Contribution of carbohydrate pools to the variations in leaf mass per areawithin a tomato plant

The contribution of the starch and soluble carbohydrate pools to the diurna l variations of leaf mass per unit area (LMA) has been investigated in toma to leaves. A glasshouse experiment was carried out with plants pruned to tw o or five fruits per truss. Leaflets were sampled at sunrise, noon and suns et at different positions within the leaf (basal or terminal), and on diffe rent sympods along the stem. Carbohydrate contents and LMA were significant ly higher in the terminal than in the basal leaflets, except at sunrise. Du ring the day, differences in starch accumulation between terminal and basal leaflets increased with leaf height on the plant. Among sympods, the solub le carbohydrate content of the terminal leaflets did not vary significantly , whereas at 13.00 h the LMA was minimum in the middle of the plant and max imum at the top, and the leaf starch content significantly increased half-w ay up the plant. The plant fruit load had only small and non-significant ef fects on the LMA and carbohydrate contents. The response of LMA and carbohy drate contents to changing source activity was observed under controlled cl imatic conditions. The starch pool of fully expanded leaves was rapidly fil led and emptied under increasing and decreasing source activity. In young e xpanding leaves, this pool was hardly filled during daylight. On average th e soluble carbohydrates did not contribute significantly to the diurnal var iations in LMA, whereas fluctuations in starch explained c. 70% and 44% of these variations in the upper and lower leaves, respectively. The results a re discussed with respect to the modelling of LMA at the level of individua l tomato leaves or sympods.