Genetic variation in dry weight of carnation cuttings: its causes and consequences

Summary Carnation cultivars vary considerably in the average weight of their cuttings. Cutting weight shows a high negative correlation with mean relative growth rate. This was initially attributed to differences in age of the cuttings when taken from the stock plants, age being defined as time elapsed since start of visible shoot growth. Shoots on a stock plant are removed as cuttings when they have reached an adequate size and number of leaf pairs. Cuttings from cultivars with a lower rate of growth are removed later, usually resulting in a higher dry weight. This could explain why a high cutting weight is associated with a low relative growth rate.The validity of this tentative explanation is tested in 13 carnation cultivars grown under controlled conditions in two trials, one with plants grown from pinched cuttings and one with plants grown from unpinched cuttings for early flowering (forcing). An analysis of the data for plant weight and leaf pair number at successive stages of development suggests that the primary cause of the negative correlation between cutting weight and mean relative growth rate is not genetic variation in age of the cuttings, but variation in the size of the fully developed axillary bud on the stock plant. Larger buds produce thicker shoots that grow more slowly. Their later harvesting as cuttings only enhances their initial weight advantage, and also the negative correlation with mean relative growth rate.When plants are pinched 20 days after planting, as is normal in a summer-grown crop, the newly developing shoots still show variation in age. This is attributed to variation in the developmental stage of the dormant axillary buds at the time of pinching, causing variation in the time interval between pinching and the start of visible shoot growth.