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Senay Ozgen, James S. Busse, and Jiwan P. Palta

The important roles of calcium on plant growth and development including cell division and cell elongation is well documented. The purpose of the present study was to determine the impact of root zone calcium on the growth and health of potato apical meristem and on the maintenance of apical dominance. For this purpose, single-node potato cuttings (Solanum tuberosum L. cv. Dark Red Norland) were grown in sterilized modified Murashige and Skoog (MS) media containing varying concentrations of calcium (1 to 3000 μM). After 13 to 30 d of growth, plantlets were harvested and data for height of the main shoot and for the number of axillary shoots produced were recorded. Plantlets were ashed and tissue calcium concentration was determined. Shoot height was retarded with decreasing concentration of calcium in the media. Calcium deficiency induced shoot tip injury and loss of apical dominance. Tip injury was followed by the development of axillary shoots. The number of axillary shoots increased from one to 21 as calcium concentration in the media decreased from 3000 to 1 μM. At calcium concentration of 1500 μM or higher, there was a single main shoot with no axillary shoots. Addition of ethylene glycol tetra acetic acid (EGTA), a calcium chelator, to the media with 2720 μM calcium (sufficient calcium) resulted in the development of shoot injury and in the formation of axillary shoots. Calcium deficiency injury symptoms were prevented by the addition of a calcium analog, strontium, to MS media deficient in calcium. Strontium has been reported to strongly bind to plant cell walls and the inclusion of strontium prevented injury in shoots of plants grown on calcium-deficient media. These results suggest that strontium is able to mimic the role of calcium in the maintenance of cell wall integrity and supports previous studies that showed that calcium deficiency results from cell wall collapse of the subapical cells.

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Sandra E. Vega, Jiwan P. Palta, and John B. Bamberg

Two major components of frost resistance are freezing tolerance in the nonacclimated state (growing in normal condition) and capacity to cold acclimate (increase in freezing tolerance upon exposure to chilling temperatures). In addition to these two major components, numerous factors contribute to frost survival. Although the rate of cold acclimation and deacclimation have been recognized as important factors contributing to frost survival, very little information about them is available. Our objective was to determine if there is variability in the rate of cold acclimation and deacclimation among tuber-bearing wild potato species: S. acaule Bitter, S. commersonii Dunal, S. megistacrolobum Bitter, S. multidissectum Hawkes, S. polytrichon Rydb., S. sanctae-rosae Hawkes, and S. megistacrolobum subsp. toralapanum (Cárdenas & Hawkes) Giannattasio&Spooner. Relative freezing tolerance of these species was measured after 0, 3, 6, 9 and 12 days of cold acclimation and after 12 and 24 hours deacclimation. Our results showed there were differences in the rates of cold acclimation and deacclimation among these species. With respect to the rate of acclimation we found these species can be divided into four groups: (i) early; (ii) late acclimators; (iii) progressive acclimators, and (iv) nonacclimators. Likewise, a wide range of cold deacclimation behavior was found. Some species showed as low a loss of 20% of their freezing tolerance, others showed as much as >60% loss after 12 hours of deacclimation. Significant deacclimation was observed in all cold acclimating species after 1 day. These results demonstrate that the rates of cold acclimation and deacclimation were not necessarily related to the cold acclimation capacity of a species. Rapid acclimation in response to low temperatures preceding a frost episode and slow deacclimation in response to unseasonably warm daytime temperatures could be advantageous for plants to survive frost events. Thus, in addition to nonacclimated freezing tolerance and acclimation capacity, it would be very desirable to be able to select for rapid acclimation and slow deacclimation abilities. Results demonstrate that variability for these two traits exists in Solanum L. (potato) species.

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Mustafa Ozgen, Jiwan P. Palta, and Stephen B. Ryu

Ethephon [2-(chloroethyl) phoshonic acid] is used widely to maximize the yield of ripe tomato fruits. However, ethephon causes rapid and extensive defoliation, overripening, and promotes sunscald damage to the fruit. Recent studies from our laboratory have provided evidence that lysophosphatidylethanolamine (LPE) can reduce leaf senescence. We investigated the potential use of LPE to reduce damaging effect of ethephon on tomato foliage and influence on the activity of phospholipase D (PLD). Disruption of membrane integrity has been suggested as a primary cause of senescence in plants. PLD is known to be a key enzyme, which initiates the selective degradation of membrane phospholipids in senescing tissues. Two-month-old tomato plants (`Mountain Spring') grown in greenhouse condition were sprayed with water, 200 ppm LPE, and 1000 ppm ethephon. In addition, LPE spray prior to ethephon or mixture with ethephon were also tested. Leaves were sampled after 0, 2, 5, 24, 72, and 168 h of spray application, for PLD activity measurements. Spray of LPE prior to ethephon spray or inclusion of LPE in the ethephon spray reduced foliar injury by ethephon. Activity of soluble PLD was increased dramatically in leaves sprayed with ethephon initially and than dropped by 7 days. We also found that LPE-treated leaves had lower PLD activity than the ethephon-treated leaves. Plants treated with LPE-ethephon mixture also showed significantly lower PLD activity. These results suggest that LPE treatments mitigate ethephon injury to tomato plants. Furthermore, it appears that this mitigation involves modulation of the activity of PLD.

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Rita A. Teutonico, Jiwan P. Palta, and Tom C. Osborn

Identification of the genes involved in freezing tolerance in oilseed Erussica could lead to genetic improvement of winter survival of this crop and other species, as well as provide greater understanding of the basis of cold stress tolerance in plants. We developed a genetic linkage map for B. rapa using restriction fragment length polymorphisms (RFLPs) and identified molecular markers which are linked to genes controlling vernalization requirement and freezing tolerance. We mapped the location of a group of cold-regulated (`cor') genes from Arabidopsis thaliana in this population and determined their association with freezing tolerance and vernalization requirement. We developed genetically fixed, recombinant inbred lines of B. rapa to assay the physiological processes involved in these cold responses. Specifically, we measured the differences in lipid composition of the plasma membranes of acclimated and nonacclimated plants of a subset of this population. We will determine if the genes involved in the physiological responses to cold temperature are also associated with the acquisition of freezing tolerance.

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James S. Busse, Senay Ozgen, and Jiwan P. Palta

Shoot tip necrosis has been attributed to calcium deficiency in in vitro cultures, resulting in death of the stem tip, the loss of apical dominance, and axillary branch development. Using an in vitro shoot culture system with Solanum tuberosum L. cv. Dark Red Norland, we studied the development of injury symptoms at the microscopic and tissue levels at a range of media calcium concentrations varying from 6.8 to 3000 μm. Light and electron microscopic studies revealed that the primary injury due to calcium deficiency was the death and collapse of expanding pith cells below the shoot apex. The structure and organization of the shoot apical meristem was the same when plants were cultured on sufficient or suboptimal media calcium concentrations. However, the apical meristem senesced following subapical shoot tissue collapse. Death of the shoot apical meristem was a secondary effect of calcium deficiency, resulting in loss of apical dominance. Studies with 45Ca indicated that calcium was distributed in a gradient along the shoot, with highest concentration at the base and the lowest at the apex. Shoot tip necrosis developed after 20 days of culture on the suboptimal calcium concentration medium. The development of these symptoms and axillary shoot growth was associated with the lack of calcium accumulation in the shoots. Our results provide evidence that a primary injury of calcium deficiency is localized in the expanding pith cells below the shoot apical meristem and this injury results in the collapse of subapical cells. Death of the shoot apical meristem is a secondary injury resulting from calcium deficiency.

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Beth Ann A. Workmaster, Jiwan P. Palta, and Michael Wisniewski

Infrared video thermography was used to study formation of ice in leaves, stems, and fruit of cranberry (Vaccinium macrocarpon Ait. `Stevens'). Ice formed on the plant surface at -1 or -2 °C by freezing of a droplet of water containing ice nucleation-active bacteria (Pseudomonas syringae van Hall). Samples were then cooled to a minimum of -8 °C. Observations on the initiation and propagation of ice were recorded. Leaves froze only when ice was present on the abaxial surface. Once initiated, ice propagated to the stem and then readily to other leaves. In both unripe and ripe fruit, ice propagation from the stem to the fruit via the pedicel was not observed. Fruit remained supercooled for up to 1 hour after ice was present in the stem. Fruit could only be nucleated when ice was present at the calyx (distal) end. Red (ripe) berries supercooled to colder temperatures and for longer durations than blush (unripe) berries before an apparent intrinsic nucleation event occurred. These observations provide evidence that leaves are nucleated by ice penetration via stomata. The ability of fruit to supercool appears to be related to the presence of barriers to extrinsic ice propagation at both the pedicel and fruit surface. Stomata at the calyx end of the fruit in the remnant nectary area may provide avenues for extrinsic ice nucleation.

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James S. Busse, Senay Ozgen, and Jiwan P. Palta*

Calcium deficiency in the potato shoot results in sub-apical necrosis. This is a physiological condition whereby necrotic lesions form a few millimeters below the shoot apex ultimately causing shoot tip senescence, loss of apical dominance, and the release of axillary buds. Using a Dark Red Norland tissue culture system, we studied the relationship of root zone calcium levels to shoot tip maintenance. Root zone calcium levels lower than 50 ppm resulted in shoot tip death and prolific branching from axillary buds. Chelator studies with EGTA and tracer studies with 45 Ca, indicated a direct involvement of calcium at the shoot tip for shoot tip maintenance. Interestingly, low root zone calcium deficiency syptoms could be mitigated with 0.001 to 0.01 μM of the auxin analog NAA. Developmental studies of calcium deficiency symptoms indicate no anatomical relationship with shoot tip necrosis as xylem conducting elements were found near the shoot apex regardless of the root zone calcium level. These results have important implications for potato shoot development especially during the early development stage from the seed piece.

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Juan M. Quintana, Helen C. Harrison, James Nienhuis, Jiwan P. Palta, and Michael A. Grusak

To assess nutritional potential, pod yield, and Ca concentration of pods and foliage were determined for a snap bean population, which included sixty S1 families plus four commercial varieties. The experimental design was an 8 × 8 double lattice, repeated at two locations (Arlington and Hancock, Wis.). Snap beans were planted in June 1993 and machine harvested in August 1993. Calcium analyses were made using an atomic absorption spectrophotometer. Significant differences were detected in pod Ca concentration and yield among the S1 families. Pod size and Ca concentration were inversely correlated (R 2 = 0.88). Distinct differences between the locations were not observed, and higher Ca genotypes remained high regardless of location or pod size. Low correlation (R 2 = 0.21) between pod and leaf Ca concentration was found. Pods of certain genotypes appeared to have the ability to import Ca more efficiently than others, but this factor was not related to yield.

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Juan M. Quintana, Helen C. Harrison, Jiwan P. Palta, and James Nienhuis

To understand physiological factors associated with genetic differences for pod Ca concentration between snap bean genotypes, flow rate and Ca uptake of sieve sap were measured, as well as pod Ca concentration. Measurements for flow rate and Ca uptake were done at three developmental stages (fl owering and 1 and 3 weeks after) in two commercial snap bean cultivars (Hystyle and Labrador) grown aeroponically. Pods were collected 2 weeks after flowering only. Flow rate and Ca uptake sampling began 4 weeks after transplanting and consisted of: 1) decapitation of the plant at the first node; 2) covering the stem with pre-weighed dry cotton; and 3) removing the cotton, reweighing it, and saving it for Ca determination. Flow rate was defined as the difference in cotton weight (expressed as ml) per 17 hr divided by foliage mass. Ca uptake was defined as mg of Ca per total volume of sieve sap after 17 hr divided by foliage mass. Ca determinations were made using an atomic absorption spectrophotometer. A positive correlation between flow rate and total Ca uptake of sieve sap (R 2 = 0.90), flow rate and pod Ca concentration (R 2 = 0.47), and Ca uptake and pod Ca concentration (R 2 = 0.42) were found. Hystyle reflected 1.5 times more flow rate and pod Ca concentration than Labrador. Significant differences between genotypes for pod Ca concentration, Ca uptake, and flow rate were observed. Results were consistent across developmental stages.

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Juan M. Quintana, Helen C. Harrison, James Nienhuis, and Jiwan P. Palta

We have previously observed significant variation for pod Ca concentration among snap bean genotypes. In the present experiment, we compare pod Ca concentration between snap bean and dry bean genotypes. Eight snap bean cultivars and eight dry bean cultivars were chosen to be evaluated for pod Ca concentration in summers of 1995 and 1996 at Hancock, Wis. The experimental design consisted in randomized complete blocks with three replications in 1995 and six in 1996. Snap and dry beans were planted in June and hand-harvested in August for both experiments. Soil analysis showed 430 ppm of Ca in soil at time of planting. No additional Ca was applied. Plots consisted of 10 plants each. Harvesting was made by collecting a pooled sample of medium size pods from the 10 plants. Ca determinations were made using an atomic absorption spectrophotometer. Data was presented as mg of Ca per gram of dry weight, pooled from both years, and analyzed using SAS. Results reflected significant differences between genotypes. Checkmate (5.5) showed the highest pod calcium concentrations and Labrador (3.9) the lowest among snap beans. G0122 (5.1) resulted in the highest and Porrillo (3.6) the lowest within dry beans Results were consistent across years. Snap beans (4.6) presented significantly higher pod calcium concentration than dry beans (4.2). Apparently, snap bean genotypes have the ability to absorb calcium from the soil more efficiently than dry bean genotypes, and this phenomenon is not significantly influenced by environmental factors.