Impact of E-genes on Soybean (Glycine max L. [Merr]) Development, Senescence and Yield

Genetic improvement of a number of crops including soybean (Glycine max L. [Merr]) has been associated with stay-green. Research on stay green genes has focused primarily on genes involved with photosynthesis and chlorophyll degradation. The current study explores the impact of a group of developmental genes, known as the E gene series, on the rate of soybean leaf senescence. The objective of this experiment was to determine the role of E-genes in the control of leaf senescence in soybean. The experiment was conducted in a split-plot design with three replications. The main plots were two photoperiods imposed following R1; i) natural day length (Amb) and ii) incandescent day length extension of 3 hours (Amb+3). The split plots were five E-gene near-isogenic lines (NILs), planted on different dates to obtain synchronous flowering. Phenology, photosynthesis, normalized difference vegetative index (NDVI) and fluorescence measurements were taken including, dark adapted photosynthetic efficiency (Fv/Fm), electron transport rate (ETR), and leaf chlorophyll concentration (SPAD). Leaf tissues were also analyzed for gene expression patterns among Harosoy isolines. Yield parameters like dry matter accumulation, harvest index and grain yields were recorded. The leaf net photosynthesis was more closely related to ETR than to SPAD values, suggesting that visual observation of stay-green may not be as effective in evaluating functional senescence as measurement of ETR. Cultivars with the dominant E1 allele maintained functional photosynthesis for longer, such that full senescence was delayed by 10-15 days in these cultivars. This phenomenon was observed under both photoperiod treatments and irrespective of the genetic background (Clark and Harosoy) in which the alleles appeared. Maintenance of functional photosynthesis by the E1 dominant allele can be attributed to maintenance of high ETR, and Fv/Fm, as well as delayed decline in leaf chlorophyll concentrations. Expression of senescence related genes were delayed in the isoline which had delayed leaf senescence phenotype. Consistent with the effect on leaf senescence, the dominant alleles also reduced the rate of phenological development, such that R5 occurred later in genotypes with dominant alleles and under the Amb+3 treatment. Cultivars with the dominant E1 allele under extended photoperiod treatment accumulated more biomass and had decreased apparent harvest index which caused no change in grain yields. The dominant E allele may delay leaf senescence directly or indirectly, through its delay of reproductive development