Muskmelon (Cucumis melo L.) is a high-value horticultural crop. The prediction of node appearance is an important part of simulation models of crops with decumbent growth like muskmelon and other cucurbits. One way to predict node appearance is by using the concept of plastochron, defined as the time interval between the appearance of successive nodes on a stem or vine. Time can be expressed as thermal time, and in this case, the plastochron has units of °C day node-1. Previous studies have demonstrated that the plastochron in muskmelon varies with genotype and planting date. The objective of this study was to estimate the plastochron in muskmelon grown inside a plastic greenhouse at different transplanting dates. Twelve planting dates were performed inside a 10m X 25m greenhouse covered with polyethylene in Santa Maria, RS, Brazil. The HY-MARK hybrid was used. Twelve plants in each planting date were tagged with colored wire. The number of visible nodes (NN) on the main vine of the tagged plants was measured twice a week. Daily degree-days (DDD, °C day) were calculated, with cardinal temperatures for node appearance in muskmelon (10, 34, and 45°C). Accumulated thermal time (TT, °C day) from transplanting was calculated by accumulating DDD. The NN was linearly regressed against TT. The plastochron was estimated by the inverse of the angular coefficient of the linear regression. Plastochron varied among planting dates, from 13.4 to 21.8°C day node-1, with an average of 18.6 (±2.3)°C day node-1. This difference in plastochron values among planting dates may represent several calendar days when NN is used to estimate the date of developmental stages in muskmelon such as flowering of the first male and hermaphrodite flowers.
temperature; thermal time; node appearance; plant development