Associations between partitioning of carboxylase activity and bundle sheath leakiness to CO₂, carbon isotope discrimination, photosynthesis, and growth in sugarcane

Abstract

Genetic and environmental variation in carbon isotope discrimination (Δ), photosynthetic gas exchange, growth and activities of phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase (Rubisco) were studied in four sugarcane clones grown under three different irrigation regimes for 65 d in a greenhouse. Δ of the uppermost fully expanded leaf increased with decreasing irrigation frequency and exhibited genetic variation at all irrigation frequencies. Concurrent measurements of leaf gas exchange characteristics suggested that variation in A was attributable to variation in bundle sheath leakiness to CO 2 (Φ) because the ratio of intercellular to ambient partial pressure of CO 2 remained relatively constant except at the lowest level of water availability. Carboxylase assays performed on two genotypes revealed significant genetic variation in Rubisco activity, but no genetic variation in PEPC activity. The Rubisco :PEPC activity ratio, an index of C 3 :C 4 cycle activity, therefore varied genetically. Rubisco :PEPC activity declined significantly with decreasing irrigation frequency. Both Δ and Φ were negatively correlated with genetic and environmental variation in Rubisco :PEPC activity. Taken together, these results suggest that variation in A in sugarcane is governed largely by relative leakage of CO 2 from the bundle sheath mediated by changes in Rubisco :PEPC activity. CO 2 assimilation (A) and shoot growth rate (SGR) were negatively correlated with Δ and Φ, but positively correlated with Rubisco : PEPC activity. Thus, A can serve as an integrated measure of underlying variation in photosynthetic biochemistry associated with variation in performance at the whole plant level in sugarcane and probably other C 4 species.