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Bjorn H. Karlsson and Jiwan P. Palta*

Supplemental calcium application has been shown in our previous work to improve tuber quality and reduce internal defects. We evaluated the response under field conditions of five commerically significant cultivars to a combination of calcium nitrate, calcium chloride and urea (168 kg·ha-1 per season) over three seasons. We were able to determine that the cultivar with the greatest response to supplemental calcium for reduced bruising, `Atlantic' had the lowest levels of tuber tissue calcium. Conversely, cultivars with least response to supplemental calcium, `Dark Red Norland' and `Superior', had the highest levels of tuber tissue calcium. `Snowden' was both intermediate in response to calcium and tuber tissue concentration. Based on data for 3 years, we determined that across cultivars the calcium concentration at which tubers no longer respond is ≈250 ppm and ranges for individual years from 195 to 242 ppm. These results suggest that seasonal variation for individual cultivars may affect the tuber need for calcium for reduced bruising. Although the exact mechanism is not known, we believe that calcium supplemented to bulking tubers may lead to improved cell membrane stability, increased wall structure or enhanced ability of tubers to repair following injury. The results of our study show that supplemental calcium fertilization has the ability to significantly reduce the incidence of tuber bruising for several cultivars.

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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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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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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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Beth Ann A. Workmaster and Jiwan P. Palta

Recent work in our laboratory has shown that pre- and postharvest applications of lysophosphotidylethanolamine (LPE) retard senescence processes in several fruit and flower species (apple, tomato, carnation). Banana was selected to develop a rapid bioassay to test the effects of LPE and other substances on various processes associated with senescence. Excised peel pieces from fully yellow `Grand Nain' bananas (Musa AAA) were incubated in petri dishes containing LPE solution (0, 25, 50, and 100 ppm) for 4 days. Fresh weight and ethylene production was measured daily. At the end of the experiment, tissue density, ion leakage, and soluble protein leakage was measured. Ion and soluble protein leakage was significantly lowered with 100 LPE. The 100 ppm LPE also significantly inhibited ethylene production after only 2 hours of treatment and this low level was maintained during the experiment. Peel tissue from the 100 ppm LPE remained firm and intact while tissue from the other treatments expanded and lost integrity. By day 2, peel from the 0, 25, and 50 ppm LPE gained significantly in fresh weight, while tissue treated with 100 ppm initially lost and then only slightly gained in fresh weight. Our results suggest that LPE is able to protect membrane function in senescence. Furthermore, these results provide evidence that LPE may also be retarding senescence by modulating the ethylene pathway.

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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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Sandra E. Vega and Jiwan P. Palta

Previous studies in our laboratory both in pine needles and potato leaves have shown evidence of an increase in 18: 2 (linoleate) in the purified plasma membrane fraction during cold acclimation. This increase was reversible on deacclimation, thereby suggesting a link between the accumulation of 18: 2 and acquisition of freezing tolerance. These studies suggest that the activity of specific desaturases may be modulated during cold acclimation. This study was aimed at studying the possible involvement of stearoyl-ACP desaturase (delta9) in potato cold acclimation response. Our approach was to study the induction of delta9 desaturase at the transcript level by using potato delta9 desaturase gene specific primers and reverse transcriptase. For this purpose, mRNA from S. tuberosum (cold sensitive, unable to acclimate) and S. commersonii (cold tolerant, able to cold acclimate) was extracted before and after acclimation. Sequence analysis confirmed that the amplified band was delta9 desaturase. Our results show that there is an increase in delta9 desaturase gene transcripts during cold acclimation and that this increase is associated with the cold acclimation response in potato. These results together with previous reports on the increase in 18: 2 in the plasma membrane during cold acclimation give more evidence toward the involvement of stearoyl-ACP desaturase (delta9) in the potato cold response.

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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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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.