Ivy geranium (Pelargonium peltatum L.) is an important floriculture crop but it does not tolerate the high temperatures of southeastern U.S. summers. Under heat stress, the newly developing leaves of ivy geranium are partially or completely white, expand less, and cup upward. This physiological disorder is referred to as “bleaching.” Plants may resume developing green leaves when the temperatures moderate. Observational evidence indicated that applying Fe-chelate before bleaching helped reduce the extent of bleaching, which is thought to be Fe-deficiency induced by elevated temperatures (Harvey Lang, Syngenta Flowers, Inc., Boulder, CO, personal communication). Neither the direct cause of this problem nor the role of Fe in bleaching is known.
High root-zone temperatures above 30 °C profoundly affect plant growth by reducing root growth (Mathers, 2003). Roots of bentgrass are more sensitive to high temperatures than shoots and a decline in root growth has been found to precede that of shoots under heat stress (Carrow, 1996; Xu and Huang, 2000). Solanum species screened for heat tolerance indicated considerable variation in the degree of chlorosis developing as a response to high-temperature stress (Reynolds et al., 1990).
High temperatures inhibit photosynthesis (Sharkey et al., 2001) and the response to high temperatures can vary significantly within a species (Reynolds et al., 1990). High temperatures influenced chloroplast development, chlorophyll biosynthesis, and the greening process (Mohanty et al., 2006). Air temperatures of 41.2 °C induced chlorosis in tree fern (Cyathea cooperi) and resulted in reduced chlorophyll (Chl) concentration, reduced Chl a:b ratio, fewer grana, and almost completely absent stroma thylakoids (Doley, 1983).
Iron uptake decreased with increasing root zone temperatures (Raeini-Sartaz and Barthakur, 1995; Tagliavini et al., 1991). Lettuce grown with high ambient root-zone temperatures normally show symptoms of Fe deficiency (He and Lee, 1998a, 1998b). Translocation of Fe within the plant is dependent both on root temperature and the presence of Fe (Schmidt and Steinbach, 2000). Elevated levels of Fe in the nutrient solution increased tissue chlorophyll content in Pelargonium ×hortorum (Lee et al., 1996) and chile peppers (Anchondo et al., 2001). Fe deficiency is accompanied by a decrease in all membrane components, including the electron carriers in the photosynthetic electron transport chain (Spiller and Terry, 1980) and light-harvesting pigments, chlorophylls, and carotenoids (Morales et al., 1990, 1994).
Although considerable research has been conducted on Fe nutrition and heat stress, the role of Fe and heat stress in bleaching of ivy geranium is unknown. The objectives of this study were to determine if elevated root-zone or air temperatures cause bleaching of ivy geranium and to determine the effectiveness of Fe-chelate in preventing bleaching of ivy geraniums.
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