Abstract
Abstract With the aim of refining methods of determining plant N status of cotton, we measured nitrate concentrations in petioles and xylem sap in field experiments over three seasons. Main plots were rotation/tillage treatments which were split for fertilizer N. Petiole nitrate concentration was about 30 g N/kg at appearance of flower buds (squares). Rotation and N rate had highly significant effects on petiole nitrate concentration in every season. There was a linear decline in petiole nitrate with stage of development (measured in day degrees, base 12°C), particularly between 600 and 900 day degrees from sowing. The relationship between fertilizer N requirement and petiole nitrate concentration was exponential, with greatest precision when high fertilizer rates were required. The relationship between fertilizer N requirement and the rate of decline in petiole nitrate was linear, giving equal precision over the N fertilizer range. Optimum plant N status was represented by a petiole nitrate concentration...
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