Leaf Extension Rate in Temperate Pasture Grasses in Relation to Assimilate Pool in the Extension Zone

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Abstract
In two growth cabinet experiments the leaf extension rate (LER) was studied under a 14 h photoperiod followed by prolonged darkness, in tillers of the perennial temperate pasture grasses Phalaris tuberosa L. cv. Sirosa and Dactylis glomerata L. cv. Currie. Levels of soluble non-structural carbohydrates and total α-amino nitrogen were periodically determined in the extension zone, as well as water, osmotic and turgor potentials in the youngest fully expanded leaves. In the light period, decreases in LER were associated with decreases in water potential in the blade. Turgor potential was positively correlated with the decreases in LER. LER decreased curvilinearly as total soluble sugars decreased in response to prolonged dark period. The fall in LER started 3–4 h after the programmed ‘lights-on’ point. Even when the preceding photoperiod was reduced from 14 h to 9 h, the fall in LER started after at least 1 h beyond the ‘lights-on’ point. The shortening of the preceding photoperiod resulted in decreases in the pool of total soluble sugars in the extension zone by 53% of fresh weight in the cv. Sirosa and 32% in cv. Currie. However, since LER remained unaffected in the initial period of prolonged darkness even after reducing the assimilate pool, it was concluded that the concentration of sugars does not control growth in these pasture grasses.

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