Gas Exchange, Carbon Isotope Discrimination, and Chloroplast Ultrastructure of a Chlorophyll‐Deficient Mutant of Cowpea

Abstract

Chlorophyll‐deficient mutants provide an opportunity for studying relationships among chlorophyll composition, chloroplast ultrastructure, and plant function. A chlorophyll‐deficient mutant of cowpea [Vigna unguiculata (L.) Walp.] and its normally pigmented parent were grown in glasshouse and field conditions. The chlorophyll (a + b) content of the mutant was 35 to 48% less per unit leaf area and the chlorophyll a:b ratio was 62 to 74% greater than the parents. Chloroplasts of the mutant had distended thylakoids and substantially reduced granal stacking, supporting the hypothesis that chlorophyll content, chlorophyll a:b ratio, and chloroplast lamellar organization are developmentally related. The mutant had significantly higher CO2 assimilation rates per unit leaf area (+13%) at high photon flux densities and a quantum requirement that was not significantly different from the parent. Apparently, the mutant's ability to assimilate CO2 was not adversely affected by the substantial changes in chlorophyll content and composition, and chloroplast ultrastructure, compared with the parent. Significant differences in 13C discrimination for leaves sampled from field‐grown plants led to the prediction that plants in a drier treatment had higher intrinsic water‐use efficiency than well‐watered plants, which was consistent with significant differences observed in leaf gas exchange by the same plants. The mutant had significantly greater 13C discrimination than the parent; however, leaf gas exchange data indicated no significant differences in intrinsic water‐use efficiency.