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

Dep. of Botany and Plant Sciences, Univ. of California, Riverside, CA 92521

^* Corresponding author.

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.

Received for publication November 6, 1987.