Response of physiological growth indices and water use efficiency of soybean to mycorrhiza and zinc sulfate application method under water stress conditions

Soybean (Glycine max L.) is one of the most important oilseeds in the world due to its 18 to 22% seed oil and high protein content (36-38%). Meanwhile drought stress is one of the important factor limiting its growth and yield. Drought stress limits root growth and reduces the mobility of important micro-nutrients especially zinc in the soil. Application of zinc sulfate fertilizer increases physiological growth parameters, grain yield and water use efficiency in soybeans. Also mycorrhizal colonization in plants increases growth, improves yield, and increases the plants resistance to drought stress. This study aims to evaluate the effect of mycorrhiza and zinc sulfate fertilizer application method on some soybean growth physiological indices and its water use efficiency under soil moisture deficit stress.

The experiment was performed as a factorial experiment in the form of randomized complete blocks with three replications. First factor was moisture stress at three levels of no-stress, mild stress and severe stress (irrigation after 60, 90 and 120 mm evaporation from Class A pan, respectively). Second factor was arbuscular mycorrhiza (Funneliformis mosseae) in two levels of application and no-application and third factor was the application method of zinc sulfate fertilizer in three levels of no-application, soil application and foliar feeding. To evaluate the response of growth indices of soybean, plant samples were prepared during the growing season and their leaf area and dry matter were measured. Water use efficiency was also obtained from the ratio of grain yield to the volume of water consumed. During flowering stage, the percentage of mycorrhiza colonization was measured and grain yield was obtained by harvesting two square meters from each plot.

In this experiment, severe moisture stress shortened the growth period of soybeans. The results showed that at all levels of moisture stress, inoculation with mycorrhiza increased leaf area index, crop growth rate, water use efficiency and grain yield. Application of mycorrhiza and foliar feeding of zinc sulfate increased maximum leaf area index, maximum crop growth rate and water use efficiency by 69, 98 and 112%, respectively. In the absence of mycorrhiza, soil application of zinc sulfate had no effect on maximum leaf area index and maximum crop growth rate, but the use of mycorrhiza led to a 15 and 20% increase in these indices by soil application of zinc sulfate compared to no zinc sulfate use. Severe and moderate moisture stresses in no-application of mycorrhiza and no zinc sulfate reduced the total dry matter by 47.22 and 63.04%, respectively, but the application of mycorrhiza in these treatments has reduced the severity of these effects and led to increase of 111.20% in total dry matter. The highest colonization of mycorrhiza occured in moderate moisture stress (65.33%). Soil application of zinc sulfate reduced the colonization of mycorrhiza even compared to the no-application of zinc sulfate fertilizer. In this study, the highest percentages of mycorrhizal colonization occured in moderate moisture stress and foliar application of zinc sulfate by 64 and 59%, respectively. Foliar feeding of zinc sulfate compared to no-zinc sulfate, under severe moisture stress increased grain yield by 68% (1100 kg ha-1). No-application of mycorrhiza led to no difference between soil use and no-application of zinc sulfate in soybean yield. Mycorrhiza in no-moisture stress, moderate stress and severe stress increased water use efficiency by 80.49, 183.33, 275% respectively, compared to no-application of mycorrhiza.

According to the results, foliar feeding of zinc sulfate is more efficient than soil application of zinc sulfate, but this efficiency decreases during moisture stress. Application of mycorrhiza in addition to reducing the effects of moisture stress can increase the efficiency of zinc sulfate fertilizer especially under foliar feeding conditions.