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  • Author or Editor: Edward L. Proebsting x
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In Dec. 1990, sweet cherry (Prunus avium L.) selections varied in floral bud kill from 9% to 92% following exposure to severe cold. In the following winter, the hardiness of two hardy and two tender selections was analyzed by differential thermal analysis (DTA) to screen selections for hardiness. In a mild winter, when buds remained at their minimum hardiness level, the hardy selections consistently were > 2C hardier than the tender selections. About one-half of that hardiness difference was associated with differences in tissue water content, the other half with unknown factors. Buds of the tender selections began to develop earlier and bloomed earlier than the hardy selections. DTA analysis of floral bud populations separated selections that clearly differed in floral bud hardiness.

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Flower buds of 20 Prunus species showed quite different strategies to cope with low temperatures. Buds of most species deep supercooled. The two hardiest species, both from the subgenus Padus (P. padus L. and P. virginiana L.), did not supercool and survived -33C with no bud kill. Prunus serotina J.F. Ehrh., also in Padus, did supercool. Prunus nigra Ait., P. americana Marsh, P. fruticosa Pall., and P. besseyi L.H. Bailey had a low minimum hardiness level (MHL), small buds, and a low water content. Exotherms were no longer detectable from the buds of these species after 2 days at -7C and some buds survived -33C. Prunus triloba Lindl. and P. japonica Thunb. were similar to that group, but no buds survived -33C. Prunus davidiana (Carriere) Franch., P. avium L., and P. domestica L. had a relatively high MHL but hardened rapidly when the buds were frozen. Prunus persica (L.) Batsch., P. subhirtella Miq., P. dulcis (Mill) D. A. Webb, and P. emarginata (Dougl. ex Hook) Walp. deep supercooled, had large flower buds and a high MHL, and were killed in the Dec. 1990 freeze. Prunus salicina Lindl., P. hortulana L.H. Bailey, P. armeniaca L., and P. tomentosa Thunb. were in an intermediate group with a moderately low MHL and a moderate rate of hardiness increase while frozen. Prunus dulcis and P. davidiana had a low chilling requirement and bloomed early, whereas P. virginiana, P. fruticosa, P. nigra, and P. domestica had high chilling requirements and bloomed late.

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Flower buds of Prunus serotina Ehrh. produced high temperature exotherms (HTEs) and low temperature exotherms (LTEs). Supercooling in P. serotina occurred during full dormancy in December and early January but disappeared thereafter, whereas no supercooling was observed in P. padus L. or P. virginiana L. Both intact and detached lnflorescences of P. serotina supercooled and froze as a unit and not as individual florets. Exotherms in dehydrated parts occurred at lower temperatures than in hydrated parts. Dormant buds of P. serotina lost the detectable exotherms when kept at -7C for 2 days, while buds stored at 3C exhibited LTEs between -20 and -26C. Dormant Morescences of P. serotina were filled with elongated procambium and pith cells. In contrast, P. padus and P. virginiana had differentiated xylem vessel elements (XVE) the entire length of the inflorescence and did not supercool. Bud scales and bud axis of P. serotina were the flower parts where water apparently migrated during storage at 3 and -7C. This was not observed for P. padus and P. virginiana flower buds. The rate of water migration from the inflorescences to bud scales and axis probably plays a role in the freezing behavior of P. serotina.

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Abstract

The development of deep supercooling in sweet cherry (Prunus avium L. ‘Bing’) and peach [Prunus persica (L.) Batsch ‘Redhaven’] flower buds during late summer and early autumn was measured by differential thermal analysis. The capacity to deep supercool developed under ambient conditions during September in both 1982 and 1983. It developed 1-2 weeks earlier in sweet cherry than in peach flower buds. Exposing a sweet cherry limb in situ to daily 15° day/5°C night temperatures beginning in early Aug. 1983 advanced the development of the flower buds’ capacity to deep supercool by 4 weeks. Sweet cherry flower buds whose capacity to deep supercool was advanced by exposure to these cool temperatures displayed a median low-temperature exotherm (LTE50) near - 20°. This demonstrated the potential capacity of sweet cherry flower buds to avoid injury during moderate freezes as early as mid-August.

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Abstract

Seasonal changes in the temperature of the median low-temperature exotherm (LTE50) of dormant sweet cherry (Prunus avium L. cv. Bing) flower buds were significantly correlated with the preceding minimum air temperature in the orchard and the water content of the flower primordia. When buds were exposed to temperatures just below the high temperature exotherms, water migrated from the primordia to the bud scales. Under these conditions, the LTE50 decreased almost 5°C during the first day, but only 1°/day thereafter. The minimum LTE50 was near −32° after the buds were frozen for 10 days. Thawing buds at 0° or above increased the LTE50 about 1°/hr, to −20° to −21°. The LTE50 did not increase above these temperatures until the buds were exposed to 20° for 1-2 days following the completion of rest. During deacclimation, both the LTE50 and temperature range of the low temperature exotherms (LTEs) increased. These changes were accompanied by fluctuations in the capacity of the flower buds to exhibit deep supercooling, expressed as the fraction of an LTE produced per flower primordium (LTE/primordium). Even on days when the LTE/primordium was low, the temperature required to injure 50% of the flower primordia was similar to the LTE50.

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Drought stress was imposed in two `Delicious' apple (Malu×domestica Borkh.) orchards on a sandy loam soil of different soil depths (0.8 and 1.2 m) in the semi-arid environment of central Washington by withholding irrigation all season or from 3, 5, 7, 9, 11, 13, 15, or 17 weeks before harvest. Total pan evaporation was 1005 mm and precipitation was negligible from May through Sept. Soil of the control trees was near field capacity all season, and stem water potential (Ψstem) averaged -1.29 MPa. Total available soil water (TAW) declined after irrigation was terminated for each treatment. As TAW declined to 35%, the TAW that commercial growers are recommended to allow soil to dry to before irrigating, Ψstem was 93% of the controls, fruit growth rate was 97% of the controls, and leaf senescence did not exceed the control trees. As TAW decreased below 30%, leaves senesced acropetally starting with transition leaves near the bud-scale scar. Soil moisture of nonirrigated trees was depleted in July in the orchard on shallow soil and in late August in the orchard on deep soil. Normal June drop was reduced in the driest treatments, but crop load was not affected in the other treatments. There was no difference in drought response between the two rootstocks studied (M.7 and MM.111), but nonspur-type trees exhibited slightly greater symptoms of drought stress than the smaller spur-type trees. A Crop Water Deficit Index (CWDI) based on Ψstem measurements was linearly related to fruit weight at harvest (r 2 = 0.87). All trees were well-watered the following year and yield was reduced only for trees that were severely stressed the previous year.

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`Delicious' apple (Malus domestica Borkh.) trees received regulated deficit irrigation (RDI) early in the growing season to determine if fruit quality and storage life would he altered compared to well-watered trees. Soil moisture and leaf water potential were lower in RDI trees than in those that did not receive RDI most of the season. Internal ethylene concentration increased logarithmically earlier in RDI apples. At harvest, RDI fruit were smaller and had a higher soluble solids concentration (SSC) and lower titratable acidity. Starch degradation was delayed in RDI fruit, and their color was not affected. Firmness was not affected when the effect of size on firmness was removed. The SSC of RDI apples remained higher during storage, but starch content, titratable acidity, firmness, and color were similar.

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A standard fruit growth curve, used commercially as an aid to hand thinning, was compared to periodic volume measurements of apple fruit (Malus domestica Borkh. `Delicious') subjected to early season regulated deficit irrigation (RDI) to determine when to end RDI, which is used to control vegetative growth and save water. RDI suppressed stem water potential, stomatal conductance, and fruit growth rate compared to the trickle- and furrow-irrigated controls, which wetted about one-half and the entire soil volume, respectively. Full irrigation was restored to RDI trees by trickle and microsprinklers, which wetted about one-half and the entire soil volume, respectively, after terminal buds set. Stem water potential, stomatal conductance, and fruit growth rate of RDI trees increased to that of the controls, except for RDI/trickle trees, which had 80% the stomatal conductance of the other treatments. Fruit weight at harvest was affected by an interaction of irrigation treatment and cropload. RDI trees had similar or less vegetative growth and similar or higher yield efficiency than the controls. We recommend ending RDI before fruit growth declines below the standard curve.

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The techniques of task analysis and task allocation were applied to the problem of decision support system development in tree fruit production. The task of midwinter freeze protection of peach and nectarine [Prunus persica (L.) Batsch] flower buds was chosen as the model system. Sixty-five tasks and subtasks were identified as necessary components of the freeze-protection activity at the testing and subsequent management activity levels. Of these, 45 were done exclusively in the orchard where we wished to focus our efforts to benefit the broadest group of growers. Of these 45 tasks or subtasks, 13 were judged suitable for computerization. These 13 tasks were prioritized in order of importance to growers through a self-administered mail survey that asked how often they would use a computer to perform each task. Based on a 77% rate of return, peach and nectarine growers indicated that they were most likely to use a computer to monitor weather forecasts for general weather and freeze alerts, monitor real-time orchard temperatures, and estimate critical temperature ranges for flower-bud damage. This close interaction also produced additional design and use information for the proposed knowledge-based system, such as data presentation requirements, the presence of a variety of farming styles that often determined how the critical temperature data were produced and used, and the challenges of developing suitable validation data for the users.

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There has been an explosion of interest in the development of computer-based Decision Support Systems (DSS) in agriculture. Humans factor, which is the design and evaluation of a system to optimize human and total system performance, offer tools to improve the usefulness of DSS. Task analysis, a formal human factors approach to study human-machine interaction, identifies all of the physical and psychological tasks which must be completed by either the human or the machine in order to meet the various system performance requirements and constraints. Our study focuses on the tasks associated with mid-winter stone fruit freeze protection. Using this technique we have identified work load and output requirements of current critical temperature estimation procedures, additional information needed to improve critical temperature estimates and training needs of fruit industry personnel making critical temperature determinations. This information will be used to produce a requirements specification for a freeze protection DSS.

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