Efficient irrigation and drainage management are highly required for increasing crop productivity in paddy rice and upland crop rotation. However, conventional management does not sufficiently consider the water status of the plants and soil in the root zone. The aim of this study was to evaluate whether the hydraulic resistance of soil (R_s) or plant (R_p) principally controlled transpiration in rotational paddy fields (RPFs) located in humid regions. To achieve this, we conducted field measurements of soil water conditions, evapotranspiration rate, and leaf water potential in RPFs cropped with soybean after the flowering stage and calculated R_s and R_p based on the theory of root water uptake. After the flowering stage, the soil was sometimes saturated owing to intermittent precipitation, and thus R_s was maintained at a low value. By contrast, R_p gradually increased over time and ranged between 5.1 × 10⁸ and 10.3 × 10⁸ s, which was one to three orders of magnitude higher than R_s. The ratio of the actual to the potential transpiration rate decreased throughout the investigation period and hardly reached 1.0, even though the soil was sufficiently wet. These results indicate that R_p, which probably increases with continuous soil saturation, controls crop transpiration in RPFs under humid climates. Our results suggest that drainage systems are essential in RPFs to avoid a change in R_p and improve crop productivity.

高效的灌溉和排水管理对于提高水稻和旱作轮作的作物生产力非常需要。然而，常规管理没有充分考虑根区植物和土壤的水分状况。本研究的目的是评估土壤 ( Rs ) 或植物 ( Rp )_的水力阻力是否主要控制位于潮湿地区的轮作稻田 (RPF) 的蒸腾作用_。为实现这一目标，我们对开花期后种植大豆的 RPF 的土壤水分状况、蒸散率和叶片水势进行了实地测量，并计算了R _s和R _p基于根系吸水理论。花期后，由于间歇性降水，土壤有时会饱和，因此R _s保持在较低值。相比之下，R _p随时间逐渐增加，范围在5.1 × 10 ⁸和10.3 × 10 ⁸  s 之间，比R _s高一到三个数量级。在整个调查期间，实际蒸腾速率与潜在蒸腾速率之比一直在下降，几乎没有达到 1.0，即使土壤足够湿润也是如此。这些结果表明R _p，这可能随着持续的土壤饱和度而增加，在潮湿气候下控制 RPFs 中的作物蒸腾。我们的结果表明，排水系统对于 RPF 至关重要，可以避免 Rp 的变化并_提高作物生产力。