Overall sales of garden roses have been declining over the past 20 years in the United States due, in part, to the lack of widely adapted cultivars to heat, drought, and salt stress in landscape environments (Byrne et al., 2010). Conversely, an increase in sales of shrub-type rose cultivars e.g., (Rosa ×hybrida L.) Knock Out® rose ‘RADrazz’, which are widely adapted to abiotic stress conditions and biotic stress such as black spot (Diplocarpon rosae Wolf), has occurred (Hutton, 2012).
High temperature stress is a major limiting factor to growing agronomic and horticultural crops worldwide (Wahid et al., 2007). High temperature stress, the rise in temperature beyond a threshold level for a period of time sufficient to cause irreversible damage to plant growth and development (Wahid et al., 2007), is a factor of both intensity and duration of elevated temperature. High temperature tolerance is a plant’s ability to grow and produce an economically viable yield under such conditions (Wahid et al., 2007).
Elevated production temperatures resulted in decreased flower quality of greenhouse-grown cut rose flowers by reducing size (Shin et al., 2001), color (Dela et al., 2003), and postharvest life (Marissen, 2001; Moe, 1975). To our knowledge, little is known about the effect of high temperature stress on garden roses and how to efficiently evaluate high temperature susceptibility on garden roses. Currently, no method, apart from field observations, has been developed for phenotyping high temperature susceptibility in garden roses. At the Texas A&M Rose Breeding Program, landscape performance was quantified on a 1–5 scale. Landscape performance was influenced by the ability of the rose to maintain healthy foliage and perpetual flowering (Aggie Horticulture, 2014).
High temperature susceptibility of plants has been a topic of research as early as the 19th century. Sachs (1864) stressed leaves of various plants in a water bath followed by scoring the extent of necrotic lesions in the days to follow. The photosynthetic apparatus in plants is sensitive to temperature (Berry and Björkman, 1980). High temperature stress in plants is associated with reductions in photosynthetic activities and has been verified for some members of the Rosaceae, including red raspberry (Rubus idaeus L.), by recording the net photosynthetic rate of leaves at different temperatures (Fernandez and Pritts, 1994). Chlorophyll fluorescence is a nonintrusive measurement (Krause and Weis, 1991) used as a physiological parameter, which correlates with thermal tolerance to both high (Camejo et al., 2005; Weng and Lai, 2005; Yamada et al., 1996) and low temperatures (Stoddard et al., 2006).
When light energy enters the cell, and drives photochemistry, it is dissipated as heat, or is reemitted as fluorescence. Chlorophyll fluorescence is usually measured as the ratio of variable fluorescence (Fv) to maximum fluorescence (Fm) (Krause and Weis, 1991). Dark fluorescence (F0) is the fluorescence emitted when all the reaction centers in photosystem II are open. Fm is the fluorescence emitted when all the reaction centers in photosystem II are closed. Fv is the maximum variable fluorescence (Fv = Fm – F0) (Krause and Weis, 1991). The ratio of Fv over Fm is expected to decrease under high temperature stress conditions. The nonintrusive method of measuring CFL makes it a desirable approach to screen large numbers of individuals within breeding populations (Srinivasan et al., 1996).
Bilger et al. (1984) reported a significant correlation (r = 0.87) between the temperature resulting in 50% necrosis and the temperature at which F0 starts to increase. To our knowledge, CFL has not been investigated for high temperature tolerance in roses; however, CFL measured on detached leaves has been successfully used as an indicator of low temperature tolerance among 13 rose genotypes. The slope of Fv reduction among different genotypes accurately grouped genotypes as very resistant, resistant, or sensitive (Hakam et al., 2000). These Fv groupings correlated with previously reported visual scores of necrosis because of chilling injury (CI).
High temperature injury can result in heat-induced loss of the semipermeability of the plasma membrane, the tonoplast, or other membranes within the cell (Berry and Björkman, 1980). Cell MTS makes use of a conductivity test to measure the amount of electrolyte leakage from leaf disks. Cell membrane thermostability has been successfully used as an indicator of high temperature tolerance on field crops such as wheat (Triticum aestivum L.) (Ibrahim and Quick, 2001a), 20 different species of vegetables (Kuo et al., 1993), tomato (Solanum lycopersicum L.) (Camejo et al., 2005), food legumes (Srinivasan et al., 1996) including cowpeas (Vigna unguiculata L.) (Thiaw and Hall, 2004), and ornamental plants such as chrysanthemum [Dendranthema ×grandiflora (Ramat.) Kitam.] (Wang et al., 2008; Yeh and Lin, 2003).
Srinivasan et al. (1996) compared CFL and MTS as methods for phenotyping high temperature tolerance on four different food legumes previously characterized for high temperature tolerance and found both methods to be successful, with the correlation between CFL and MTS ranging between 0.57 and 0.87. Both CFL and MTS were successful in distinguishing between a high temperature tolerant and susceptible tomato line (Camejo et al., 2005). The variables typically employed in evaluating either CFL or MTS as indicators of stress tolerance for various crops are usually based on stress temperature, duration, and the age of plant tissue. These variables, in combination with sound experimental design, must be optimized for each crop and technique before they can be applied effectively.
Currently, no rapid laboratory screening method has been developed for phenotyping high temperature susceptibility in garden roses. The objectives of the study described herein were to develop a rapid screening technique for phenotyping high temperature susceptibility in garden roses and to compare the efficacy of CFL and MTS as indicators of high temperature tolerance. A protocol for rapid screening is presented as well as guidelines on the power of experiments applying the proposed protocol.
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