A Growth Model for Leaf Lettuce under Greenhouse Environments

Abstract

A growth model for leaf lettuce grown in a greenhouse environment was developed. The experiments were conducted in a plant production facility located at Ibaraki University, Japan from January to November 2003. Each experiment took 37 days, namely 7 days for germination and 30 days after transplanting to a hydroponic system. The combinations of photosynthetic photon flux density of 140 to 200 μmol m⁻² s⁻¹ and CO₂ concentration of 400 and 1200 μmol mol⁻¹ were varied as the experimental treatments. Lettuce weights were measured 2 to 3 times a week, and the weight and number of leaves at harvest, 30 days after transplanting, were measured. A model was developed to fit the growth data using the Gompertz function, which has three coefficients. We found two of the three coefficients can be fixed at certain values within the range of our experimental conditions, and the last coefficient is closely related to fresh weight at harvest. An accurate prediction of lettuce weight at any number of days after transplanting can be achieved. The developed growth model shows a typical growth curve to thirty days as transplants respond to the input environmental conditions. Using this model, growers can identify when their plants will attain the desired target weight based on their greenhouses' environmental settings. This model may be applicable as a tool for planning production schedules and to help management strategy.