Calcium plays important roles in plant growth and development. Calcium ions are essential for cell wall strength and cell–cell adhesion (Marry et al., 2006; Marschner, 1995). Calcium bound to the outer surface of the plasma membrane maintains membrane stability and cell integrity (Hanson, 1984; Hirschi, 2004; Palta, 1996). Calcium is known to be transported in xylem making transpiration the main force for calcium transport (Busse and Palta, 2006; Clarkson, 1984; Kratzke and Palta, 1986). Thus, calcium deficiency symptoms are observed in tissues with a low transpiration rate including young expanding leaves, enclosed shoot tissues, fruits, underground tubers, and in portions of the plant principally fed by the phloem rather than the xylem (White, 2001; White and Broadley, 2003).
A physiological disorder termed “shoot-tip necrosis” observed in in vitro culture of potato (Solanum tuberosum) has been hypothesized to be a calcium deficiency symptom (McCown and Sellmer, 1987; Sha et al., 1985). This condition is typified by browning and death of the shoot tip, loss of apical dominance, and axillary shoot development in an in vitro shoot culture. Transpiration is limited during in vitro culture by the high humidity that occurs in closed culture vessels. Therefore, uptake and transport of calcium ions, which is dependent on transpiration, is limited during in vitro culture (Williams, 1993). In addition to in vitro potato shoot culture, shoot-tip necrosis resulting from calcium deficiency has been observed in shoot cultures of Pistacia vera (Abousalim and Mantell, 1994). Although these authors were able to show a relationship between shoot-tip necrosis and the medium calcium concentration, they were not able to measure effects at the individual plant level owing to the interplant competition in the culture vessels.
Calcium and strontium are closely related elements and have been shown to have similar behavior in plants (Mengel et al., 2001). Early studies by Queen et al. (1963) suggested that strontium can replace calcium during plant growth. Hutchin and Vaughan (1968) showed that plant uptake and distribution of these two elements is similar but not identical. Myttenaere (1964) found that strontium is deposited in the cell wall to a much greater extent than calcium. Results from these early studies suggest that strontium may be able to mitigate calcium deficiency symptoms, especially in cell wall development. No study to our knowledge has investigated the mitigation of calcium deficiency injury in shoot cultures using strontium.
Recently, we have provided a detailed description of the injury by calcium deficiency at the cellular level in potato shoot cultures (Busse et al., 2008). We demonstrated that primary injury from calcium deficiency is localized in the expanding pith cells below the apical shoot meristem and injury is characterized by the collapse of the walls of subapical cells in potato. These results suggest that strontium may be able to prevent the cell wall collapse in potato shoot cultures grown in calcium-deficient media.
In the present study, we provide further evidence for the role of calcium in shoot tip necrosis by 1) simulating the injury symptoms by including EGTA in the tissue culture media containing sufficient calcium; and 2) alleviating these symptoms by incorporating strontium in calcium-deficient media. By placing a single culture in each vessel, we were able to do studies at an individual plant level without interplant competition.
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