Responses of the apple plant to CO₂ enrichment: changes in photosynthesis, sorbitol, other soluble sugars, and starch

Abstract

There is no information on the effects of elevated [CO₂] on whole-plant photosynthesis and carbohydrate metabolism in apple (Malus domestica Borkh.) and other sorbitol-translocating plants. Experiments were conducted in controlled growth chambers to evaluate how increases in [CO₂] affect plant photosynthesis and carbon partitioning into soluble sugars and starch in apple leaves. Apple plants (cv. Gala), 1-year-old, were exposed to [CO₂] of 200, 360, 700, 1000, and 1600 µL L^-1 up to 8 d. Whole-plant net photosynthetic rates were analysed daily after [CO₂] treatments. Newly expanded mature leaves were sampled at 1, 2, 4, and 8 d after [CO₂] treatments for sorbitol, sucrose, glucose, fructose, and starch analysis. Midday whole-plant net photosynthetic rates increased linearly with increasing [CO₂], but the differences in whole-plant photosynthesis between CO₂-enrichment and ambient [CO₂] treatments were less significant as apple plants acclimated to high atmospheric [CO₂] for 8 d. Increases in [CO₂] significantly increased sorbitol and starch, but did not affect sucrose concentrations. As a result, the ratios of starch to sorbitol and starch to sucrose at 8 d after [CO₂] treatments were increased from 0.05 and 0.06 to 0.8 and 1.6 as [CO₂] increased from ambient [CO₂] (360 µL L^-1) to 1000 µL L^-1 [CO₂], respectively. The sorbitol to sucrose ratio also increased from 1.3 to 2.2 as [CO₂] increased from 360 to 1000 µL L^-1. Elevated [CO₂] enhanced the photosynthesis of apple plants and altered carbohydrate accumulation in mature leaves in favour of starch and sorbitol over sucrose.