Effects of the `Stay Green' Genotype GGd1d1d2d2 on Leaf Gas Exchange, Dry Matter Accumulation and Seed Yield in Soybean (Glycine max L. Merr.)

Abstract

The homozygous combination of the recessive mutations d1 and d2, i.e. d1d1d2d2, causes retention of chlorophyll, chlorophyll-binding proteins and Rubisco in senescing leaves of soybean (Glycine max L. Merr.). Together with G (a gene that preserves only chlorophyll in the mature seed coat), d1d1d2d2 prolonged photosynthetic activity and increased seed yield in growth chamber experiments. The objective of this work was to test the effects of GGd1d1d2d2 (abbreviated to Gd1d2) on leaf gas exchange, growth and seed yield in soybean plants cultured outdoors during the normal growing season. Despite preservation of the photosynthetic machinery in Gd1d2, photosynthesis during the seed filling period was similar in Gd1d2 and its near-isogenic wild type line `Clark'. The main factor limiting photosynthesis in the mutant appeared to be stomatal conductance, which was substantially lower in Gd1d2 than in `Clark'. In Gd1d2 the rate of dry matter accumulation during the seed ®lling period was similar or lower than in the wild type. At maturity, Gd1d2 had fewer nodes, fruiting nodes, fruits and seeds per plant, and therefore its seed yield was reduced by 10±20% compared to `Clark'. Thus, pleiotropic effects of G, d1 and/or d2 affecting stomatal conductance and seed number appear to be major limitations to the yield potential of Gd1d2. These pleiotropic effects suggest that G, d1 and/or d2 have regulatory functions in addition to the control of chloroplast disassembly during senescence.