The Effects of Nitrate Supply and Grain Reserves on Fraction I Protein Level in the First Leaf of Barley

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Abstract
The timing and rate of nitrate application to barley seedlings grown under control and shade conditions can appreciably affect the maximum amount of Fraction I protein attained in the first leaf lamina. In unshaded seedlings early application results in a higher maximum amount of Fraction I protein per lamina, but not per unit lamina fresh weight. Late application of nitrate however delays the age at which Fraction I protein reaches a maximum both in absolute terms and as a proportion of total soluble protein. For both control and shade-grown material early and higher rates of nitrate supply increase the maximum amount of soluble protein in the leaf but not the proportion represented by Fraction I protein. Lower rates of nitrate application have much less effect on first-leaf protein synthesis when application is given late. This is thought to be due to competition from the rapidly developing second leaf. Studies on the soluble protein content of shaded first-leaf laminae have shown that although grain size affects the maximum amount of Fraction I protein attained it does not alter the age at which this is attained; nor is the proportion of the total soluble protein accounted for by Fraction I protein affected by grain size or grain nitrogen content. A model is proposed to explain the contribution made by grain reserves and exogenous carbon and nitrogen supply to the development of the soluble protein content of the first leaf.

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