Phenophasic variation in cotyledon chlorophyll content and chlorophyll fluorescence: An insight into germinating cowpea

Cowpea is an important grain and vegetable legume that is grown under water deficit-prone tropical and subtropical regions. These regions require genotypes of cowpea that can be established rapidly at the seedling stage. A better understanding of the phenomenon that happens during cowpea germination can help us to develop the varieties that have traits to establish early in the soil. The present study was conducted to investigate the phenophasic variation in cotyledon chlorophyll content and chlorophyll fluorescence in the germinating cowpea at different developmental stages because both are indicators of photosynthetic capacity and compared to gas exchange parameters; they are high throughput, speedy and simple in nature. Total chlorophyll content and chlorophyll fluorescence parameters viz. maximum fluorescence (Fm), the maximum quantum efficiency of PSII (Fv/Fm), potential photosynthetic activity of PSII (Fv/F0), steady-state relative fluorescence decline ratio (Rfd_Lss) and steady-state coefficient of photochemical quenching based on lake model (qL_Lss) were measured in the cotyledons of germinating cowpea seedlings at the three phenophasic stages that are named as closed, opened and unfurled primary leaf (UPL) cotyledon stages. It was revealed that total chlorophyll content, Fm, Fv/Fm, Fv/F0, Rfd-Lss and qL_Lss are significantly higher in UPL cotyledons as compared to closed cotyledons across all the cowpea genotypes which are the reliable indicators of photosynthetic efficiency. Moreover, total chlorophyll content in cowpea cotyledon was also found to be significantly and positively correlated with chlorophyll fluorescence parameters. The present study suggeststhat UPL cotyledons have better-developed photosynthesis apparatus, and plants tend to be independent of cotyledonary storage for their growth at this phenological stage. Apart from that, it was also evident that there is a natural variation of chlorophyll fluorescence parameters in the cowpea cotyledons with a genetic basis that can be utilized in breeding and selection programs of early establishing cowpea genotypes where chlorophyll fluorescence can be used as an selection tool.