Shading and Thinning Effects on Seed and Shoot Dry Matter Increase in Determinate Soybean during the Seed-Filling Period

Abstract

under drought conditions (Kobata et al., 1992). Both cereal crops clearly change their growth phase fromThe relationship between seed and shoot dry matter increase (R S/W ) during the seed-filling period (SFP) can reflect the balance between vegetative to reproductive stages, and the potential S the assimilate supply and the sink capacity of harvest organs. In a barely changes once the SFP begins (Evans, 1996). In 2-yr experiment, the R S/W in determinate soybean [Glycine max (L.) soybean, however, active stem and leaf growth continue Merr.] during SFP was investigated under various growing conditions after onset of SFP because flowering starts with vegetainduced by shading and thinning for plants grown under both standard tive growth earlier in the growing season than in most and reduced densities. The R S/W approximated a positive linear regrescereal crops (Ojima and Fukui, 1966). Hence, in soysion in both experiments. Slopes of the lines were less than 0.5 and did not vary within the same year even if plant density was reduced.bean, the potential S may change with the W. If the The shoot dry matter increase (W) hence seemed to directly determine potential S changes with W, the S should not reach a seed dry matter increase (S). A positive relationship between total limit such as the plateau observed in Case 1. If so, the pod number and W or S was observed in both years. Only minimal R S/W may be typed as three cases (Fig. 1, Cases 2-4). In changes in seed numbers per pod and individual seed weight wereCase 2, S during the SFP is equal to W (S ϭ W), and observed, regardless of the treatment applied. A positive relationshipthere is no net transfer of assimilate reserves to the also existed between total pod number and the number of branch nodes, and hence W increased S via increase in the number of branch seed. In Case 3, S is greater than W due to net transfer node bearing pods. Half the current assimilate product during SFP of assimilate reserves from the stem to the seed. Conappeared to be used for increase of vegetative plant parts. This deterversely, in Case 4, S is less than W because excess dry mined total pod number and hence potential S. Determinate soybean matter is stored in the stem rather than in the seed. is thus very different from cereal crops, in which almost all the currentThe soybean harvest index (HI), the ratio of seed assimilate during the SFP is appropriated to S. The stability of the yield to total biomass at maturity, has been considered harvest index in soybean reflects the important effect of the W on pod establishment during SFP.

Relationship between Phosphorus Accumulation and Dry Matter Production in Soybeans

“…4 and 8). Although harvest index in soybeans is unstable when shoot dry matter at maturity is markedly low, it is stable in many cases, including interplant competition, photoperiod, the timing of drought stress, irrigation rate, and planting density or shading conditions (Spaeth et al, 1984;Kakiuchi and Kobata, 2004). Our results under various P conditions are in agreement with these reports.…”

The Relationship between Dry Matter Increase of Seed and Shoot during the Seed-Filling Period in Three Kinds of Soybeans with Different Growth Habits Subjected to Shading and Thinning

“…The R S/W was around 0.41 0.54, which suggests that approximately half of W was accumulated in vegetative organs. There is a positive correlation between the total pod number and S or W, and between the total pod number and the number of branch nodes, and hence W increased S via the increase in the number of branch-node bearing pods (Kakiuchi and Kobata, 2004).…”

“…In the determinate soybean, cv. Tamahomare, the ratio was almost fi xed under various growing conditions including shading and thinning of plants and different years (Kakiuchi and Kobata, 2004). The R S/W was around 0.41 0.54, which suggests that approximately half of W was accumulated in vegetative organs.…”

“…Dry matter production during the seed-fi lling period had a signifi cant effect on seed dry-matter increase and seed yield in determinate soybeans (Kakiuchi and Kobata, 2004) as well as in many other crops (Evans, 1996). The ratio of dry matter increase in the seed (S) to that in the shoot (W), referred to as R S/W hereafter, during the seed-fi lling period can refl ect the balance between the assimilate supply and the sink capacity.…”

“…HI in soybean has been considered to be invariable irrespective of growth habit (Spaeth et al, 1984). However, when plants suffer from water (Schapaugh and Wilcox, 1980) or shading (Kakiuchi and Kobata, 2004) stress during the seed-fi lling period, the percentage of the dry weight before the fl owering stage to that of the fi nal plant dry weight becomes higher reducing HI. Reduction of HI by low radiation after the fl owering stage has also been observed in the Pacifi c side of the Tohoku district in Japan (Arihara, 2000).…”