Transpiration reflects the plant water status and leaf photosynthetic capacity. To improve the photosynthetic capacity or drought tolerance, it is important to evaluate the transpiration rate rapidly among large scaled populations. Leaf temperature, measured by the thermal image in many cases, is a useful indicator of the transpiration rate. When evaluate the transpiration among large scaled populations, however, the bottleneck remains that leaf temperature is unstable under the field conditions.

In this study, we evaluated the temperature of the primary leaf (T_PL) of soybean genotypes grown under the controlled environment. T_PL measured at the 2 weeks after sowing showed a clear difference among genotypes. Notably, this genotypic difference showed the significant correlation with the stomatal conductance. These results strongly suggest that there is a considerable genotypic variation in the transpiration rate in soybean at the very early stage of the growth, and that the variation is easily detected by the thermal image. The relationship between T_PL and the leaf size, stomatal density will also be reported. The physiological meaning of T_PL waits for the further research, but we suspect that there is some relationship between T_PL and the transpiration characteristics under the field conditions in soybean.

We proposed the rapid evaluation of the leaf temperature by targeting the primary leaf in soybean. The measurement can be conducted within 2 weeks after sowing. Since the seeds can be sown at very high density, many plants can be observed at the same time using the thermal image. These features may contribute the rapid screening or selection of the soybean materials based on their transpiration characteristics.