Effect of Shade Treatment after the Beginning of Bloom Stage on Seed Production of Soybean Cultivar ‘Fukuyutaka’ and ‘Misato-zairai’ without Branching Vegetation

‘Misato-zairai’ is a local soybean cultivar in Mie Prefecture. Seed size is relatively large; for example, 100seed weight of ‘Fukuyutaka’, a popular cultivar in western Japan, is 30 g and that of ‘Misato-zairai’ is 45─50 g (Nose et al., 2008; Nagasuga et al., 2011). The seeds of ‘Misatozairai’ are commercially available in parts of Mie Prefecture; owing to their sweet taste, these seeds are used for processed foods such as tofu, soybean curd, and kinako, roasted soybean flour. However, this cultivar is difficult to grow and its seed yield is unstable (Nose et al., 2008); these drawbacks prevent ‘Misato-zairai’ from becoming the predominant cultivar grown by farmers. Maximum seed yields of ‘Misato-zairai’ and ‘Fukuyutaka’ are similar levels, but the yield components and light intercepting characteristics differ (Nagasuga et al., 2011). As mentioned above, ‘Misato-zairai’ seeds are larger and it has fewer seeds and pods than ‘Fukuyutaka’ (Nose et al., 2008; Nagasuga et al., 2011). Because of vigorous vegetative growth, leaf area index of ‘Misato-zairai’ is often higher than that of ‘Fukuyutaka’ from the beginning of bloom stage (Nagasuga et al., 2011; Nagasuga et al., 2014). Additionally, the stem of ‘Misato-zairai’ at maturity is often heavier, and these caused lower seed-stem ratio (Nagasuga et al., 2011). In the Tokai area (central Japan), vigorous vegetative growth often results in lodging, and partial leaf thining before the beginning of bloom stage is recommended even in ‘Fukuyutaka’ (Hayashi et al., 2008). Restriction of vigorous vegetation through breeding or cultivation is important for stable seed production, particularly in ‘Misato-zairai’. Vigorous vegetation also negatively affects canopy light interception. ‘Misato-zairai’ has dense foliage in the upper layer of the canopy (Nagasuga et al., 2011; Nagasuga et al., 2013b) and its leaf inclination angle is lower than in ‘Fukuyutaka’ (Nagasuga et al., 2011; Nagasuga et al., 2013a). However the two cultivars do not differ significantly in the light interception coefficient, an indicator of canopy light interception (Nagasuga et al., 2011; Nagasuga et al., 2013b). These observations suggest that unstable seed production of ‘Misato-zairai’ results from its inferior utilization of light energy for leaf photosynthesis, of photosynthates for seed production, or both. We have previously measured seed weight and yield components stratified by 10-cm intervals above the ground and their correlation with relative light intensity for the leaflets elongated from each layer at the beginning of seed development in these two cultivars (Nagasuga et al., 2019). Seed weight per node number was greater in higher layers (brighter light), and there was a significant linear relationship between seed weight per node number and relative light intensity in ‘Fukuyutaka’. A similar tendency was found in ‘Misato-zairai’, but the regression was logarithmic and the increase in seed weight per node number was small at high relative light intensity. We concluded that light utilization for seed production of ‘Misato-zairai’ is inferior to that of ‘Fukuyutaka’, particularly at high light intensity. In soybean, both vegetative and reproductive growth continue after the beginning of bloom stage. Japanese soybean cultivars are determinate, so the growth of shoot apices on the main stem finishes after the beginning of bloom stage. Yet, branching continues; as a result, pod growth