kg ha−1 where this increase was 61.1% higher than control.