Nitrogen response surface models of zucchini squash,head lettuce and potato

Plants grown at 5 nitrate (N) levels ranging from 2–36 mmol L^–1 for lettuce and 2–43 mmol L^–1 for zucchini squash and potato were harvested over the growth period to maturity at a minimum of 2 week intervals. Gamma × cubic response surface models fitted actual dry matter growth data quite precisely (R²>0.98) from which growth and dry matter partitioning could be derived.Total dry matter growth was very responsive to N and maximum growth was predicted to occur at an N level of 14.2 mmol L^–1 for lettuce, 18.1 mmol L^–1 for zucchini squash and 11.6 mmol L^–1 for potato. Growth declined at higher N levels for all species. For zucchini squash, both high and low N levels which reduced growth increased partitioning of dry matter to fruit. For potato, partitioning of dry matter to tubers tended to increase at high levels of N, particularly at maturity. For lettuce, the N level producing the highest dry matter yield partitioned the highest ratio of dry matter to head.The highest fresh yields of zucchini squash fruit, lettuce head and potato tubers were recorded at N levels of 14, 5 and 11 and 7 mmol L^–1 respectively. The effect of N on quality followed a similar trend.