Interactive Effects of Duration of Long-day Period and Plant Density on External Quality of Cut Chrysanthemum

To quantify the effect of duration of long-day (LD) period and plant density on several chrysanthemum external quality aspects, a greenhouse experiment was conducted under summer conditions. Chrysanthemum ‘Reagan Improved’ was planted in May and grown under three durations (2, 9 and 16 days) of LD period combined with three plant densities (48, 64 and 80 plants m-2). Plant height linearly increased with duration of LD period. Decreasing LD period from 16 to 2 days resulted in 25% shorter plants, but a marketable height was always reached (> 65 cm). Plant height showed an optimum response to plant density, however this effect was rather small (= 7%). Total number of flowers and flower buds per plant (NoF) increased with increased assimilate supply, measured as total plant fresh mass (TFM), whereas individual flower size (area and dry mass) was not affected. For example, plants that received 16 days of LD period and grown at 48 plants m-2 were the heaviest (91.5 g plant-1) and obtained the highest NoF (28 flowers plant-1). In contrast, 2 days of LD period and 80 plants m-2, resulted in 57% lighter plants with 67% less flowers. A model to predict NoF was developed based on TFM (NoF= 0.353 TFM – 5.66; R2= 0.93). It was concluded that a similar TFM could be obtained using several combinations of plant density and duration of LD, without affecting either NoF or individual flower size. A photosynthesis-driven crop growth model was validated and used to quantify this trade-off, when aiming at a certain TFM. It was shown that such trade-off is dependent on the planting date throughout the year