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Recent studies, including those from our laboratory, have provided evidence that by improving tuber Ca level we can reduce tuber internal defects and improve its storability. The purpose of the present study was to determine the influence of supplemental Ca fertilization on tuber size and tuber number. For this purpose, plantlets of Solanum tuberosum cv Russet Burbank raised in tissue culture were planted in 20 L pots filled with sandy loam field soil with the pH of 6.9 and soil Ca level of 350 ppm. All treatments received the same total amount of N (at the rate of 280 kg.ha). Four treatments were evaluated: 1) non-split N (from ammonium nitrate), 2) split N (from ammonium nitrate), 3) split N (from liquid N (50%ammonium nitrate+50% urea)) + calcium chloride, and 4) split N (from calcium nitrate). The total Ca was applied at the rate of 168 kg.ha. Supplemental Ca was applied on a split schedule (equally split at four, six, and eight weeks after planting). Four months after planting, tubers were harvested and evaluated. In general all Ca treatments had lower tuber number and greater tuber size as compared to the nonsplit N control. Although total yield was unaffected the percentage of total tubers (number) as well as the percentage of the total yield from the grade A tubers was increased by all Ca applications. Present study provides evidence that tuberization signal can be influenced by Ca level in the soil. These results suggest that the Ca content of the soil can influence both potato tuber number and tuber size and that by increasing soil Ca one may increase average tuber size and decrease tuber number. We suggest that soil Ca influences tuberization by altering the hormonal balance at the stolon tip. INTRODUCTION Calcium is one of the most abundant elements in soil (Mengel and Kirkby, 1987) and this element is known to play several important roles in plant membrane structure and function (Marschner, 1995; Palta, 1996). Calcium contributes to maintenance of cell membrane stability and wall structure (Marschner, 1995; Palta, 1996). Calcium deficiency has been linked to many disorders especially in fruit and storage organs (Bangerth, 1979) and has been shown to improve quality and yield of these plants. For example, calcium application in the zone of peanut formation can increase the Ca content of peanut fruit and improve peanut yield and grade (Coffelt and Hallock, 1986; Walker and Csinos, 1980). Both peanuts and potatoes are underground storage organs with low rates of transpiration. Low Ca concentration often occurs in organs with low rates of transpiration, such as potato tubers. There are many potato disorders, such as brown center, hollow heart, that were thought to be related to tuber Ca level (Bangerth, 1979). A number of recent studies have shown that the Ca content of tubers may affect their susceptibility to bacterial soft rot caused by Erwinia caratovora (Cother and Cullis, 1992; McGuire and Kelman, 1984; Kleinhenz et al., 1995; Kratzke, 1988) and internal defects of tubers (Kleinhenz et al., 1999; Ozgen et al., 2000; Gunter et al., 2000; Karlsson et al., 2001). Proc. XXVI IHC  Potatoes Healthy Food for Humanity

556 Influence of Supplemental Calcium Fertilization on Potato Tuber Size and Tuber Number