Studies on the Expansion of the Leaf Surface: II. THE INFLUENCE OF LIGHT INTENSITY AND DAYLENGTH

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Abstract
The parts played by constant amounts of visible radiation per day and its two components—daylength and intensity—in influencing the growth of Cucumis sativus have been investigated. The amount of radiation per day had a far greater influence than either of its components per se. Nevertheless, small significant effects of photoperiod were found, leaf expansion and dry weight increase being greatest at daylengths between 10 and 15 hr. rather than with longer days which, with similar daily totals, would be expected to give the greatest amounts of assimilation. Rates of leaf production and appearance were greatest with the highest amounts of radiation, but the rates of expansion of individual leaves and their maximum areas were greatest with intermediate amounts of radiation. This response resulted in an optimum curve relating the leaf surface and the dry weight attained after a given period to radiation. The amount of radiation giving the maximum leaf surface and dry weight decreased with age and with external nutrient supply, but at any one age was higher for increase in dry weight than for increase in leaf surface; stem and root tissues responded more to high radiation than did the leaf surface. The net assimilation rate was a linear function of visible radiation over the range of 15–120 cal. cm.-2 day-1 explored, the highest value of radiation used representing the intensity at which photosynthesis would be expected to be maximal over a 12–15 hr. day. The inhibitory effect of high radiation on leaf expansion and the resultant influence on the growth of the plant are explained in terms of the number and intensity of ‘sinks’ for carbohydrate and mineral nutrients within the plant.

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