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Jack D. Early Jr. and George C. Martin

Photoperiod is an important environmental signal for regulating developmental patterns in many plant species. In several species, photoperiodic regulation of gibberellin A1 biosynthesis has been implicated as the mechanism by which photoperiod may alter development. To examine this phenomenon in strawberry, Fragaria virginiana plants grown under long day (LD) and short day (SD) conditions with equivalent total PAR were examined to determine changes in vegetative growth and GA1 biosynthesis.

LD conditions (16 hr) promoted vegetative growth. Runner production, total leaf area, area of individual leaves, and petiole lengths, all increased under LD conditions. No runner production occurred under SD conditions (8 hr); however, the number of branch crowns increased.

Gibberellins A44, A19, A20, and A1, all from the GA1 biosynthetic pathway, were identified in plants under both LD and SD conditions. However, SD conditions appeared to affect the 2β-hydroxylation of GA20 to GA1. Whereas levels of most GAs decreased under SD conditions, levels of GA20 increased, and only trace amounts of GA1 were found, indicating a possible blockage of the pathway at this point. As GA1 is considered the active component of the pathway, blockage of GA20 conversion under SD conditions may explain the concomitant reduction in vegetative growth.

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Han Ping Guan and Harry W. Janes

Light/dark effects on growth and sugar accumulation in tomato fruit were studied on intact plants (in vivo) and in tissue culture (in vitro). Similar patterns of growth and sugar accumulation were found in vivo and in vitro. Fruit growth in different sugar sources (glucose, fructose or sucrose) showed that sucrose was the primary carbon source translocated into tomato fruit. Darkening the fruit decreased growth about 40% in vivo and in vitro: Light-grown fruit took up 30% more sucrose from the same source and accumulated almost twice as much starch as that in dark-grown fruit. The difference in CO2 exchange rate between light and dark indicated that light effects on fruit growth were due to mechanisms other than photosynthesis. Supporting this conclusion was the fact that light intensities ranging from 40 to 160 μmol/m2/s had no influence on growth and light did not increase growth when fruits were grown on glucose or fructose. A possible expansion of an additional sink for carbon by fight stimulation of starch synthesis during early development will be discussed.

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Kenneth L. Steffen, John L. Sims, and Lowell P. Bush

This study examined the effect of shifts in growth temperature on: 1) the partitioning of carbohydrate into starch or sucrose; and 2) the differential responses of tobacco types which differ in starch accumulation capacities. Six-week-old tobacco seedlings of Speight G28 (G28), a fluecured cultivar, and Ky 14 (K14), a burley cultivar, were acclimated for 9 days in growth chambers to a 14 h photoperiod of 300 μmol/s-m2 PAR at 27/22 C (day/night) and a relative humidity of between 70-80%. Temperature was then shifted to 15/10 C for 13 days and then back to 27/22 C for 8 days. At all points, sucrose, starch and protein content was higher in G28. Both cultivars demonstrated significant increases in dry matter accumulation per area, 1 day after the shift to 15/10 C. Dry matter increased steadily through day 13 in G28, but increased to day 5 and then leveled off at day 13 in K14. Nearly identical patterns of sucrose accumulation were observed in both species, with marked increases to day 5 and then a dramatic decline at day 13. Starch content increased steadily from day 1 to day 13 in G28, but leveled off in K14 after day 5. At day 8, soluble protein content increased only slightly in K14, but increased nearly 2-fold in G28. Within 1 day of the return to 27/22 C, starch and sucrose levels in both cultivars dropped 2 to 5-fold, to pre-temperature shift levels. The significance of these finding will be discussed.

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Chris B. Watkins, Juan-Pablo Fernández-Trujillo, and Jacqueline F. Nock

Susceptibility of apple fruit to CO2 can be affected by cultivar and postharvest treatment with diphenylamine (DPA). To study possible metabolic reasons for CO2 injury development, `Cortland' and `Law Rome' apple fruit were either untreated or treated with DPA at harvest, and then exposed to air or 45 kPa CO2 for up to 12 days. Fruit were sampled at 3-day intervals during treatment, and peel and flesh samples were taken for organic acid and fermentation product analysis. Additional fruit were removed to air and stored for 25 weeks for evaluation of injury. `Cortland' apple fruit had more external CO2 injury, but less internal CO2 injury than `Law Rome'. DPA treatment markedly reduced incidence of both external and internal injury. Fermentation products increased in peel and flesh of both cultivars with increasing exposure to CO2. However, acetaldehyde concentrations were ≈10 times higher in peel and flesh of `Law Rome' than `Cortland' apples. Ethanol concentrations in the flesh were similar in both cultivars, but were about twice as high in `Cortland' than `Law Rome' peel. Neither acetaldehyde nor ethanol concentrations were affected consistently by DPA treatment. Cultivar or DPA treatment did not affect accumulation of succinate, often regarded as the compound responsible for CO2 injury. These results do not indicate that acetaldehyde, ethanol, or succinate accumulation is responsible for CO2 injury in apple fruit.

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Dwight S. Fujii and Abhaya M. Dandekar

Many tree crops belonging to the Rosaceae family translocate and metabolize sorbitol. We have determined that some species of bacteria belonging to the genus Agrobacterium, Pseudomonas, and Erwinia pathogenic to the Rosaceae demonstrate the ability to metabolize sorbitol while those that were isolated from other hosts could not utilize sorbitol. Employing cellulose acetate electrophoresis (CAE) we have been able to demonstrate the presence of isoenzymes of sorbitol dehydrogenase (SDH) that correlate with the ability to metabolize sorbitol in these organisms. In order to study the properties of SDH in these organisms we carried out a detailed enzymatic analysis of the enzyme from A. tumefaciens. We found that the enzyme displayed activity when mannitol or xylitol were used as substrates, in addition to sorbitol. Michaelis constants (Km) were 32.8 mM, 0.19 mM, and 38.2 mM for sorbitol, mannitol, and xylitol respectively. To further distinguish the reactions with the different substrates the enzymatic extracts were further characterized on CAE using different substrates to visualize patterns of isoenzymes for a particular sugar alcohol. These analyses revealed the presence of unique isoenzymes for SDH. In addition we observed the presence of mannitol dehydrogenase (MDH) representing in most species a non-specific polyol dehydrogenase.

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Jack W. Buxton, Donna Switzer, and Guoqiang Hou

Marigold seedlings, 3 weeks old, were grown in natural light growth chambers at 3 day/night temperature regimes, 8°N/16°D, 13°N/20°D and 18°N/24°D, in a factorial combination with ambient and 1000-1500 ppm CO2. Seedlings were harvested at regular intervals during a 24 hr period and were analyzed for soluble sugars (reducing sugars and sucrose) and starch. Neither temperature nor CO2 concentration affected the accumulation of soluble sugars or starch during the day or night. The soluble sugar concentration ranged from 3% of dry weight at sunrise to 6% at mid-day; the concentration changed little during the night. Light intensity was different during replications of the experiment. Increased light intensity appeared to cause a slight increase in the soluble sugars maintained by the seedling during the day. Accumulated starch increased 6% to 8% from sunrise to late afternoon. Preliminary results indicate that light intensity greatly affected the concentration of starch. On the higher light intensity day, starch accumulated to a maximum of 18% of dry weight; whereas on the lower light intensity day the maximum concentration was 10%. During the night following the lower light intensity day, the starch concentration decreased to approximately 3% by the end of the night; following a brighter day the starch content was 13% at the end of the night.

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D. Mark Hodges, Gene E. Lester, Robert D. Meyer, Vivian E. Willmets, and Michele L. Elliot

Consumption of phytochemicals has been associated with reduced risks of human health dysfunctions such as cancers and heart disease. Such information has led to increased sales of fruits and vegetables. For example, in the United States, an estimated 23% increase in melon consumption (up to 13.2 lbs/capita/annum) has been recorded over 16 years. However, some health issues have been attributed to cantaloupe due to bacteria such as Salmonella attaching to inaccessible sites, such as the rind netting. Honeydew melons do not have a netted rind. The purpose of this study was to compare concentrations of antioxidants between cantaloupe and both green- and orange-fleshed honeydew melons during 14 days of storage to determine if orange-fleshed honeydew melon would represent a feasible alterative to cantaloupe to the increasingly health/food safety-conscious consumer. Cantaloupe (`Cruiser'; C), green-fleshed Honeydew (`HoneyBrew'; HB), and orange-fleshed Honeydew (`OrangeDew'; OD) melons were harvested in Texas at the beginning and at the end of the production season. β-carotene content was highest in OD, followed by C; no β-carotene was detected in HB. β-carotene levels did not change during storage. Phenolic levels increased in all three melon species during storage, whereas total ascorbate content declined in OD and in early harvest HB. Ascorbate peroxidase activities were lowest in OD, perhaps due to the lower ascorbate levels; little difference between the melon species in activities of the other ascorbate-associated enzymes were observed. Based on the phytochemicals measured in this study, choosing non-netted OD over netted C in order to reduce potential exposure to pathogens would not represent a less healthy food choice.

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Douglas D. Archbold* and Marta Nosarszewski

Acquiring sufficient carbohydrate is essential for successful apple fruit set. Sorbitol may be the dominant carbohydrate imported by growing fruit, and the rate of sorbitol accumulation may be a function of NAD-dependent sorbitol dehydrogenase (SDH; EC activity. Prior work indicated that SDH activity from whole fruit (seeds plus cortex) increased for 2 or 3 weeks after initiation of fruit growth and then declined through 5 weeks. Using SDH activity assays, an SDH-specific antibody, and SDH-specific probes in Northern analyses, it is evident that SDH is expressed and is active in both apple seed and cortex tissue during the first few weeks of fruit growth. On a per unit protein basis, SDH activity in seeds increased by the pattern described above while that in fruit was generally lower and constant. During this same period of time, the sorbitol content of the expressed sap of apple shoots was analyzed. The sorbitol concentration was 50- to 100-fold higher than the sucrose concentration. The concentrations of both carbohydrates changed in parallel to the change in SDH activity of whole fruit and seeds. The lowest SDH activity and sap sorbitol levels preceded and/or coincided with the beginning of the natural fruit drop (or June drop) period.

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Lili Zhou and Robert E. Paull

Papaya (Carica papaya L.) fruit flesh and seed growth, fruit respiration, sugar accumulation, and the activities of sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) were determined from anthesis for ≈150 days after anthesis (DAA), the full ripe stage. Sugar began to accumulate in the fruit flesh between 100 and 140 DAA, after seed maturation had occurred. SPS activity remained low throughout fruit development. The activity of SS was high 14 DAA and decreased to less than one-fourth within 56 DAA, then remained constant during the remainder of fruit development. AI activity was low in young fruit and began to increase 90 DAA and reached a peak more than 10-fold higher, 125 DAA, as sugar accumulated in the flesh. Results suggest that SS and AI are two major enzymes that may determine papaya fruit sink strength in the early and late fruit development phases, respectively. AI activity paralleled sugar accumulation and may be involved in phloem sugar unloading.