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Abstract
Translocation of 14C-photosynthetic products from source leaves to fruit segments in direct vertical alignment to them was examined in ‘Hamlin’ sweet orange [Citrus sinensis (L.) Osbeck] and ‘Marsh’ grapefruit (Citrus paradisi Macf.). A single source leaf nearest (or next nearest) to a fruit was exposed for 1 hour to 14CO2 and then flushed with ambient air for 5 hours. Source, path, and sink tissues were separated and fruit was longitudinally quartered with respect to source-leaf attachment position. About 20% of the total 14C-photosynthate was translocated into the fruit within 6 hours. The quarter aligned directly to the source leaf was allocated 77% and 85% of these assimilates in sweet oranges and grapefruit, respectively.
Fruit were removed 8, 16, and 24 weeks after peak bloom from 3- and 4-year-old, `Hamlin' orange [Citrus sinensis (L.) Osbeck] trees on `Carrizo' citrange root-stock [C. sinensis (L.) Osbeck ×Poncirus trifoliata (L.) Raf.], planted as bare root or containerized trees, to determine if fruit removal enhanced vegetative growth. Bare-root trees had a greater stem diameter and tree height than containerized trees at planting and after years 3 and 4. Fruit of bare-root trees had lower fresh and dry weights, refixed less of their respiratory CO2 during development, and lost water less rapidly after harvest. In addition, fruit removal treatments did not increase growth of either bare-root or containerized trees relative to trees from which no fruit had been removed. In fact, tree diameters were slightly higher when fruit were not harvested. Carbon cost of fruit production may have been countered by other factors under field conditions, such as known enhancement of photosynthetic rates by fruit load and/or diurnal contributions by fruit to leaf water demands.
Three commercially employed treatments to force scion bud growth were compared using greenhouse-grown `Carrizo' citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.] seedlings budded with `Hamlin' orange [Citrus sinensis (L.) Osb.]. Scion buds were forced either by 1) cutting off (removing the rootstock shoot above the bud union); 2) lopping (cutting half-way through the rootstock shoot above the bud union and breaking over the rootstock stem); or 3) bending (bending the rootstock shoot to its base and tying it in position). Plants were harvested, dried, and weighed at various times after scion shoot emergence. Plants on which rootstock shoots remained attached (lopping or bending) had the largest scion leaf area and gained the most scion and whole-plant dry weight. Bending rootstock shoots also resulted in a longer scion, more leaves, and higher root dry weight than did removal of rootstock shoots by cutting off. Few differences in overall growth were observed among trees retaining their rootstock shoots after two scion growth flushes. Removal of rootstock shoots after the first scion growth flush reduced leaf area and dry weight gain of the second scion growth flush; however, it did not affect total scion leaf area or dry weight. Analysis of 14C-photosynthate transfer from labeled rootstock leaves showed that bending allowed the greatest movement of labeled assimilates to other plant parts during the 24 hours after 14CO2 fixation. Radiolabeled photosynthates from rootstock leaves were partitioned primarily to shoots during scion growth flushes and to roots during periods between growth flushes.
Peach-to-nectarine mutations are associated with broad pleiotropic effects. The present study addresses the heritability of nectarine-specific effects in three hybrid families. A comparison of peach and nectarine siblings showed that nectarine fruit were smaller (less fresh weight), rounder, darker, redder, and had higher levels of sugars and organic acids. These heritable characteristics are similar to effects of spontaneous peach-to-nectarine mutations described previously.
Two peach-to-nectarine mutants were compared with their peach progenitors to quantify physical and biochemical characters associated with this conversion. Both nectarine mutants showed pleiotropic effects that included smaller, rounder, and more-dense fruit with redder skin and altered sugar and organic acid composition relative to those found in their peach progenitors. In addition, one of the nectarine mutants exhibited a later bloom time, a less pronounced change in fruit size, a shorter fruit development period, and an associated capacity to develop red fall leaf color.
Water and carbon budgets of individual citrus fruit were determined throughout their growth to quantify the demand for sucrose and water relative to developmental changes. Fruit transpiration, water accumulation, photosynthesis, respiration, and C gain were measured during this period for grapefruit (Citrus paradisii Macf.) and calamondin (Citrus madurensis Lour.). On a whole-fruit basis, estimated rates of grapefruit transpiration and mean daily water inflow decreased after the first third of development, whereas water apparently was lost freely throughout growth of the smaller, thin-peeled calamondins. Estimates of daily fruit C import remained relatively similar during the majority of grapefruit growth, increasing rapidly only as fruit neared maturation. A similar trend was observed in calamondins, although rates were more variable. Overall, estimated mean daily water inflow into “developing grapefruit decreased relative to that of sucrose inflow, resulting in a progressively higher ratio of sucrose transport to net water inflow. Values for these ratios rose from ≈; 10 to >300 g sucrose/liter of water, reaching levels of net water and sngar transfer that could both be accommodated by citrus phloem alone. Any additional entry into grapefruit appears to have been offset by xylem back-flow, because no other net water influx was observed.
Variability in sucrose levels and metabolism in ripe fruit of several Vaccinium species were examined. The objective was to determine if sufficient variability for fruit sucrose accumulation was present in existing populations to warrant attempts to breed for high-sucrose fruit, which potentially would be less subject to bird predation. Three-fold differences in fruit sucrose concentration were found among species, ranging from 19 to 24 mg·(g fw)-1 in V. stamineum and V. arboreum to about 7 mg·(g fw)-1 in cultivated blueberry (V. ashei and V. corymbosum) and V. darrowi. Soluble acid invertase activity was negatively correlated with fruit sucrose concentration. There was no apparent correlation between fruit sugar concentration and either sucrose phosphate synthase or sucrose synthase activities, both of which were low for all species studied. The degree of variability in fruit sucrose accumulation among Vaccinium species supports the feasibility of developing high sucrose fruit, which would be a potentially valuable addition to current strategies of minimizing crop losses to birds.
Immunolocalization of sucrose synthase in cotton ovules was conducted to clarify the relationship between this enzyme and sucrose import during seed and hair development. Although rapidly growing plant organs are known to have high activities of sucrose synthase, information on specific localization has been minimal at the cell level. Cotton ovules were examined in the present work as representatives of developing dicot seeds, and because their expanding hairs would have a locally high demand for cell wall synthesis. Initial analyses focused on ovules immediately before and after anthesis/pollination. Heavy immunolabel was observed solely in the nucellar tissue 1 day before anthesis. Immediately following anthesis/pollination, however, sucrose synthase became evident in cells of the external (but not internal) integument, expanding from the chalazal to micropylar end of this tissue. Little sucrose synthase was directly associated with very young epidermal hairs prior to their most rapid expansion. Immunolabel appeared to be localized in cells associated with the vascular bundle of this external integument. The shift in spatial/temporal localization of sucrose synthase in cotton ovules following pollination indicates a close association between sucrose synthase and probable sites of sucrose import at a cellular level during early seed development.
In this study, the effects of grafting with interspecific hybrid rootstocks on field-grown tomato fruit quality were evaluated over a 2-year period. Fruit quality attributes from determinate ‘Florida 47’ tomato plants grafted onto either ‘Beaufort’ or ‘Multifort’ rootstocks were compared with those from non- and self-grafted controls. Grafted plants had higher fruit yields than non- and self-grafted plants, and increased production of marketable fruit by ≈41%. The increased yield was accompanied by few major differences in nutritional quality attributes measured for these fruit. Although grafting with the interspecific rootstocks led to consistently small, but significant increases of fruit moisture (≈0.6%), flavor attributes such as total titratable acidity (TTA) and the ratio of soluble solids content (SSC) to TTA were not significantly altered. Among the antioxidants evaluated, ascorbic acid concentration was reduced by 22% in fruit from grafted plants, but significant effects were not evident for either total phenolics or antioxidant capacity as assayed by oxygen radical absorbance capacity (ORAC). Levels of carotenoids (lycopene, β-carotene, and lutein) were similar in fruit from grafted plants with hybrid rootstocks compared with non- and self-grafted controls. Overall, the seasonal differences outweighed the grafting effects on fruit quality attributes. This study showed that grafting with interspecific hybrid rootstocks could be an effective horticultural technique for enhancing fruit yield of tomato plants. Despite the modest reduction in ascorbic acid content associated with the use of these rootstocks, grafting did not cause major negative impacts on fruit composition or nutritional quality of fresh-market tomatoes.