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“The feasibility of using an over-tree microsprinkler irrigation system for spring freeze protection of `Loring' peach trees [Prunus persica (L.) Batsch] was evaluated under a range of meteorological conditions during Winter 1988-89. Microsprinklers were attached to the underside of polyethylene laterals 2.5 m above ground level and centered over the tree rows. Irrigation rates of 0, 27, 36, and 44 liters/hour per tree were tested on trees trained to an open-center habit using microsprinklers that produced a circular wetting pattern. Microsprinkler irrigation maintained average bud temperature above -2C and 2 to 5C above those of nonirrigated trees under calm conditions, but provided no protection under windy conditions. Flower bud temperatures of irrigated trees were similar for 36 and 44 liters·hour-1, but were slightly lower for 27 liters·hour-1 under conditions typical of spring freezes. Limb breakage due to ice loading was negligible for all application rates, even under advective freeze conditions. Calculated water and energy consumption were reduced by at least 50% and 88%, respectively, by the microsprinkler system, compared to a typical overhead sprinkler system.

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Photosynthesis is the very essence of agriculture. Previous photosynthetic and transpirational studies of onion (Allium cepa) have been limited to specific developmental stages. Our study measured photosynthesis and transpiration in sixteen plants of a single short-day cultivar over an eleven week period containing both non- and bulb inductive photoperiods. Differences in weekly means for photosynthesis, leaf conductance, water use efficiency, and intercellular CO, were highly significant. Weekly photosynthetic means increased under a non-inductive photoperiod and peaked one week after initiating a bulb inducing photoperiod. A decrease and leveling period occurred as bulbs developed followed by a decrease as foliage lodged. Weekly photosynthetic and leaf conductance means were correlated and highly significant. Water use efficiency and intercellular CO, means remained fairly constant throughout the study suggesting that photosynthesis in unstressed onions was controlled by internal mechanisms instead of stomata.

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Solarization and chemical alternatives to methyl bromide (MeBr) soil fumigation for strawberry (Fragaria {XtimesX} ananassa) were evaluated in a 3-year study in Savannah, Ga. Solarization using clear or black plastic, metam sodium (Sectagon), dazomet (Basamid), 1,3-dichloropropene and chloropicrin (Telone C-35), MeBr, and untreated control treatments were used. Solarization produced maximal soil temperatures of 55 to 60 °C (131 to 140 °F) at the 2.5 cm (1 inch) depth, and 42 to 48 °C (108 to 118 °F) at the 15 cm (6 inch) depth. Clear and black plastic were generally equally effective in heating the soil. A double layer of clear plastic raised soil temperatures 1 to 2 °C (2 to 4 °F) above those under a single layer of clear at the 2.5 cm depth, although this occurred less frequently at the 15 cm depth. MeBr treatment increased yield by 46% and 128% in the first and second years, respectively, compared to the untreated control, but all treatments were similar in yield in year three. Season average fruit size differed among treatments in only the first year, with MeBr resulting in fruit 13% to 25% larger than other treatments. Yield for the metam sodium treatment in the first year was 34% lower than for MeBr, but comparable to MeBr in the other 2 years. Solarization treatment yields were similar to those of MeBr in the first and third years, but could not be analyzed in the second year due to plot damage. Dazomet treatment yields were similar to those of MeBr, metam sodium, and the untreated control in its single year of testing, but logistics of application and high costs may disfavor this treatment. The 1,3-dichloropropene/chloropicrin treatment performed as well as MeBr in its single year of testing. Three treatments-metam sodium, 1,3-dichloropropene/chloropicrin, and solarization with black plastic-offer viable, lower cost alternatives to MeBr.

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The peach [Prunus persica (L.) Batsch (Peach Group)] fruit is a sink organ comprised of different types of tissue, which undergoes three distinct developmental stages during the growth season. The objective of this study was to characterize the activity and partitioning of sorbitol and sucrose catabolism within `Encore' peach fruit to determine whether the two forms of translocated carbon play different roles in the various fruit tissues and/or stages of development. Sorbitol catabolic activity was defined as the sum of NAD-dependent sorbitol dehydrogenase (SDH) and sorbitol oxidase (SOX) activities, whereas sucrose catabolic activity was defined as the sum of sucrose synthase (SS), soluble acid invertase (AI), and neutral invertase (NI) activities. Partitioning of sorbitol and sucrose catabolism in each tissue was calculated as percentage of total sorbitol or sucrose catabolic activity in the entire fruit. At cell division, sorbitol catabolic activity was similar in the endocarp and mesocarp, but lower in the seed. However, sorbitol catabolism was mostly partitioned into the mesocarp, due to its large size compared to that of other tissues. SDH was more active in the mesocarp, while SOX was more active in the endocarp. Sucrose catabolism was most active and partitioned mainly into the endocarp. At endocarp hardening, both sorbitol and sucrose catabolic activities were highest in the seed, but despite this, sucrose catabolism was partitioned mostly in the mesocarp. At cell expansion, sorbitol and sucrose catabolic activities were still higher in the seed only when expressed on a weight basis and similar in mesocarp and seed when expressed on a protein basis. Both sorbitol and sucrose catabolism were partitioned mostly into the mesocarp. Sorbitol and sucrose contents were generally higher in the tissues that exhibited lower catabolic activities. All carbohydrates were always partitioned mostly into the mesocarp. Our results show that, at the cell division and endocarp hardening stages, sorbitol and sucrose catabolism are partitioned differently in the fruit and that SDH activity may play an important role in mesocarp cell division and final fruit size determination.

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In peach [Prunus persica (L.) Batsch (Peach Group)], both sorbitol and sucrose are used for source to sink carbon (C) transport, yet their specific functions in fruit growth and development remain unclear. Growth rate (GR), respiration rate (R), carbohydrate content, and the activities of sorbitol dehydrogenase (SDH), sorbitol oxidase (SOX), sucrose synthase (SS), acid invertase (AI), and neutral invertase (NI) were determined in `Encore' peaches to study the specific functions of sorbitol and sucrose in each phase of fruit development (an early period of rapid cell division, a relatively inactive intermediate stage where endocarp (pit) hardening occurs, and a final swelling due to cell expansion). Fruit growth and respiration rates (mol C/fruit per day) were always positively correlated, but the growth coefficient (gc) relating them was significantly higher at cell division, when maintenance respiration (Rm) was nearly absent. Sorbitol and sucrose appeared to participate equally in growth and maintenance respiration. Contents of sorbitol and sucrose both correlated positively to GR, and their rates of accumulation increased from early to late growth stages in similar fashion. SDH activity was always positively correlated with sink strength and GR, but with R only at endocarp hardening (r = 0.632). SOX activity was also correlated with sink strength and GR in the early (r = 0.514 and 0.553) and late (r = 0.503 and 0.495) growth phases, but not at endocarp hardening, and was correlated with R in two of three growth phases. Among sucrose cleavage enzymes, AI activity was positively correlated with sink strength, GR, and R more strongly than the others (r = 0.51 to 0.80), but only in the cell division and cell expansion periods. SS activity was correlated with sink strength and R only at endocarp hardening, and NI activity was generally not correlated to sink strength, GR, or R. We conclude that sorbitol and sucrose play similar roles in fruit development, and the enzymes associated with their metabolism work in concert to produce the observed changes in growth and respiration.

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Abstract

Leaf water relations and soil-to-leaf resistance were studied in 3-month-old pecan [Carya illinoenis (Wangenh.) C. Koch] seedlings as soil dried progressively to minimum water potentials of −0.3, −0.6, and −1.1 MPa in three separate tests. Leaf conductance, transpiration, and predawn leaf water potential declined with increasing soil water deficits, and only predawn leaf water potential fully returned to pre-stress levels after rewatering. Reduced levels of leaf conductance following water stress were apparently caused by internal factors other than leaf water potential. Leaf conductance of well-watered seedlings decreased logarithmically and with increasing leaf-to-air vapor pressure gradient. Soil-to-leaf resistance to water flow varied diurnally and generally increased following water stress at minimum soil water potentials of −0.6 and −1.1 MPa. Osmotic adjustment and changes in the distribution of water between the apoplast and symplast in leaves did not occur in response to soil water potentials of −0.6 MPa.

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Along with sucrose, sorbitol represents the major photosynthetic product and the main form of translocated carbon in peach. The objective of the present study was to determine whether in peach fruit, sorbitol and sucrose enzyme activities are source-regulated, and more specifically modulated by sorbitol or sucrose availability. In two separate trials, peach fruit relative growth rate (RGR), enzyme activities, and carbohydrates were measured 1) at cell division stage before and after girdling of the shoot subtending the fruit; and 2) on 14 shoots with different leaf to fruit ratio (L:F) at cell division and cell expansion stages. Fruit RGR and sorbitol dehydrogenase (SDH) activity were significantly reduced by girdling, whereas sucrose synthase (SS), acid invertase (AI), and neutral invertase (NI) where equally active in girdled and control fruits on the fourth day after girdling. All major carbohydrates (sorbitol, sucrose, glucose, fructose and starch) were reduced on the fourth day after girdling. SDH activity was the only enzyme activity proportional to L:F in both fruit developmental stages. Peach fruit incubation in sorbitol for 24 hours also resulted in SDH activities higher than those of fruits incubated in buffer and similar to those of freshly extracted samples. Overall, our data provide some evidence for regulation of sorbitol metabolism, but not sucrose metabolism, by photoassimilate availability in peach fruit. In particular, sorbitol translocated to the fruit may function as a signal for modulating SDH activity.

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Rabbiteye blueberry (Vaccinium ashei R.) flowers often suffer slight freeze damage that prevents fertilization and fruit development. To determine if gibberellic acid (GA3) might be useful in rescuing freeze-damaged flowers the following treatments were applied before anthesis to two cultivars at different locations: 1) undamaged control, 2) approximately two-thirds of the corolla and most of the style removed, 3) approximately half of the style removed, and 4) ovules lanced with an insect pin by driving it through the equator of the undeveloped berry until the point came out the other side. Half the bushes were not sprayed, and half were sprayed with GA3 (312 ppm, v/v) the night following treatment. `Climax' at Chula, Ga., had good fruit set for treatment 1 with and without GA3 (70% to 85%). Good fruit set also occurred for treatment 2, 3, and 4 where GA3 was applied (47% to 54%), but poor fruit set without GA3 (4% to 16%). `Tifblue' at Chula had significantly better fruit set for treatment 1 with GA3 (54% vs. 27%). Excellent fruit set occurred for treatment 2, 3, and 4 where GA3 was applied (81% to 96%), and poor fruit set without GA3 (6% to 7%). `Tifblue' fruit set by GA3 sized better than `Climax' fruit set by GA3. The experiments provide corroborative evidence that flowers that have suffered freeze damage to the stigma, style, corolla, and perhaps ovules can be set with GA3.

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Ovary temperatures of upward and downward facing flowers of `Junegold' Peach (Prunus persica (L.) Batsch) were measured on 5 nights in March 1991 to determine whether differential survival of ovaries following frost was related to flower orientation. Flowering twigs were removed from mature trees and positioned horizontally ≈ 1.5 m above ground level prior to occurrence of low temperatures (0-5C). Thermocouples were inserted through the hypanthium to contact ovaries of 10 upward and 10 downward facing flowers, and temperature and meteorological data were logged every five minutes. Under clear, calm conditions, temperature of upward facing flowers averaged 0.33C lower than that of downward facing flowers during the coldest period of the night, with maximal differences of 0.77C. Under cloudy, calm conditions, temperature differences between upward and downward facing flowers were less frequently observed and lower in magnitude (0.08 - 0.15C). Under windy conditions (>2.5 m/s), no temperature difference between upward and downward facing flowers occurred, despite strongly negative net radiation. Based on known values of ovary cold tolerance, it is concluded that differences in survival of Up to 38% could occur due to flower orientation when air temperature reaches critical values.

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The effect of specific ion toxicity during salt stress was tested in the present study. The experiment was repeated twice, in 1996 and 1998, with `Nemaguard' peach seedlings and rooted cuttings grown in hydroponics under two NaCl concentrations (50 and 30 mm). Foliage was separated in symptomatic and symptomless leaves and the amount of sodium (Na+) and chloride (C1) was determined. Significantly higher Na+ content was found in symptomatic than in symptomless leaves in both experiments, whereas in only two of the six cases was Cl content higher in symptomatic than in symptomless leaves. The Na+ threshold for leaf scorch was somewhere between 4 and 6 mg·g–1 dry weight. Results indicated that Na+ accumulation, rather than Cl accumulation, was associated with the familiar marginal and interveinal scorch symptoms seen in salt-stressed peach leaves.

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