Effect of using drip irrigation on the growth, yield and its components of soybean grown in a low rainfall region in Japan

Field experiments were carried out for 3 years to assess the effect of using drip irrigation on the growth and yield of two Japanese soybean cultivars in Kagawa Prefecture, which has the second-lowest rainfall in Japan. The treatments were drip irrigation (Drip) and rainfed (Rainfed) from the blooming stage to the full-seed stage. The largest seed yield was achieved in 2017, followed by 2018 and 2016. This order corresponded to the total water input (TWI, the sum of effective rainfall and irrigation) throughout cultivation. TWI was the main factor affecting the variation of yield and its components among years. Similarly, the higher TWI in Drip than in Rainfed contributed to the higher yield in Drip than in Rainfed within each year. ANOVA detected a significant effect of drip irrigation on total seed yield, above-ground dry matter (AGDM) at maturity, and numbers of branches, nodes, and fertile pods. AGDM had a significant correlation with the mean crop growth rate (CGR) during the treatment, and CGR was closely correlated with the mean net assimilation rate (NAR). Significant positive correlation among NAR, radiation use efficiency, and leaf water potential suggested that drip irrigation prevented the decrease of plant water status that contributed to maintain dry matter production. The advantage of using drip irrigation for soybean cultivation at the experiment site would be suppressing the decrease in yield in years with low rainfall rather than achieving higher yield than standard in years with normal or high rainfall. Abbreviations: AGDM, aboveground dry matter; CGR, crop growth rate; CISR, cumulative intercepted solar radiation; DAS, days after sowing; ETa, actual evapotranspiration; ETr, reference evapotranspiration; FIPAR, fraction of intercepted photosynthetically active radiation; LAI, leaf area index; Ψ L , leaf water potential; mLAI, mean leaf area index; NAR, net assimilation rate; PAR, photosynthetically active radiation; PCC, percent of canopy coverage; RUE, radiation use efficiency; TWI, total water input