Carbon partitioning into sorbitol,sucrose, and starch in source and sink apple leaves as affected by elevated CO2

Experiments were conducted in controlled growth chambers to evaluate how increases in CO₂ concentration ([CO₂]) affected carbon metabolism and partitioning into sorbitol, sucrose, and starch in various ages of apple leaves. Apple plants (Malus domestica), 1 year old, were exposed to [CO₂] of 200, 360, 700, 1000, and 1600 μl l⁻¹ up to 8 days. Six groups of leaves (counted from the shoot apex): leaves 1–5 (sink), 6–7 (sink to source transition), 8–9 (sink to source transition), 10–11 (nearly-matured source), 21–22 (mid-age source), and 30–32 (aged source), were sampled at 1, 2, 4, and 8 days after [CO₂] treatments for carbohydrate analysis. Increases in [CO₂] from a sub-ambient (200 μl l⁻¹) to an ambient level (360 μl l⁻¹) significantly increased the concentrations of sorbitol, sucrose, glucose, and fructose tested in all ages of leaves. Continuous increase in [CO₂] from ambient to super-ambient levels up to 1600 μl l⁻¹ also increased sorbitol concentration by ≈50% in source leaves, but not in sink and sink to source transition leaves. Increases in [CO₂] from 360 to 1600 μl l⁻¹, however, had little effect on sucrose content in all ages of leaves. Starch concentrations increased in all ages of leaves as [CO₂] increased. Rapid starch increases (e.g. 5-, 6-, 20-, and 50-fold increases for leaf groups 1–5, 6–7, 10–11, and 21–22, respectively) occurred from 700 to 1600 μl l⁻¹ [CO₂] during which increases in sorbitol concentration either ceased or slowed down. Our results indicate that changes in carbohydrates were much more responsive to CO₂ enrichment in source leaves than in sink and sink to source transition leaves. Carbon partitioning was favored into starch and sorbitol over sucrose in all ages of leaves when [CO₂] was increased from 200 to 700 μl l⁻¹, and was favored into starch over sorbitol from 700 to 1600 μl l⁻¹ [CO₂].