Growth Rate and Water Relations of Citrus Leaf Flushes*

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Abstract
Growth rates of seasonal leaf flushes of ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] were measured and water relations characteristics of young (new) and over-wintered (old) citrus leaves were compared. New flush leaves had lower specific leaf weights and lower midday leaf water potentials than comparably exposed old leaves. Spring and summer flush new leaves had higher osmotic potentials than old leaves. These differences became non-significant as the new leaves matured. During summer conditions, water-stressed new leaves reached zero turgor and stomatal conductance also began to decrease in them at higher leaf water potentials than in old leaves. Old leaves were capable of maintaining open stomata at lower leaf water potentials. Opened flowers and new flush leaves lost more water, on a dry weight basis, than flower buds, fruit or mature leaves. The results illustrate differences in leaf water potential and stomatal conductance which can be attributed to the maintenance of leaf turgor by decreases in leaf osmotic potentials as leaves mature. These changes in citrus leaf water relations are especially important since water stress resulting from high water loss rates of new tissues could reduce flowering and fruit set.

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