Effect of source-sink manipulations on the crassulacean acid metabolism of Kalanchoe pinnata

The effect of manipulations of the sink-source at the above-ground level and girdling of source leaves was measured in 4-month-old plants of the CAM species Kalanchoe pinnata (Lam.) Pers. At this age plants developed five pairs of leaves. The upper fourth and fifth leaf pairs were not fully expanded and behaved as carbohydrate sinks. Removal of the developing leaves induced a progressive accumulation of glucans and sugars in the mature leaves. The titratable acidity increased during the second week but accumulation was less than in the control plants three to four weeks after sink removal. Similar, but more rapid, results were observed in mature leaves with girdled petioles. Up to the second night after girdling dark CO2 fixation increased, but decreased steadily afterwards. CAM Phase 4 (afternoon CO2 fixation) however, was more sensitive to girdling, being reduced by 38% on the first day, and disappearing completely 3 d after girdling. The glucan and sugar contents of girdled leaves increased continuously after treatment, but day-night changes ceased completely on the fifth day. Girdling also caused a considerable increase in chloroplast area, with up to 80% of their internal space occupied by starch grains. leading to grana distortion. In girdled leaves, or in source leaves in plants lacking aerial carbohydrate sinks, dawn-dusk changes in titratable acidity started to decrease when the leaf glucan content exceeded 1.0 mol equivalent hexoses kg-1 dry weight. Increased sink strength through shading of all leaves except one source leaf did not affect its CAM activity. The titratable acidity and non-structural carbohydrate content of the shaded mature leaves was reduced by around 55%. Removal of all the mature source leaves accelerated the maturation process of sink leaves, increasing titratable acidity at dawn and synthesis of glucans during the light period. The results support the hypothesis that CO2 fixation in a CAM plant is controlled by accumulation of glucans in chloroplasts.