Fruit photosynthesis

by M M Blanke, F Lenz
Plant, Cell and Environment ()


concentration of HCO^/COj and malate. Malate is stored in the vaeuole as malic acid, consti- tuting 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"^ PEP and 0.2 mol m ' HCO,, and large Ki= 1 lOmol m^-^ HCOj. In fleshy fruit, chlorophyll and chloroplasts are unevenly distributed; they resemble the C, sun- type and are concentrated in the perivascular tissue. With stnaller chloroplasts, fewer grana per chloroplast and a larger degree of vacuolation than commonly found in a leaf of the same speeies. Eruit photosyn- thesis often compensates for respiratory CO, loss in the light. However, due to respiration in the dark, CO, loss is in excess of photosyntheiie gain in the light, such that a continual loss of COi was observed in the diurnal cycle and which is maintained throughout fruit development. The rate of COj exchange deereases on a fresh weight or surfaee basis, but increases with fruit ontogeny on a per fruit basis, causing aeeumulation of several percent CO2 in the mternal eavity. Stomata are present in the outer epidermis of those fruits examined, but wilh a 10-to 100-fold lesser frequency than in the abaxial epidermis of leaf of the same species. The number of stomata is ^et 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 trans- tormed into lentieels during fruit ontogeny, thereby decreasing the permeability of the outer epidermis. The discrepancy between the CO^-concentrating mechanism provided by PEPC analogous to C4/CAM photosynthesis and the kinetics of fruit PEPC, charac- teristic of C,/non-autotrophic tissue, suggests the defi- nition of a new type of 'fruit photosynthesis' rather Ulan its eategorization within an existing type

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