Effects of Solar Radiation Regimes on Growth and N2 Fixation of Soybean, Cowpea, and Bushbean1

Abstract

AbstractLegumes are commonly grown in the tropics under reduced solar radiation due to dense cloud cover and shading from plantation or tall intercrops. This paper examines the growth and N2 fixation of several grain legumes common to the tropics on an Oxic Haplustoll in Hawaii. Plantings were made in April (warm season) and November (cool season) of soybean (Glycine max L. Merr.) ‘Kahala’; cowpea (Vigna unguiculata Walp.) ‘TVu 1190’ (April) and ‘TVu 4557’ (Nov.); and bushbean (Phaseolus vulgaris L.) ‘Burpee Tenderpod’. Polypropylene netting was used to obtain 27, 45, 70, and 100% of the incoming solar radiation through the entire crop cycle. Consistent shading effects were with the three legumes during both warm and cool seasons even though absolute yields were lower in the cool season due to daylength and temperature sensitivity in soybean and cowpea. Nitrogen fixation parameters generally followed dry matter (DM) and seed yields. Total nodule activity (TNA) was very low in all the legumes at 27% sun. With the exception of bushbean at 27% sun, shading caused an 34% average reduction in DM and grain yields, vs a 50% reduction in TNA, and a 76% reduction in nodule mass. Soybean TNA peaked 3 to 4 weeks after flowering, cowpea (cool season) TNA peaked at flowering, and bushbean TNA peaked slightly before or at flowering. There was a tendency for a delay (≈ 2 weeks) at 70, 45, and 27% sun in the final stages of TNA, for summer and cool‐season soybean and bushbean. These delays were not noted for cowpea or cool‐season bushbean. Of the three legumes, cowpea was the least shade tolerant. It produced more DM at full sun than either soybean or bushbean, but at 27% sun, DM yields were similar for all three crops. The response of soybean to shade was intermediate with no reduction in DM and grain yield at 70% sun. Bushbean was the least affected by shading; in fact, maximum yields were obtained at 70% sun. At full sun, their leaves were yellow and seed weight was low; this problem could be corrected by N fertilization.