Juice tissues of citrus lack phloem; therefore, photosynthates enroute to juice sacs exit the vascular system on the surface of each segment. Areas of extensive phloem unloading and transport (vascular bundles + segment epidermis) can thus be separated from those of assimilate storage Uuice sacs) and adjacent tissues where both processes occur (peel). Sugar composition, dry weight accumulation, and activities of four sucrose-metabolizing enzymes (soluble and cell-wall-bound acid invertase, alkaline invertase, sucrose synthase, and sucrose phosphate synthase) were measured in these transport and sink tissues of grapefruit (Citrus paradisi Macf.) to determine more clearly whether a given enzyme appeared to be more directly associated with assimilate transport versus deposition or utilization. Results were compared at three developmental stages. Activity of sucrose (per gram fresh weight and per milligram protein) extracted from zones of extensive phloem unloading and transport was significantiy greater than from adjacent sink tissues during the stages (II and ll) when juice sacs grow most rapidly. In stage 11 fruit, activity of sucrose synthase also significantly surpassed that of all other sucrose-metabolizing enzymes in extracts from the transport tissues (vascular bundles + segment epidermis). In contrast, sucrose phosphate synthase and alkaline invertase at this stage of growth were the most active enzymes from adjacent,

The role of sucrose synthase in translocation and sucrose partitioning remains unresolved despite extensive study of its association with elevated carbon import. Although growing interest has centered on its involvement in sucrose metabolism by importing cells (2, 17), additional evidence also supports a possible vascular function (1,3,6). Recently, Yang and Russell (19) suggested that the promoter for one of the sucrose synthase genes in maize (shrunken 1 gene) was "phloem specific" in transgenic tobacco plants. However, other studies have indicated expression of sucrose synthase genes may be more complex and appears to be sensitive to changes in metabolism and/or environment (14, 15). Nonetheless, immunohistological evidence has indicated a greater abundance of sucrose synthase protein in vascular areas of young roots (1). The levels of total protein also tend to be greater in vascular tissues (13), however, leaving the extent of sucrose synthase activity in vascular tissues unresolved.The possibility that sucrose synthase activity may be greater in vascular bundles has been difficult to address because few opportunities exist for isolation of vascular strands (4). Most plant tissues have fine networks of vascular bundles that cannot be effectively separated from adjacent parenchymal cells. Hawker and Hatch (6) MATERIALS AND METHODS'Marsh' grapefruit (Citrus paradisi Macf.) were sampled from the outer, southern canopy of 55-year-old trees in a commercial orchard in Lake Wales, FL, during late August to early September (before completion of the expansive phase of growth) during four consecutive growing seasons. Samples for immediate dissection were also obtained during the fourth growing season from 10-year-old containerized trees grown in Gainesville, FL. Results from these fruit were similar to those from the orchard-grown trees and were included in overall means.Fruit were washed, sectioned longitudinally, peeled, and separated into individual segments. These were dissected into tissues pictured in Figure 1

Carbon distribution and metabolism by northern red oak seedlings (Quercus rubra L.) were followed for 72 h after a 30-min photosynthetic exposure to (14)CO(2). Approximately 50% of the assimilated carbon was lost during this time, presumably through respiration. Most of the (14)C recovered in the plant remained in the source leaf. Most of the exported (14)C was found in stems and especially roots. Position of the source leaf did not affect distribution of translocated (14)C; however, upper source leaves retained slightly more (14)C than lower source leaves. Most (14)C in all tissues was found initially in sugars. Labeled carbon in this fraction decreased rapidly and increased in other chemical fractions, particularly residue (structural carbohydrates and lignin). More (14)C was incorporated into residue than into any other chemical fraction, indicating continued growth by each of the tissues during the lag stage of seedling development. Labeled carbon increased in proteins for 6 to 12 h after treatment, then remained essentially constant for the remainder of the transport period, indicating both rapid incorporation of the pulsed (14)C into, and slow turnover of, the labeled protein components. In source leaves, (14)C incorporated into starch in the light was lost during the dark period, a typical diurnal storage pattern. In stems, (14)C in starch increased for 12 h, then decreased during the dark period, and then increased during the rest of the transport period. This pattern indicates that stems have both diurnal and long-term storage pools of starch. In roots, (14)C in starch increased rapidly for 12 h, then remained constant for the rest of the transport period, indicating primarily long-term storage in this tissue. Peak (14)C in amino acids in taproot preceded that in stem tissue, indicating recycling of (14)C from the roots to the shoot in amino acids.

To explore the physiological mechanisms underlying ozone-induced growth reductions in loblolly pine (Pinus taeda L.), seedlings were exposed to sub-ambient (charcoal-filtered), ambient or twice-ambient ozone in open-top chambers for three growing seasons. In the final year of exposure, current-year needle fascicles were labeled with (14)CO(2) and the incorporation of (14)C into biochemical fractions was followed for 48 hours. Irrespective of ozone treatment, losses of (14)C-assimilates from foliage to respiration and translocation were minimal during the first 3 hours, whereas more than 60% of the label was lost during the next 45 hours. Radiolabel in sugar decreased rapidly after a lag period, roughly paralleling the pattern of total (14)C loss. The amount of (14)C label in starch and lipids plus pigments remained constant throughout the 48-hour chase period, whereas the amount of (14)C label in other fractions showed a net decrease over the 48-hour chase period. Ozone treatments altered foliar carbon dynamics in two ways: (1) ozone exposure increased foliar (14)C retention up to 21% for the first 5 hours after labeling, but not thereafter, and (2) ozone exposure decreased partitioning of (14)C into starch and increased partitioning of (14)C into organic acids, residue, and lipids plus pigments, indicating an intensified partitioning of carbon to injury and repair processes. Both short-term carbon retention and diversion of carbon from storage compounds to repair processes are foliar mechanisms by which ozone exposure could decrease growth in loblolly pine seedlings.

The episodic or flushing growth habit of northern red oak (Quercus rubra L.) has a significant influence on carbon fixation, carbon transport from source leaves, and carbon allocation within the plant; however, the impact of episodic growth on carbon partitioning among chemical fractions is unknown. Median-flush leaves of the first and second flush were photosynthetically labeled with 14CO2, and partitioning of 14C into lipids and pigments, sugars, amino acids, organic acids, protein, starch, and structural carbohydrates of source leaves, stem, and roots was determined. In addition, four chemical fractions (sugars, starch, amino acids, and total structural carbohydrates) were quantitatively analyzed in leaves, stems, and roots. Chemical changes in source leaves reflected leaf maturation, changing sink demand during a growth cycle, and leaf senescence. Starch and sugar storage in leaves, stems, and roots during lag and bud growth stages indicate a feedback response of these tissues to decreasing sink strength and temporary storage of both starch and sugar in these plant tissues. Northern red oak, with episodic shoot growth patterns, provides an experimental system in which large changes in sink strength occur naturally and require no plant manipulation. Metabolic changes in leaf, stem, and root tissue of red oak have broad application for other oak species and for both temperate and tropical tree species with cyclic growth habits.