Phenotypic expression of a chlorophyll mutant in cowpea ( Vigna unguiculata): Environmental influences and effects on productivity

Abstract

Phenotypic expression of a chlorophyll-deficient mutation in cowpea was studied to determine the value of this trait for plant breeding. The mutant and its parent were grown under two irradiances in growth chambers, and under two contrasting temperature regimes in glasshouses. In addition, grain-yield and biomass productivity of the mutant and parent were compared under field conditions. Under optimal temperatures, the leaves of the mutant exhibited significantly lower chlorophyll content per unit leaf area and higher chlorophyll- a: b ratio than the parent when grown in low, moderate, or high photon-flux density. Leaf thickness and leaf chlorophyll content per unit leaf area of both genotypes were less for plants grown under low irradiance, but the chlorophyll- a: b ratio was not inlfuenced by irradiance. In contrast, when grown under hot day/night temperatures of 36/29°C and high photon-flux density, the mutant exhibited similar chlorophyll content and chlorophyll composition as the parent. There was an association between higher chlorophyll content and lower, more normal, chlorophyll- a: b ratios. This association was also seen in field conditions, where the mutant produced greener leaves during hot weather in mid-summer. In sunny field conditions the mutant produced equivalent average grain-yields over 6 years, and identical biomass to the parent, even though the leaves of the mutant had substantially less chlorophyll during flowering and pod-filling. Instead of considering the mutant to be chlorophyll-deficient, it may be more appropriate to state that the parent, and many other cowpeas, have an excess of chlorophyll for growth under sunny conditions.