Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. II. Changes in the Weight and Chemical Composition of Single Leaves During Their Growth and Death

Abstract

Small communities of S24 ryegrass were grown under supplementary lights in a glasshouse at 20°C, and abundantly supplied with a complete nutrient solution containing 300 p.p.m. of nitrogen, until they had a leaf area index of 5 and were fully light intercepting. Half were then given a solution containing only 3 p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m. (HN). The HN plants subsequently produced marginally more leaves, which elongated more rapidly to a greater final length and area, on a third more tillers than did the LN plants. Leaves 5, 6 and 7 on the main stem were examined in more detail. In both the HN and the LN plants the d. wts of both laminae and sheaths fell by about 30 per cent between their full expansion and death. Changes in acid extractable carbohydrate (AEC) very largely accounted for the changes in leaf weight, particularly in the LN plants. With increased nitrogen deficiency, AEC contents rose from less than 10 per cent for leaf 5 to peak values of 20 and 45 per cent for the lamina and sheath of leaf 7, as against 10 and 15 per cent in the nitrogen sufficient leaves. Conversely, the nitrogen content of the deficient plants fell from values of 5·8 and 4·8 per cent for the lamina and sheath of leaf 5 to 3·0 and 1·2 per cent for leaf 7. It was striking that while the HN leaves lost nitrogen only when they aged and died, the LN leaves started losing nitrogen before they had reached full expansion—70 per cent of the N initially present was remobilized by the time the leaves were dead. The significance of these findinp to estimates of leaf death and total biomass production in the field, and to our understanding of the achievement of ceiling yield, are discussed.