Supplemental Calcium Application Influences Potato Tuber Number and Size

Tuberization in potato is known to be under complex biochemical control involving hormones. A number of studies have provided evidence for a critical role of GA in tuberization. There is also evidence that GA in plants can be modulated by a Ca/calmodulin pathway. The purpose of the present study was to determine the influence of supplemental Ca fertilization on tuber size and tuber number. Plantlets of Solanum tuberosum ‘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 exchangeable soil Ca level of 350 ppm. All treatments received the same total amount of N (equivalent to the rate of 280 kg·ha–1). Four treatments were evaluated: nonsplit N (from ammonium nitrate), split N (from ammonium nitrate), split N+Ca (from calcium nitrate), split N+Ca (50% N from urea, 50% N from ammonium nitrate and Ca from calcium chloride). The total Ca was applied at the rate equivalent to 168 kg·ha–1 on a split schedule (equally split at four, six, eight and ten weeks after planting). Four months after planting tubers were harvested and evaluated. As expected tuber tissue Ca was increased by Ca application from 144 to 245 μg·g–1. In general, the two Ca treatments had significantly lower tuber number per plant as compared to the nonsplit and split N treatments. A plot of mean tuber Ca and tuber number for individual plants showed a significant negative relationship. Both Ca treatments produced tubers with higher mean tuber weight compared to nonsplit N. This increase in tuber size with Ca application was not apparent when compared with split N treatment. These results show that Ca application to soil can decrease tuber number suggesting that soil Ca may influence tuberization in potato. Tuberization in potato (Solanum tuberosum L.) plants is a complex process that is known to be influenced by photoperiod, temperatures, and N nutrition (Batutis and Ewing, 1982; Ewing and Struik, 1992; Ewing and Wareing, 1978; Koda and Okazawa, 1983; Li, 1985; Menzel, 1985; Jackson, 1999; Snyder and Ewing, 1989). For example short days (longer than a critical length of the night period) promote tuberization (Batutis and Ewing, 1982; Ewing and Wareing, 1978; Jackson, 1999) whereas high temperatures and N inhibits tuberization (Li, 1985; Menzel, 1983, 1985). Although the exact mechanism of how environmental and nutritional factors affect tuberization is not known, many studies have implicated the role of plant hormones in modulating tuberization in potato (see for review Ewing, 1995). Among the known hormones, the most convincing case for a critical role in the control of tuberization, has been made for gibberellin (Ewing, 1995; Jackson, 1999; Koda and Okazawa, 1983). High GA level inhibited tuberization and tuberization was promoted by reducing GA level. For example, Jackson and Prat (1996) were able to induce tuberization in long days (otherwise noninducing conditions) by inhibiting GA biosynthesis with ancymidol. Inhibition of tuberization by high temperatures has been linked to enhanced GA level and reduction in inhibitors such as ABA (Menzel, 1980). In follow up studies Menzel (1983, 1985) found that high temperatures promoted GA synthesis in the buds which reduced tuberization. Similarly the inhibition of tuberization by high N was explained in terms of increased GA level (Krauss, 1985; Krauss and Marschner, 1982). A recent study by Xu et al. (1998) also demonstrated a decrease in GA during tuber initiation in vitro cultured single node cuttings. From these studies the authors concluded GA to be a dominant regulator of tuber initiation and growth. There is some evidence indicating the involvement of Ca in tuberization (Balamani et al., 1986). Tuberization was inhibited in a single node leaf cutting by Ca chelator EGTA and Ca ionophore A 23187. Tuberization was restored by including CaCl 2 in the medium. Poovaiah et al. (1996) developed a transgenic plant over expressing PCM1, a potato calmodulin isoform. These plants had reduced tuberization and exhibited a phenotype reminiscent of GA treated potato plants. Other studies on the changes in barley aleurone during germination contain evidence for the modulation of GA by Ca/calmodulin pathway (Bush et al., 1993; Gilroy and Jones, 1993). These studies have provided evidence for a powerful interaction between cytosolic Ca and GA action. Free cytosolic Ca has also been demonstrated to be a major metabolic regulator participating in the signal transduction (Hepler and Wayne, 1985; Marme, 1982; Poovaiah, 1985; Poovaiah and Reddy, 1987). In addition, the role of Ca in the maintenance of membrane integrity and cell wall strength is well established (Clarkson and Hanson, 1980; Marschner, 1995; Palta, 1996). There is also evidence for the presence of Ca-dependent and calmodulin-independent protein kinase which is thought to modulate plant growth and development (Roberts and Harmon, 1992). Thus, it is possible that Ca could regulate tuberization process in potato. In fact a recent review by Jackson (1999) suggested Ca/calmodulin to be a signaling pathway regulating potato tuberization. In the present study we report the influence of root zone Ca on tuberization. Materials and Methods Plants of ‘Russet Burbank’ were raised from micropropagated stem cuttings. For this purpose, single node cuttings were transferred to a sterile MS (Murashige and Skoog, 1962) culture media for 21 d under continuous light with about 60 μmol·m·s photosynthetic photon flux (PPF) from cool white fluorescent lamps (Steffen and Palta, 1986). Uniform rooted and single stemmed micropropagated plantlets were transplanted to 5 × 6 cm plant containers filled with Jiffy Mix (JPA, Chicago, Ill.). Transplants were kept covered with clear plastic for the first 2 d to minimize transplant shock and avoid desiccation. Two weeks after transplanting, plants were transferred to 20-L (30cm-diameter) pots containing 1:1 (by volume) Plainfield loamy sand (sandy, mixed, mesic, Typic Udipsammet) and perlite. The soil for this purpose was collected from the top 15 to 20 cm layer at the University of Wisconsin–Madison Hancock Agricultural Research station. This soil is typical of that commercial production of potatoes in Wisconsin. Soil analyses gave values of 2 meq/100 g cation exchange capacity, 0.7% organic matter, and nutrients (mg·kg soil) 66 (P), 160 (K), 350 (Ca), and 100 (Mg) (Standard soil analysis performed by University of Wisconsin Soil and Forage Laboratory). Based on a plant population of 35,960 plants/ha, the N P K requirement per plant were calculated on individual plant basis. This amounted to 7.81 g of N, 4.3 g of P and 4.3 g of K per plant. For each pot all of the P and K and some of N (1.09 g) was mixed with soil before planting. The experiment was conducted in a greenhouse at the University of Wisconsin-Madison. A randomized complete block design with four nutrient treatments and eight replications per treatment was used. Each replication for every nutrient treatment consisted of one plant. Daily minimum and maximum temperatures were about 20/18 oC during the experimental period. The photoperiod was 14 h with photosynthetically active radiation of 400 to 600 μmol·m·s at the top of the plant canopy from alternating high pressure sodium and metal halide lamps. Two weeks after planting in the 20-L pots 3.12 g of N in the form of ammonium nitrate HORTSCIENCE 40(1):102–105. 2004. Received for publication 11 Dec. 2003. Accepted for publication 3 Mar. 2004. This research was supported by the College of Agriculture and Life Sciences, Univ. of Wisconsin-Madison, and by a grant from the Wisconsin State Potato Industry Board. Graduate research assistant. Campbell-Bascom professor of horticulture. To whom reprint request should be addressed; e-mail [email protected].