Genotypic variation in energy efficiency in greenhouse crops: underlying physiological and morphological parameters

Greenhouse horticulture in The Netherlands is a highly sophisticated form of crop production, resulting in high production levels and good product quality. However, it also requires high energy inputs, representing 15 to 20% of the production costs in most crops. It is important that energy efficiency (amount of product produced per unit energy input) is improved. Breeding for cultivars of greenhouse crops that are better adapted to temperatures below the current economical optimum temperature would contribute significantly to an improvement in energy efficiency. This thesis aims at quantifying and understanding the possibilities for improving energy efficiency of greenhouse crops by plant breeding. Tomato and chrysanthemum are used as model crops. The main emphasis is on exploring possible differences in growth, development and ultimately yield within tomato or chrysanthemum cultivars in response to sub-optimal temperature, but attention is also given to improving yield at optimal temperature. In the present study it was found that there is much more variation for temperature response between chrysanthemum than between tomato cultivars. Therefore it is important that for tomato alternative sources for variation are explored, like wild related species.When breeding for lower temperature demanding cultivars it is important to take into account the reduction in growth at sub-optimal temperature due to a decrease in specific leaf area (SLA). Furthermore, attention should be given to the delay in development at sub-optimal temperatures. Energy efficiency can also be increased by creating higher yielding cultivars. In chrysanthemum the whole above ground plant is harvested and therefore a shift in biomass partitioning might not be profitable unless plant quality is improved, resulting in a higher economical value. In tomato changing partitioning from leaves in favour of the fruits might be possible, if cultivation is adapted such that leaf area index stays high enough. For both crops it is important that light use efficiency (LUE) is further increased. Furthermore, a better light interception early in the cultivation, for example by increasing SLA, can be beneficial for growth of both chrysanthemum and tomato although for tomato, for a large part of the cultivation period light interception can be kept already at a near optimum level with modern cultivation practices.