A theoretical model for the decline in the protein content in plants during growth

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Abstract
SUMMARY: The protein contents of plants decline with increasing size and age even when the plants are adequately supplied with nitrogen. The quantitative nature of this decline needs to be defined for use in models for nitrogen response and to improve diagnostic criteria for detecting nitrogen deficiency. For this reason we have considered the use of photosynthate for the synthesis of the organic constituents within a plant and for maintenance purposes. It has led us to deduce thatIn (P/P0) = (δβ/a) In (F/F0) – βMG(T–T0),which in the special case of exponential growth approximates toIn (P/P0) = (δβ/a) In (F/F0),where P is the critical protein content at time T, F is the total weight of nitrogen-free organic matter at time T, P0 and F0 are the corresponding weights at some initial time T0; δ is a coefficient related to the relative rates of production of protein and the nitrogenfree constituents of plants. β and α. are energy coefficients for the conversion of glucose into protein and nitrogen-free organic matter and MGis the maintenance respiration rate in g of glucose/g of protein/day.The values for β and α depend on the chemical composition of the crops but for forage crops appear to be about 0·62 and 0·76 respectively over a wide range of temperatures. MG is temperature dependent and experimental evidence suggests that it is about 20 mg of glucose/g of protein/day at 15 °C. Less information is available about δ although when there are ample supplies of nitrogen it is probably near unity.