Effects of planting date and plant density on crop growth of cut chrysanthemum

The effects of planting date (season) and plant density (32, 48 or 64 plants m-2) on growth of cut chrysanthemum (Chrysanthemum (Indicum group)) were investigated in six greenhouse experiments, applying the expolinear growth equation. Final plant fresh and dry mass and number of flowers per plant decreased with plant density and this decrease was larger in summer than in winter. Stem length hardly responded to plant density and total dry mass production per m2 linearly increased with plant density. Plant dry-matter content was not influenced by plant density. Plant dry-matter content, plant fresh and dry mass, number of flowers per plant, stem length and biomass production per m2 were all higher in summer than in winter. Final plant fresh mass (y; g) showed a linear relationship with cumulative incident photosynthetic active radiation (PAR) per plant (x; MJ plant-1) (y = 16.7x 28.0, R2 = 0.97). This relationship implies proportionality between incident light per plant and plant density (at the same final plant fresh mass), showing that plant density effects were primarily mediated through competition for light. Dry-mass production in time could be described accurately by the expolinear growth equation with three regression parameters: maximum relative growth rate (rm; assumed to be independent of plant density), maximum absolute growth rate (cm) and lost time (tb). rm was 2.4 times higher and cm was 4.1 times higher in summer than in winter and no effect of plant density on cm was observed, whereas to decreased linearly with increased plant density. Using these parameters and measured maximum leaf area index (LAI) for calculating dynamic growth patterns of LAI, resulted in large over- or under-estimations, except for summer-grown crops. When an extended expolinear growth function was fitted simultaneously on dry-mass production and LAI in time, accurate time curves for LAI were obtained, whereas dry-mass production was accurately described only for the summer crops. Only under rather constant radiation levels during a cultivation (summer), could accurate descriptions for both dry mass production and LAI increase in time be obtained. Light use efficiency (LUE), the slope of the linear relationship between crop growth and cumulative intercepted PAR, varied between 3.4 g MJ-1 in summer and 5.3 g MJ-1 in winter and LUE slightly increased with plant density