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  • Author or Editor: Mark Rieger x
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Root and shoot characteristics related to drought tolerance were studied for Prunus persica, P. andersonii, P. besseyi, P. maritima, P. subcordata, and P. tomentosa. In general, shoot characteristics were more closely associated with drought adaptation than root characteristics across species. The most xeric species, andersonii, had the most xerophytic leaf morphology, highest rates of leaf gas exchange, high root length/leaf area and root weight/leaf area ratios, but had root length and hydraulic conductivity similar to that of more mesic species. Water use efficiency (WUE) increased as water potentials (ψ) dropped to -3.0 to -4.0 Mpa during a 5-7 day drought for the xeric andersonii and subcordata. However, after an initial increase, WUE decreased with declining ψ in the other 4 species, indicating that carboxylation was affected by stress in the -1.5 to -3.0 range of ψ for besseyi, maritima, persica and tomentosa. CO2 assimilation (A) decreased linearly with ψ during drought in all species, but the ψ at which A reached zero was not well correlated with drought adaptation. Root hydraulic conductivity was similar for all species, indicating a lack of importance of this parameter for drought tolerance. The data suggest that introduction of xerophytic shoot characteristics into commercial cultivars of Prunus would improve drought tolerance to a greater extent than using drought tolerant species as rootstocks.

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Pressure-induced flow (PIF) and transpiration-induced flow (TIF) methods were used to obtain estimates of root hydraulic resistance (Rroot), stem hydraulic resistance (Rstem), and total hydraulic resistance (Rtotal = Rroot + Rstem) of ‘Lovell’, ‘Nemaguard’‚and ‘Siberian C’ peach [Prunus persica (L.) Batsch] rootstocks. Estimates of Rtotal did not differ significantly between the PIF and TIF methods, and Rroot differed between methods for ‘Siberian C only. However, Rstem was about three times lower and velocity of flow was greater in the PIF than in the TIF method for all three rootstocks. Lower Rstem in the PIF than the TIF method was not due to water moving in non-xylem pathways (i.e., pith, cortex) of stems in PIF determinations. Rroot and Rtotal were highly correlated in all cases, except TIF determinations with ‘Nemaguard’. Rroot accounted for 64% to 73% of Rtotal in TIF determinations, but significantly more (90% to 91%) of Rtotal in PIF determinations.

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One-year-old kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang et R. Ferguson var. deliciosa] vines were grown under 8- and 16-hour photoperiods to study the influence of photoperiod on cold acclimation and determine the potential level of hardiness that young vines attain. Vines were acclimated by reducing growth chamber temperature at 2-week intervals, beginning at 31/20C (16 hours/8 hours) and ending with 15/5C after 8 weeks. Vines receiving an 8-hour photoperiod were more cold hardy than vines receiving a 16-hour photoperiod after 4 weeks of acclimation as determined by electrolyte leakage from stem tissues. Moreover, vines receiving an 8-hour photoperiod survived freezing at – 9C at the end of the 8-week acclimation period, whereas those receiving a 16-hour photoperiod were killed at – 6C. Vine survival and electrolyte leakage of sterns were highly correlated (r = – 0.79 to – 0.90).

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Estimates of root hydraulic conductivity (Lp) were obtained on intact peach (Prunus persica × P. davidiana `Nemaguard') and sour orange (Citrus aurantium L.) rootstock over a broad range of transpiration rates. Within a species, Lp was lower when estimated using the Ohm's law analog than the reciprocal of the slope of the linear regression between transpiration (E) and stem xylem water potential (Ψ). Nonzero y-intercepts in linear regressions of Ψ vs. E resulted in the lack of agreement between Lp estimates. Removal of the root system caused xylem Ψ to rapidly approach zero in both species when E ≈ 0, suggesting that factors responsible for nonzero y intercepts resided within roots. Lp was 2.2 and 3.5 times lower for sour orange than peach when calculated by the Ohm's law and regression methods, respectively.

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Paclobutrazol (PBZ) was supplied in nutrient solution culture to `Nemaguard' peach rootstock [Prunus persica × P. davidiana] at concentrations of 0, 0.001, 0.01, 0.1, and 1.0 mg·liter-1. PBZ increased root: shoot ratio and decreased root length by ≈ 5-fold over the range of PBZ concentrations tested. Root tip diameter, stele diameter, and width of the root cortex were not significantly affected by PBZ. Root hydraulic conductivity decreased log-linearly with increasing PBZ concentration; however, this decrease did not affect midday leaf conductance or net photosynthetic rate. Foliar levels of N, P, K, Fe, and Mo were reduced, whereas levels of Ca, Mg, B, and Mn were increased by PBZ. The magnitude of changes in foliar nutrition were proportional to the degree of growth suppression. Chemical name used: (2RS,3RS)-l-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol).

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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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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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