Studies on the Expansion of the Leaf Surface

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Abstract
An investigation was made of the expansion of the leaf surface of cucumber at temperatures of 12°, 18°, 24°, and 30°C. with two levels of visible radiation (40 and 80 cal. cm.−2 day−1). The relative rate of expansion of the leaf surface increased with temperature up to 24° but was lower at 30° than at 24°. It was slightly greater with the higher than the lower level of radiation at the lower temperatures only. This rate was the resultant of the rate of unfolding of new leaves and the rate of expansion of the component leaves. The rate of leaf production increased with increasing temperature up to 24° and was constant there-after, but leaves unfolded from the terminal bud more rapidly with increase in temperature over the entire range. The rate of expansion of individual leaves was greatest at 24°, being less at both lower and higher temperatures. Differences in this rate between temperatures increased in the order: cotyledon, leaf 1, leaf 2. Leaf production and unfolding was greater with the higher level of radiation but the expansion of individual leaves was not influenced. These results suggested the following interpretation of the expansion the leaf surface. Its potential rate is set by the rate of unfolding of leaves from the terminal bud, which depends mainly on the temperature and the rate of assimilation by the upper leaves and the terminal bud, the demand for assimilate exceeding the supply in this region. The demand for mineral substrates by the terminal bud is low and not influenced by a wide variation in potential supply. After unfolding from the terminal bud, the leaf provides most of its own supply of carbohydrate by assimilation and this can be met at a low level of radiation. Surplus assimilate is diverted to the roots and stems which respond much more to increased radiation than does the leaf surface. The demand for mineral substrates by expanding leaves, however, is high—the greater the number expanding at any time the more likely is the demand by any one leaf to exceed the supply. This leads to a reduction in the number of cell division and, consequently, a reduced rate of expansion and a smaller leaf. The optimum level of any environmental factor is that at which the most effective compromise between these conflicting processes is reached.