Interaction Effects between Light Level and Plant Density on Plant Growth, Development and External Quality in Year-around Cut Chrysanthemum

Aims of this study are to analyze growth pattern and development of cut chrysanthemum and test simple regression models in relation to light level and plant density. Cut chrysanthemum (Chrysanthemum morifolium), cv. Reagan Improved, was grown at four different plant densities of 16, 32, 64 or 80 plants m-² combined with three different light levels of 100%, 66% or 43% of light in three greenhouse compartments in summer at Wageningen University in the Netherlands. Plant fresh and dry mass, stem strength, number of flowers and stem length had shown interactive effects between light level and plant density. All these parameters increased with increased light level and with increased plant density; however, the decrease as a result of light reduction was much larger at low plant densities except for stem length. Stem length increased with increased plant density, following a saturation-type of response, except for the lowest light level, where an optimum-response was observed, resulting in a reduced stem length at the highest density. Dry mass production per ㎡ greenhouse area and final leaf area index increased with increased light level and with increased plant density. Light use efficiency (LUE, unit dry mass per unit of cumulative intercepted PAR) was 48% higher at lower light levels than 100%. LUE also showed a quadratic response to plant density in range of 16 and 80 plant m-². Using an existing empirical linear model for plant, fresh mass and number of flowers for cut chrysanthemum showed overestimation in plant fresh mass and under estimation for number of flowers at the lowest plant density in all light level in this study. Plant shape, calculated as a ratio between stem length and a length between a node with first lateral stem to top point of plant, linearly decreased with increased LAI. Plant qualities hence strongly varied depending on crop canopy like leaf area index. Although the simple regression models are good tools for summary of experiment, it does not fit at lower plant density. To produce a constant product quality throughout the year, optimal number of plant m-² with amount of available light should be considered through the growing period.