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Sookhee Park and Jiwan P. Palta

High temperature effects potato production by reducing overall growth and partitioning of photosynthate to tubers. Recent studies from our laboratory demonstrated that these effects can be reduced by increasing rhizospheric calcium. This present study was conducted to determine if this mitigation of heat stress effect on potato is due to modulation of heat shock protein by calcium during stress. An inert medium and nutrient delivery system capable of maintaining precise rhizospheric calcium levels were used. Biomass was measured and protein samples were collected from potato leaves. Using electroblotting, heat shock proteins were detected by antibodies to Hsp21 and Hsp70 (obtained from Dr. Elizabeth Vierling). Injury by prolonged heat stress was mitigated at calcium concentration >5 ppm. The calcium concentration of leaf and stem tissues were twice as high in 25 ppm calcium-treated plant compared to 1 ppm calcium-treated plants. Total foliage fresh weight was 33% higher and dry weight 20% higher in plants supplied with 25 ppm of calcium than supplied with 1 ppm of calcium. HSP21 was expressed only at high temperature and at greater concentrations in 25 ppm calcium treatment. HSP70 was expressed in both control, 20 °C/15 °C (day/night) and heat-stressed tissue, 35 °C/25 °C (day/night) under various calcium treatments (1 to 25 ppm). Also, there were some differences in HSPs expression patterns between young and mature leaves. Young tissue responded immediately to the heat stress and started to express HSP21 within 1 day. Mature tissue started to express HSP21 after 2 days. HSP21 of young tissue disappeared sooner than mature tissue when heat stress-treated plants were returned to normal conditions. These results support our earlier studies indicating that an increase in rhizospheric calcium mitigate heat stress effects on the potato plant. Furthermore these results suggest that this mitigation may be due to modulation of HSP21by rhizospheric calcium during heat stress.

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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.