Soybean (
Glycine max
(L.) Merrill) is an important cheap source of plant protein in Nigeria. Its production is constrained by low light intensity in the southern rainforest ecology due to cloud cover during the production season. Understanding the physiological basis of performance under low light intensity is therefore central to efforts geared towards the crop's productivity improvement in the zone. Soybean plants were subjected to two weeks of 75 and 50% daylight regimes at the vegetative (V4-5); early flowering (R2); and pod filling (R5) stages of growth. Full exposure or 100% daylight served as the control. In the field and pots, the 75% and 50% light intensities (obtained by covering cages with one or two layers of 1mm mesh net, respectively) significantly enhanced the vegetative growth over plants grown under 100% light intensity. Plants grown under 50% light reduction (L2 plants) had significantly (P=0.05) higher leaf area plant
-1, stem height plant
-1 and number of branches plant
-1 than plants grown under 75% light intensity (L1 plants). Similarly, L1 plants had higher values of these vegetative parameters than plants grown under 100% light intensity (L0 plants). Generally, the results of the pot and field trials were similar. Reduced light regimes led to a higher accumulation of dry matter in the various plant parts particularly when imposed at the vegetative stage of growth. Additionally, reduced light regimes led to reduced leaf chlorophyll concentration and a general reduction in yield. The highest reduction in yield resulted from 50% light intensity imposed for two weeks at the pod filling stage of growth. In pots the seed dry weight plant
-1 of L0, L1 and L2 plants were 40.6g, 30.4g and 23.3g, respectively. While for the field trials, the seed dry weight plant
-1 for L0, L1, L2 plants were 37.3g, 34.3g and 32.5g, respectively. The seed dry weight plant
-1 for light reduction at the vegetative stage (35.5g) was similar to that at the early flowering stage (35.5g) but was significantly higher than the seed dry weight plant
-1 at the pod filling stage (33.2g). Results reveal that reduced light intensity was most damaging to soybean performance at the pod filling stage and the lower the light intensity the greater the yield reduction.