Fruit photosynthesis



Abstract. In addition to photosynthesis as in the leaf, fruit possess a system which refixes CO2 from the mitochondrial respiration of predominantly imported carbon. This pathway produces malate by the action of phosphoenolpyruvate carboxylase, PEPC, (E.C. and appears to be regulated primarily by the cytosolic concentration of HCO3/CO2 and malate. Malate is stored in the vacuole as malic acid, constituting a major carbon pool and a potential substrate for respiration. The PEPC in apple fruit proves to be an efficient form of the enzyme with low Michaelis constants, i.e. Km = 0.09 mol m-3 PEP and 0.2 mol m–3 HCO3, and large Ki= 110 mol m-3 HCO3. In fleshy fruit, chlorophyll and chloroplasts are unevenly distributed; they resemble the C3 sun-type and arc concentrated in the perivascular tissue, with smaller chloroplasts, fewer grana per chloroplast and a larger degree of vacuolation than commonly found in a leaf of the same species. Fruit photosynthesis often compensates for respiratory CO2 loss in the light. However, due to respiration in the dark, CO2 loss is in excess of photosynthetic gain in the light, such that a continual loss of CO2 was observed in the diurnal cycle and which is maintained throughout fruit development. The rate of CO2 exchange decreases on a fresh weight or surface basis, but increases with fruit ontogeny on a per fruit basis, causing accumulation of several percent CO2 in the internal cavity. Stomata are present in the outer epidermis of those fruits examined, but with a 10-to 100-fold lesser frequency than in the abaxial epidermis of leaf of the same species. The number of Stomata is set at anthesis and remained constant, while the stomatal frequency decreases as the fruit surface expands. Stomata are as sensitive as in leaves in the early stages of fruit development, but often are transformed into lenticels during fruit ontogeny, thereby decreasing the permeability of the outer epidermis. The discrepancy between the CO2-concentrating mechanism provided by PEPC analogous to C4/CAM Photosynthesis and the kinetics of fruit PEPC, characteristic of C3/non-autotrophic tissue, suggests the definition of a new type of ‘fruit photosynthesis’ rather than its categorization within an existing type.