Leaf nitrate partitioning and growth response in pumpkin as influenced by nitrogen rate

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Abstract
Pumpkin species Cucurbita moschata ''Dickenson Field'' and C. pepo ''Connecticut Field'' were grown in the greenhouse in a Plainfield sand at 8 rates of N applied as Ca(NO3)2. Petiole NO3 concentrations in recently mature and mature leaves were highly responsive to N rate. When plants were stressed for N, translocation of petiole NO3 was primarily to the corresponding blade. The levels as well as the range of NO3 concentration in the leaf blade were lower than those in the petiole. The NO3 content in the leaf blade was slower to react to N stress than that in the petiole. Variability in NO3 concentrations among leaf parts was lowest in the petiole and highest in the blade. For each leaf part, variability in NO3 concentration decreased with leaf age. Critical NO3-N concentrations in C. moschata were estimated at 18,950 and 3500 ppm in mature petioles and 14,700 and 3050 ppm in recently mature petioles at early vegetative and full flower growth stages, respectively.