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Karim M. Farag and Jiwan P. Palta

We have demonstrated that postharvest treatment of McIntosh apple fruits with Lysophosphatidylethanolamine (LPE) delays the loss of firmness. In the present study, McIntosh apples were preharvest treated by hand spray to the run off point. Fruits were sprayed on August 28, 1991 and harvested two weeks later. One half of the tree was sprayed with LPE (100 ppm) and the other half was the control. Three trees were used in this study. Periodical samples for starch test, internal CO2 and ethylene, total soluble solids and evolved CO2 and ethylene were taken to monitor the progress of ripening. At harvest, on average, LPE treated apples abscised 26% while the control trees abscised 55%. LPE treated apples colored earlier and had more uniform and intense color than the control. In a related study, we have found the LPE can delay senescence of tomato leaf and fruit tissues. The delay of the abscission of apples by LPE, found in the present study, might be due to the effect of LPE on delaying senescence of cells in abscission zone of apple fruit pedicle. These results suggest that LPE has the potential to substitute for the use of NAA on apples before harvest and at the same time LPE can improve color uniformity and density of McIntosh apples.

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Christopher C. Gunter and Jiwan P. Palta

Tuber tissue calcium has been linked to several potato quality characteristics, including internal defects and the susceptibility of tubers to decay by soft rotting Erwinia species. We were particularly interested in studying the relationship between supplemental calcium fertilization during the seed tuber production cycle to raise the seed piece calcium concentration and the impact on crop performance the following season. The role of seed tuber tissue calcium level on seed piece decay, growth, development, and performance of the plant was evaluated for cultivars Russet Burbank, Dark Red Norland, Atlantic, Superior, and Snowden. This study was performed over four growing seasons. Seed tubers were raised with varying calcium and the following season, individual tubers (over 3,000 total for 4 years of study) were sampled for calcium and hand planted in the field. They were evaluated for seed piece decay and total tuber yield during the growing season. Seed tubers raised with supplemental calcium resulted in significantly higher mean calcium content than the control tubers. In general, calcium-raised seed tubers tended to produce a more vigorous main sprout and higher tuber yield. We also found that there are significant differences among these cultivars for the characteristics measured. Consistently, in all three years, `Atlantic' responded to test conditions with the lowest decay values, and `Dark Red Norland' consistently showed the highest decay values. This suggests that there may be a genetic component involved in these two responses and these genotypic differences could be exploited to improve cultivated potatoes.

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Ahmed A. Tawfik and Jiwan P. Palta

The optimum temperature regime for Solanum tuberosum cv. Russet Burbank is usually 20/15°C day/night. We studied the impact of heat stress (30/25°C, day/night) on the growth of this heat sensitive cultivar under controlled conditions (UW-Biotron). Plants were grown in sandy-loam soil which tested at 1500 Kg/ha Ca. Plants were at the maximum temperature for 6h during the middle of the day with a photoperiod of 14 hrs. All pots received identical amounts of total N (rate: 225 Kg N ha1.). The treatments were: (1) NSN: non-split N (N application 1/2 emergence, 1/2 two wks later): (2) SPN: split-N (1/2 emergence 1/6 at 2, 5 and 8 wks later); (3) SPN+Ca: Split-N+Ca (Ca at 2, 5 and 8 wks after emergence, total Ca from CaNO3 was 113 Kg ha1). Total leaf FWT and DWT was significantly reduced in NS treatment by heat stress at 13 wks as compared to optimum conditions. However, this was not reduced in SPN and SPN+Ca. Under heat stress: (a) SPN + Ca gave the highest leaf FWT and DWT, stomatal conductance, transpiration rate, and leaflet tissue Ca content; (b) Young expanding leaflets gave higher growth rate with SPN and SPN + Ca than NSN; (c) Ca content of mature leaflet decreased progressively in both NSN and SPN but not in SPN + Ca. Our results show that application of Ca and N during heat stress can mitigate stress effects and that maintenance of a certain level of calcium in leaf tissue is important under heat stress.

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Stephen B. Ryu and Jiwan P. Palta

Lipids have been thought to be important largely in membrane structure and energy reserve. It is now evident that lipids and lipid-derived metabolites play a role in many critical cellular processes. Recent studies have shown that membrane lipid-based signaling mediated by phospholipases such as phospholipase A2 (PLA2), phospholipase C (PLC), and phospholipase D (PLD) constitutes a crucial step in plant responses to abiotic and biotic stresses. Phospholipases and their products also play a role during plant growth and development. For example, PLA2-derived lysophospholipids acted as growth regulators that retard senescence of plant tissues. Interestingly, the PLA2 products inhibited the activity of PLD, which has been suggested to be a key enzyme responsible for membrane lipid breakdown leading to plant senescence. Endogenous levels of lysophospholipids, such as lysophosphatidylethanolamine (LPE), could be increased in castor bean leaf discs by the treatment of auxin (50 μM), which is known to be a activator of PLA2. Pretreatment of leaf discs with a PLA2 inhibitor before auxin treatment nullified the auxin effect and rather resulted in accelerated senescence even compared to the nontreated control. Our recent results suggest a potential role of PLA2 products as biologically active molecules mediating hormonal regulation of growth and senescence. One such product LPE is being commercially exploited for retarding senescence and improving shelf life of fruits, vegetables, and cut flowers.

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Laurie S. Weiss and Jiwan P. Palta

At the University of Wisconsin Biotron facility potted plants of S. tuberosum were frozen slowly (cooling rate of 1°C/h) to -2°C. Following thaw, plants were subjected to either high light (400 umol m-2s-1) or low light (100 umol m-2 s-1). High light caused greater damage which appeared as bleaching of the upper leaves in 2 days following thaw. In another study excised paired leaflet halves of S. tuberosum and S. commersonii were subjected to damaging but sublethal freezing temperatures and thawed either fast (on ice) or slowly (1°C/h). Membrane damage (% ion leakage) was about 2x higher at fast thaw as compared to slow thaw in both cold acclimated and non acclimated tissue. There was greater photosynthetic impairment at slow thaw rate than fast in the non acclimated state, but following acclimation fast thaw was more damaging to photosynthetic function. Respiration in general was less sensitive to freeze-thaw stress as compared to photosynthesis and cell membranes.

Our results show that we could benefit from taking into consideration thaw rate and post-thaw light intensity in developing frost protection plans.

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Navjot K. Mangat and Jiwan P. Palta

The pericarp tissue of red mature tomato (Lycopersicon esculentum cv. Gagliano) was used to exctract polygalacturonase (PG) enzyme. The technique for assaying PG activity involves measurement of released reducing groups that were linked together in pectin. Since the crude extract of PG from pericarp will contain considerable reducing groups, we found that repeated washings of the cell wall pulp removed much of the sugars and thus minimized the background absorbance without loss of PG activity. There is an inherent perplexity concerning the selection of blank for PG assay. This is because (i) the enzyme extract contains both the substrate (pectin) and product (free reducing groups) involved in the reaction; (ii) the color development with cyanoacetamide requires heating for 10 min. Thus, even though the reaction is terminated with borate buffer (pH 9.0) the breakdown of pectin continues chemically by heat; (iii) the absorbance from both pectin and enzyme together at zero time termination was always lower than the sum of absorbances from pectin alone and enzyme alone. This suggests that when together in the same tube, the enzyme appears to protect the pectin from physical breakdown during the period of 10 min. boil needed to develop color using the cyanoacetamide. Thus, the most appropriate blank is processing separately the solutions of enzyme alone and substrate pectin alone for color development and then adding the two absorbances. Using this improved assay we found that lysophosphatidylethanolamine (LPE) inhibited tomato PG activity. This inhibition appears to depend on the ripening stage of the fruit. Our results suggest that LPE is able to impart firmness to tomato fruit by reducing the PG activity, which in turn could protect the pectin/middle lamellae from enzymic breakdown. The effects of LPE on PG activity are distinct from those of Triton X-100 and lysophosphatidylcholine.

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Beth Ann A. Workmaster and Jiwan P. Palta

Little is known about the growth and development of the cranberry plant (Vaccinium macrocarpon Ait.) in response to air and soil temperatures in the spring. During this period, marked changes in cranberry bud hardiness are known to occur (from –20 to 0 °C), with the greatest changes occuring before bud elongation. The ability to predict changes in bud phenology and hardiness in relation to thermal time would be useful to growers in making frost management decisions. To establish a working growth model, canopy air and soil temperatures were continuously recorded in 1996, 1997, and 1998 in a cranberry bed (cv. Stevens) in central Wisconsin. In spring, samples of uprights were randomly collected from several locations within the bed and sorted according to a nine stage bud classification from tight bud to bloom. Controlled freezing tests were performed on uprights from the most advanced stages present that constituted 10% or more of a sample on a given date. Heat units were calculated from hourly canopy air temperatures. Despite the varied weather conditions over the3 years, a distinct relationship existed between the accumulation of heat units and the advancement of the crop. Spring 1998 was very early and resulted in the accumulation of more heat units before initial and advanced bud swell was observed compared to the other 2 years. Initial evaluation suggests that soil temperatures between 5 to 10 °C and photoperiod may play a role in modulating the effect of air temperatures. Further refinement of this model and the predictive value for frost hardiness changes will be discussed.

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Ahmed A. Tawfik and Jiwan P. Palta

We have shown that tuber calcium can be enhanced by supplying soluble forms of calcium near the tuber stolon region during bulking. In the present study we applied calcium nitrate or N-HIB during bulking (hilling, 3 and 6 wks after hilling) by injecting these Ca sources into sprinkler irrigation line. Field plots were established with cv `Russet Burbank' in sandy loam soil containing about 1200 Kg Ca ha-1. All plots received equal amounts of nitrogen. Plots receiving only nitrogen (as NH4NO3) at the same time served as split N controls and the plots receiving complete nitrogen by the time of hilling (non-split N) served as the grower control.

In 1990 compared to non-split-N control a consistently higher tuber yield was obtained with split-N, N-HIB (113 Kg.Ca ha-1) calcium nitrate (113 Kg Ca ha-1). However, these differences were not significant. Tuber calcium contents were increased with N-HIB and calcium nitrate. After 5 months of storage the incidence of soft rot and of internal brown spot was significantly reduced. In 1991 N-HIB (113 Kg ha-1) gave significantly higher tuber yield than other treatments. Tuber calcium contents were increased with both calcium nitrate and N-HIB treatments. After four months of storage incidence of internal brown spot was reduced by calcium nitrate and N-HIB although calcium nitrate was most effective. These results demonstrate that it is possible to improve tuber calcium contents by application of 113-226 Kg Ca ha-1 during bulking even in a soil containing sufficient calcium for plant growth.

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Mustafa Özgen, Sookhee Park, and Jiwan P. Palta

Mitigation of ethylene promoted leaf senescence by lysophosphatidylethanolamine (LPE) was studied. Micropropagated `Russet Burbank' potato (Solanum tuberosum L.,) plantlets were grown on MS media in sterile culture tubes. After 2 weeks of growth, tubes were sealed and ethylene gas was applied to obtain 5 nL·L–1 final concentration in the culture tubes. Observations and measurements were taken two weeks after ethylene injection. Potato plantlets treated with ethylene showed severe leaf senescence symptoms such as epinasty, lack of growth, yellowing and axillary shoot formation. These observations indicate that apical dominance has been lost with ethylene treatment. The same experiment was repeated with different concentrations of LPE in the MS medium. Inclusion of 50 or 100 mg·L–1 of LPE in the medium mitigated the damage normally caused by applied ethylene. Leaves of plantlets exposed simultaneously to LPE and ethylene had significantly higher chlorophyll content and more healthy leaves compared to plantlets grown on medium lacking LPE. Results of this study suggest that LPE may have the potential to retard ethylene-promoted leaf senescence and may mitigate ethylene induced loss in apical dominance of micropropagated potato plantlets.

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Matthew D. Kleinhenz and Jiwan P. Palta

Micropropagated `Red Norland' plants were transferred to an inert mixture of 1 perlite: 1 medium-grain quartzite (v/v) and grown 21 days at 20°C day/15°C night on a 25% Hoagland solution without Ca(NO3)2 (Ca at 10 mg·L–1 from CaCl2, N at 35 mg·L–1 from KNO3). Thereafter, Ca treatments (Ca at 0.2, 1, 5, 25, 125 mg·L–1) were imposed for 21 days with other nutrients unchanged. Day/night temperatures were 20/15°C and 35/20°C for control and stress plants, respectively. Continuous drip supply of nutrient solution in excess of demand maintained target rhizospheric Ca levels. All experiments were conducted in controlled-environment chambers with 400-μmol·m–2·s–1 light level. The following results were obtained. 1) Stress, but not control, plants grown with Ca at 0.2 and 1.0 mg·L–1 displayed reduced leaf expansion, extreme senescence, and death of the primary shoot meristem. 2) Plants grown with Ca at 5, 25 and 125 mg·L–1 grew normally under both temperature regimens, although plants responded to temperature with different biomass partitioning. (3) Total root mass at harvest was similar under all Ca–temperature combinations but low-Ca-treated plants had comparatively darker roots with fewer branches. (4) Light microscopic evaluation revealed normal staining patterns of lignified elements in leaves and stems of all plants. These data suggest that constant rhizospheric Ca levels >1 mg·L–1 are required for continued plant growth during exposure to heat stress.