Variation Analysis of Efficiency of Intercepted Radiation Conversion in the Accumulation of Dry Matter by Soybean.

Abstract

Field experiments were conducted in 1989 on a converted paddy field at Shiga Agricultural Experiment Station (Azuchi, Shiga Prefec.), and in 1990 on an upland field at Shiga Prefectural Junior College (Kusatsu, Shiga Prefec.), in order to investigate stability of efficiency of intercepted photosynthetically active radiation conversion in the accumulation of dry matter (EPAR) by soybean and determine factors which might affect EPAR. In 1989, cultivar 'Enrei' was sown on 3 different dates in various planting patterns, and in 1990, cultivar 'Tamahomare' was grown at various plant population densities. The intercepted PAR and the top dry matter accumulation were in quite close relationship. But it was not always linear over the whole growth duration. EPAR increased in the early growth stage, then, approximately after the canopy closure, it was kept relatively stable or increased slightly until early grain filling period when EPAR began to decline. This time course of EPAR was considered to be resulted primarily from the change of crop photosynthetic activity. But in addition to that, the greater portion of light-saturated leaves at the early stage and the increasing maintenance respiration at the late growth stage also might cause lower EPAR at those stages respectively. Since almost all EPAR values calculated for the period through which those were observed to be stable, were within a small range, from 2.1 to 2.3gMJ-1 in 1989 and from 2.4 to 2.5gMJ-1 in 1990, respectively, due to sowing dates and planting patterns, it was concluded that the ability of dry matter production of soybean crop was not affected considerably by those conditions. Within the small variation of EPAR caused by different planting patterns in 1989, EPAR showed no correlation with N content per unit leaf area, while it showed relatively clear correlation (r=-0.651) with canopy light extinction coefficient KPAR.