Genotypic Variation in the Response to Suboptimal Temperature at Different Plant Densities in Cut Chrysanthemum

Energy efficiency of greenhouse cut chrysanthemum (Chrysanthemum morifolium Ramat.) may be increased by breeding. In addition to breeding for cultivars with a shorter reaction time at suboptimal temperatures, an alternative approach would be to develop cultivars that are heavier at suboptimal temperatures so that they could be grown at a higher plant density, enhancing the production per unit area. Therefore, the combined effect of temperature and plant density on growth and development of four cut chrysanthemum cultivars was investigated in three greenhouse experiments, carried out in different seasons. For growth-related traits, no interactions between temperature and cultivar were found, limiting the possibilities for breeding. At suboptimal temperatures, growth rate early in the cultivation period decreased as a consequence of a lower light interception resulting from a lower specific leaf area. Thus a higher dry mass production at lower temperature could only be explained by a longer cultivation time. Temperature also influenced external quality, but these effects were cultivar dependent. For instance, temperature affected the slope of the positive linear relationship between total dry mass and number of flowers, reducing number offlowers at low temperature for the same plant dry mass. It is concluded that there are possibilities for breeding for suboptimal temperature-tolerant cultivars. In The Netherlands, the greenhouse industry is an important user of energy, as it accounts for 7% of the national energy use (Oude Lansink and Bezlepkin, 2003). Most of this energy is used during the winter months for heating greenhouses to maintain high production levels year round. Cultivars that are better adapted to lower temperatures could contribute signifi- cantly to a reduction in energy use and consequently in CO2 emission. For breeding of more energy-efficient cultivars, genotypic variation in temperature response is necessary. Cut chrysanthemum is an important ornamental crop grown year round in heated Dutch greenhouses. Lowering the culti- vation temperature results in a longer cultivation period due to a delay in flowering time (Lepage et al., 1984). Therefore, on an annual basis, a reduced number of stems per unit area can be harvested. One focus for the breeding of more energy-efficient chrysanthemum cultivars could be to create cultivars with less or no delay in flowering time at suboptimal temperatures. Within the chrysanthemum assortment, a large variation in temperature response for flowering time has been observed (De Jong, 1978; Van der Ploeg et al., 2007a). Additionally, as some cultivars produce heavier plants at suboptimal temperatures, another strategy is to breed for heavier plants at a suboptimal temperature regime (Van der Ploeg et al., 2007a). This would allow a higher planting density at lower temperatures, thus making up for the increase in cultivation time by producing more stems per unit area. However, this strategy would only be feasible if the growth rate of the crop is not decreased at suboptimal temperatures. Plant growth can be enhanced by increasing the efficiency in which light is converted into dry mass (i.e., light use efficiency (LUE)) and/or by increasing light interception due to a higher leaf area index (LAI). Thus far, little is known about the mechanisms responsible for increasing plant biomass at suboptimal temperatures, which has been observed in some cultivars.