C-labeled Assimilates in Grapefruit: Light and Dark CO2 Fixation by Leaves and Fruit

Distribution of radiolabeled assimilates was examined at various intervals after 1 hour of light or dark 14CO2 fixation by leaves or developing fruit of grapefruit (Citrus paradisi Macf.) so that the fate of assimilates from each source could be assessed at sequential stages of fruit growth. Exported products of both light and dark 14CO2 fixation in leaves were deposited primarily in juice tissues of fruit even during periods of substantial dry weight accumulation by peel. Fruit photosynthesis, however, gave rise to assimilates that remained almost entirely in the peel (flavedo and albedo) even 7 days later, regardless of dry matter increases by other tissues. Products of dark 14CO2 fixation by intact fruit were recovered in all tissues but predominated in the peel of young fruit vs. juice tissues at later stages of growth. Comparison of dry matter gains and C-labeled assimilate distribution indicated that fruit photosynthesis likely contributed substantially to development of peel but not juice sacs. Data on dark 14CO2 fixation were consistent with its suggested involvement in organic acid synthesis by juice sacs. The vast majority of assimilates are produced by leaves during photosynthesis in light, but CO2 fixation occurs in fruit as well as leaves, and in both light and dark. Partitioning and translocation of photosynthates derived from source leaves of whole plants are relatively well-documented in various commercially important food species (Daie, 1985; Wardlaw, 1968; Wareing and Patrick, 1975). Less information is available for fleshy fruit crops, including citrus, but these fruit are known to be strong sinks for leaf photosynthates (Kadoya, 1974; Kriedemann 1969a, 1969b). The presence of fruit can increase photosynthetic efficiency of citrus leaves and partitioning of photosynthates (Lenz, 1979). Leaf photosynthates enter the fruit interior very slowly, however, and are transferred via relatively few vascular bundles and extensive areas of juice sac stalks completely lacking phloem (Koch, 1984; Koch and Avigne, 1984, 1990; Koch et al., 1986). Seasonal alteration has been reported by Brown (1974) for the extent of leaf-derived assimilates arriving in navel oranges. We found no reports examining changes that occur in photosynthate translocation or distribution within these fruit during their development, however. Photosynthesis also occurs in fruit of many crops. Studies of pea and soybean indicate that pods can provide photosynthates to developing seeds by reassimilation of respiratory CO2 (Atkins. et al., 1977; Quebedeaux and Chollet, 1975). In a young green apple or citrus fruit, photosynthetic CO2 fixation is of sufficient magnitude to conserve 20% to 80% of the CO2 released by dark respiration, depending on the stage of development (Bean et al., 1963; Kidd and West, 1947; Schaedle, 1975). Also, 25% of the C-labeled photosynthates produced in satsuma mandarin fruit were recovered 5 days later in juice tissues (Akao and Tsukahara, 1979). Fruit photosynthesis is reported to decrease toward maturity in citrus (Bean and Todd, 1960; Moreshet and for publication 6 Nov. 1989. Univ. of Florida Journal Series no. Rhis work is a portion of the thesis submitted by C-R Yen in partial nt of a PhD degree at the Univ. of Florida. The research was partially y USDA-CRGO grant no. 84-CRCR-1-1478, the Florida Institute of Agricultural Sciences, and the Council of Agriculture, Republic of se of product names does not imply an endorsement by the Univ. of he cost of publishing this paper was defrayed in part by the payment harges. Under postal regulations, this paper therefore must be hereby dvertisement solely to indicate this fact. ddress: Chia-Yi Agricultural Experiment Station, Chia-Yi, Taiwan. r. Soc. Hort. Sci. 115(5):815-819. 1990. Green, 1980; Ramakrishnan and Varma, 1959; Todd et al., 1961), and its total contribution to orange and lemon growth is believed to be relatively small compared with that of assimilates translocated from leaves (Moreshet and Green, 1980; Todd et al., 1961). Little other information is available on partitioning, distribution, or redistribution of photosynthates produced by grapefruit or other fleshy fruits. Dark CO2 fixation, an anaplerotic reaction contributing to amino and organic acid formation (Bidwell, 1983), is involved in normal growth and development of many tissues from roots (Splittstoesser, 1966) to flower buds of ‘Valencia’ oranges (Vu et al., 1985). Organic acid synthesis in citrus juice tissues may well be another example. Minimal research has been directed toward analysis of assimilates arising from nonphotosynthetic carboxylation reactions in fruit. However, greater amounts of C-labeled assimilates are found in citrus juice sacs than surrounding tissues after each is separately exposed to 14CO2 in darkness (Bean and Todd, 1960). Greater proportions of labeled organic acids (predominantly citric) and amino acids are also recovered in juice tissues after dark 14CO2 fixation by fruit than by photosynthesis of the same tissues. Dark CO2 fixation has long been considered one possible source of organic acid synthesis in citrus (Bean and Todd, 1960; Huffaker and Wallace, 1959; Young and Biale, 1968). Neither the extent of dark CO2 fixation in whole, intact fruit nor developmental changes in these reactions have been examined. We therefore compared the distribution of labeled assimilates derived from 14CO2 fixation in light and darkness by grapefruit and adjacent source leaves throughout fruit development. Materials and Methods Fruits and/or branches with source leaves adjacent to the fruit were randomly selected from between 1 and 2 m above ground level on the exterior canopy of 4-year-old grapefruit trees (’Foster’ seedless clone no. 1-26-39) on ‘Savage’ citrange or sour orange rootstock in Lake Wales, Fla. (Hearn, 1986). There were no evident differences in quality (size, composition, and development) among fruit from trees on these two rootstock (data not shown). Samples from a given tree were used for various analyses. Each tree represented one replication and four trees were used for each experiment. Fruit with 10-cm subtending branches were cut from trees on 7 May, 25 June, 27 July,