The Significance of Vascular Connection in Regulating Senescence of the Detached Flag Leaf of Wheat

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Abstract
The relationship between the accumulation of soluble substances and the senescence of a detached flag leaf of wheat in the light was investigated. With respect to transport to and from the leaf, a detached flag leaf can be considered as either an ‘open’ or a ‘closed’ system. A closed system was obtained by cutting the flag leaf at the base of the leaf sheath, thus severing the vascular connection between the leaf and the stem. An open system was prepared by excising the wheat stem, below the flag leaf insertion node, thereby preserving vascular connections between the flag leaf, stem and ear. By varying the number of nodes left on the stem, or having the ear either intact or detached, soluble carbohydrates and soluble nitrogenous compounds were induced to accumulate within the leaf blade at different rates. Treatments which restricted transport of carbohydrate and nitrogen, out of the detached leaf were found to promote senescence. Senescence was most rapid when the leaf system was ‘closed’ and was considerably delayed when the system was ‘open’ and the ear intact. The onset of senescence was closely associated with the attainment of a threshold concentration of ethanol-soluble carbohydrate in the leaf, while the rate of senescence was modified by the number of attached nodes or by exogenous cytokinin treatments.

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