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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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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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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.

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Beth Ann A. Workmaster and Jiwan P. Palta

`Stevens' cranberry (Vaccinium macrocarpon Ait.) terminal bud freezing stress resistance was assessed by nonlinear regression utilizing relative scoring of the post-thaw bud growth and development based on defined bud stages 2 weeks following controlled freezing tests. Bud stages tested were chosen based on a phenology profile from each sampling date throughout the spring season. Previous year (overwintering) leaf freezing stress resistance was evaluated after both 2 days (injury) and 2 weeks (survival). The Gompertz function with a bootstrapping method was used to estimate the tissues' relative freezing stress resistance as the LT50. Bud injury levels (LT50) were expressed as the temperatures at which the mean potential regrowth capability was impaired by 50%, as compared with the unfrozen controls. In leaves, the LT50 is the temperature at which 50% injury (2-day evaluation) or survival (2-week evaluation) was modeled to occur. Dramatic changes in terminal bud relative freezing stress resistance occurred both within and between the tight and swollen bud stages. These results clearly show that seasonal changes in freezing stress resistance do not necessarily parallel changes in crop phenology and bud development. These results indicate that some physiological, biochemical, or fine anatomical changes may explain the seasonal loss in hardiness within a visual bud stage. Previous year leaves may possess the ability to recover from freeze-induced injury, as leaf survival was found to be the most reliable indicator of cranberry leaf hardiness. Major shifts in phenology and bud and leaf hardiness coincided with the rise of minimum canopy-level air temperatures to above freezing. The nonlinear regression technique utilized made it possible to estimate LT50 with data points comprising half or more of the sigmoidal dose response curve. Our study provides precise and quantitative estimates of the cold hardiness changes in cranberry terminal buds and leaves in spring. From precise estimates we were able to define critical temperatures for the impairment of cranberry bud growth. This is the first systematic study of cranberry terminal bud cold hardiness and spring bud development in relation to changes in the soil and air temperatures under natural conditions. Our study shows that regrowth assessment of the cranberry upright inherently describes the composite effects of freezing stress on plant health.

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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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Zienab F.R. Ahmed and Jiwan P. Palta

Banana is one of the most consumed tropical fruits around the world. It is marketed nearly all year-around and has a relatively short shelf life. Fruits are harvested mature green and treated with ethylene to stimulate ripening before distribution and sale. The fruits generally ripen within 4–5 days after ethylene treatment and sold primarily at yellow stage of ripening. After turning yellow the fruit becomes unsuitable for marketing in 1–3 days. Thus 1–2 days of improvement in the fruit shelf life could enhance the market value of banana. Previous studies conducted in our laboratory have demonstrated that both pre- and postharvest application of lysophosphatidylethanolamine (LPE) can retard aging and improve shelf life of various fruits. The objective of this study was to investigate the possibility of improving shelf life of banana fruit by a postharvest dip in LPE. For this purpose, whole fruits obtained from the market at ripeness stage of 2.5 (about 75% green) were dipped in solution of 500 ppm LPE for 30 minutes and observed for a period of five days at room temperature. Each treatment was applied to 50 uniform fruits. From each hand bought, an equal number of fruits were separated for LPE and control treatments to reduce the variability. Five days after dip treatment, the fruit treated with LPE were firmer and thicker as compared with the control. Starch breakdown was also delayed in the LPE-treated fruits. LPE treatment slowed the development of brown spots on the peel tissue. Both peel and pulp tissue of LPE-treated fruits had lower ion leakage compared with the control. Fruit peel slices treated with LPE for 3 hours also showed a decrease in ion leakage and respiration rate at 24 hours after treatment. Results of the present study indicate that a postharvest dip treatment with LPE may improve shelf life of banana fruit by 1–2 days. These results suggest that LPE may improve shelf life by maintaining membrane integrity, reducing respiration, and slowing the breakdown of starch and cell walls during ripening and senescence of banana fruit tissue.

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Karim M. Farag and Jiwan P. Palta

A natural lipid, lysophosphatidylethanolamine (LPE), was used as a tomato fruit ripening agent. The effect of this compound on hastening the ripening and on the defoliation of the `Heinz 7155' processing tomato and the Glamour fresh-market tomato (Lycopersicon esculentum Mill.) was compared to the effect of ethephon. Vines were sprayed to runoff in the field with a hand sprayer and fruits were harvested 2 weeks or 20 days later in a single harvest operation. LPE (100 mg liter-1) accelerated ripening of both processing and fresh-market tomatoes without defoliation. LPE-treated tomatoes had a better shelf life than the control or ethephon-treated fruit, whether they were harvested at the breaker, pink, or red stage of maturity. The combination of LPE and ethephon (100 mg liter-1) enhanced tomato ripening without damaging the foliage, suggesting that LPE can mitigate the undesirable effects of ethephon on foliage and the fruit. The LPE-related lipid phosphatidyldimethylethanol-amine dipalmitoyl (PDED) also was able to enhance some aspects of keeping quality of tomato fruits, but was not able to enhance fruit ripening. Phosphatidylethanolamine was not as effective as LPE or PDED. It appears that the active molecule of this natural lipid is the lyso form. Our results provide evidence that LPE can enhance tomato fruit ripening and postharvest storage life of vine-ripe fruits and fruits picked at early ripeness stages.

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Björn H. Karlsson and Jiwan P. Palta

Recent studies suggest cold-regulated heat-stable proteins mitigate the potential damaging effects of low water activity associated with freezing. A proposed function of these proteins is stabilization of enzymes during exposure of plants to subzero temperatures. To test this hypothesis for tuber-bearing Solanum L. species we determined the quantitative expression of heat-stable proteins, the qualitative changes in dehydrin proteins, and the capacity of heat-stable proteins to cryoprotect a freeze-thaw labile enzyme lactate dehydrogenase (LDH). We used five tuber-bearing Solanum species (S. tuberosum L. `Red Pontiac', S. acaule Bitter, S. sanctae rosea Hawkes, S. commersonii Dunal, and S. cardiophyllum Bitter), which vary in nonacclimated relative freezing tolerance (NA RFT), acclimated relative freezing tolerance (AC RFT), and acclimation capacity (ACC). The protein fraction containing a mixture of heat-stable proteins demonstrated cryoprotective capacities greater or equal to other cryoprotective compounds (bovine serum albumin, polyethylene glycol, glycerol, and sucrose). Heat-stable proteins extracted from acclimated S. commersonii had superior cryoprotective capacity than those extracted from nonacclimated S. commersonii plants. Interestingly, in the presence of these proteins extracted from acclimated plants (in S. commersonii and S. sanctae rosea), LDH activity was elevated above that of unfrozen controls. No quantitative relationships were found between heat-stable protein concentration and NA RFT, AC RFT, or ACC among the five species. This was also true for dehydrin protein expression. Cold acclimation treatment resulted in increased dehydrin expression for acclimating and nonacclimating species. In three of the cold acclimating species (S. acaule, S. sanctae rosea, and S. commersonii), an increase in dehydrin expression may play a role in increased freezing tolerance during cold acclimation. In the cold sensitive, nonacclimating species (S. tuberosum and S. cardiophyllum), however, an increase in dehydrin level maybe related to the response of these species to changed (perhaps stressful) environment during cold treatment. By exploiting the genetic variation in NA RFT and ACC for five tuber-bearing species, we were able to gain new insight into the complexity of the relationship between heat-stable protein and cold response.

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Karim M. Farag, Jiwan P. Palta, and Elden J. Stang

The application of ethanol for enhancing effectiveness of ethephon under field conditions on cranberry (Vaccinium macrocarpon Ait.) fruit was tested during three seasons (1986 to 1988). The formulation containing ethephon plus the surfactant Tergitol (0.3% or 0.5%, v/v) and ethanol (2.5%, 5%, or 10%) consistently increased anthocyanin content in the fruit by 28% to 54% over the control. In general, fruit size was not affected by the ethephon treatment containing ethanol and Tergitol. The application of ethephon plus surfactant did not increase the anthocyanin content in the fruit. The presence of ethanol in the ethephon and surfactant mixture, however, consistently enhanced the fruit anthocyanin content by 21% to 40% as compared to ethephon plus surfactant. No adverse effect of various treatments on vine growth or appearance was noticed over the three seasons. Chemical name used: (2-chloroethyl) phosphonic acid (ethephon).

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Björn H. Karlsson, Jiwan P. Palta, and Peter M. Crump

Our previous research has provided evidence that in-season calcium applications can increase tuber calcium and improve tuber quality with reduced internal defects. To determine if increasing the tuber calcium concentration also mitigates tuber bruise incidence, five commercially relevant potato (Solanum tuberosum L.) cultivars (`Russet Burbank', `Atlantic', `Snowden', `Superior', and `Dark Red Norland') were grown during three seasons, 1999–2001. Three split applications of a calcium/nitrogen water soluble blend totaling 168 kg·ha–1calcium were made starting at hilling. All plots, including controls, received an equal amount of total nitrogen in a season. Tubers were allowed to be bruised during normal machine harvest standard to commercial production in Wisconsin. Over 100 tubers from each replication (5–10 replications/treatment) were cut and examined for the incidences of bruise and internal brown spot. Paired samples of medullary tissue were taken for measuring calcium concentration. As expected, tuber tissue calcium concentration increased significantly, in all cultivars and in all years, with in-season calcium application. Bruise incidence varied among cultivars and seasons. Although tuber calcium concentration varied among seasons, `Atlantic' and 'Snowden' consistently had the lowest calcium concentration, whereas `Superior' and `Dark Red Norland' consistently had the highest calcium concentration. Meta-analysis of pooled data for three years showed that blackspot bruise incidence was significantly reduced with calcium application in `Atlantic', `Burbank', and `Snowden'. On the other hand, `Dark Red Norland' and `Superior' had low incidence of bruise and were unaffected by calcium applications. Regression analyses of pooled data from all cultivars for three years revealed a significant quadratic relationship between blackspot bruise and tuber tissue calcium as well as between blackspot bruise and internal brown spot. A linear to plateau plot of medullary calcium concentration versus blackspot bruise incidence revealed that bruise incidence is minimized between 200 and 250 μg/kg (dry wt)–1 tuber calcium concentration. To our knowledge, ours is the first study providing evidence for reducing bruise by improving tuber calcium. Variations in the bruise incidences among cultivars generally followed tuber calcium concentration suggesting a genetic control. Given the role of calcium in improved membrane health and enhanced wall structure, and as a modulator of physiological responses, it is not surprising that internal brown spot and bruise incidences are reduced by in-season application to calcium-deficient cultivars.