High temperatures are a major factor limiting plant growth (Bibi et al., 2008; Xu and Huang, 2008; Zhao et al., 2007). Ivy geranium grows poorly in the heat of southeastern U.S. summers. Optimum temperature for growth of ivy geranium is 20 to 23 °C. Temperatures above 30 °C are quite common during summers in the southeastern states of the United States (Weather Channel, 2008). Elevated air temperatures (heat stress) have been found to cause foliar bleaching in ivy geraniums (Dhir et al., 2011). The newly developing leaves of ivy geranium turn white and lack chlorophyll, are small, and curl upward. Colloquial information and producers’ practices in ivy geranium production indicated heat-induced foliar bleaching could be alleviated using chelated Fe applications.
According to Wahid et al. (2007), heat stress is defined as the rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development. A transient elevation in temperature, usually 10 to 15 °C above ambient, is considered heat stress or heat shock.
Foliar bleaching symptoms resulting from heat stress vary based on species, foliage exposure, and foliage physiological characteristics (Vollenweider and Gunthardt-Goerg, 2005). Heat stress caused leaf yellowing in kentucky bluegrass [(Poa pratensis (He and Huang, 2007)] and rib discoloration in crisphead lettuce [Lactuca sativa (Jenni, 2005)]. Most crops are highly sensitive to heat stress, often resulting in progressively decreasing yields at temperatures above the optimum (Singh et al., 2007). Increased leaf temperatures during the summer are a potential hazard in greenhouses. Low air speed and high humidity during summer can decrease the rate of leaf cooling (Taiz and Zeiger, 2002). Leaf transpiration and internal CO2 concentration of tomato plants increased with high temperatures (Camejo et al., 2005). Maintenance of transpirational (leaf) cooling was an important factor associated with better performance of kentucky bluegrass under summer heat stress (Bonos and Murphy, 1999).
Temperature stress can lead to inhibition of photosynthesis (Haldimann and Feller, 2005; Sharkey et al., 2001). Temperatures above 30 °C reduced photosynthetic rate as a result of a reduction in photosystem II (PSII) efficiency (Kadir et al., 2006). Photosynthetic response to high temperatures can vary significantly within a species (Reynolds et al., 1990). Net photosynthesis of pea (Pisum sativum) plants decreased with increasing leaf temperature from 25 to 45 °C (Haldimann and Feller, 2005). Temperatures from 25 to 38 °C slightly decreased photosynthetic rate of lily (Lilium ×formolongo), but temperatures at 44 °C induced a significant reduction in photosynthetic rate (Luo et al., 2008). Rye (Secale cereale) plants growing at 32 °C were deficient in chlorophyll and chloroplastic 70-S ribosomes (Feieraband, 1977). Chlorophyll biosynthesis of cucumber (Cucumis sativus) seedlings was reduced to 60% on heat stress as a result of impairment of Chl biosynthetic enzymes compared with normal growth conditions (Tewari and Tripathy, 1998). The decrease in the ratio of Chl a to Chl b is an indicator of heat bleaching in Euglena gracilis (Thomas and Ortiz, 1995). In addition, pheophytin, another Chl, acts as an early electron acceptor in PSII and decreases in concentration inhibit PSII and may indicate heat stress in plants (Groot et al., 1997).
Heat-tolerant genotypes of Phaseolus vulgaris displayed differential responses to high temperatures, suggesting different genetic controls of heat tolerance (Rainey and Griffiths, 2005). ‘Acala’ genotypes of cotton (Gossypium hirsutum) were more tolerant to high temperatures than other genotypes (Bibi et al., 2008). Camellia cultivars originating from C. reticulata and its hybrids had the worst heat tolerance, whereas cultivars of C. sasanqua were more heat-tolerant (Li et al., 2006). Solanum species screened for heat tolerance indicated considerable variation in the degree of chlorosis as a stress response to high temperatures (Reynolds et al., 1990). The higher heat tolerance of Salvia splendens ‘Vista Red’ compared with other S. splendens cultivars tested suggested cultivars with thick, broad leaves and higher stomatal frequency had higher transpirational cooling, gas exchange, and CO2 fixation (Natarajan and Kuehny, 2008). Non-photochemical quenching and antioxidant enzymes were the main mechanisms in seedlings of two lily cultivars, which could effectively protect its photosynthetic apparatus against high temperatures (Luo et al., 2008).
Little is known about foliar bleaching in ivy geraniums and cultivar responses to elevated air temperatures. The objective of the present study was to compare the heat stress responses of two cultivars of ivy geraniums.
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