Studies on the Expansion of the Leaf Surface: III. THE INFLUENCE OF RADIATION ON CELL DIVISION AND LEAF EXPANSION

Abstract

The numbers of leaves and the areas and cell numbers of leaves from the first five nodes of the cucumber were determined throughout their development with three different amounts of daily radiation and two conditions of nutrient supply. The rate of leaf production was found to be constant with time for any one amount of radiation and to increase with increased radiation. The transition from the rate in darkness to that in light was sharp and occurred more quickly the higher the radiation. Provided the nutrient supply was high the ultimate areas of individual leaves were greater the higher the radiation; if mineral nutrients were depleted the maximum area of a leaf occurred with an intermediate amount of light. This arose because these factors exercised a differential effect on the various phases of growth. Each leaf commenced its life as a mass of dividing cells and the mean rate of division remained constant until it unfolded from the terminal bud. The mean rate of division was much greater at high than at low levels of radiation and was interpreted as being regulated by carbohydrate supply. Although expansion of some cells was likely before unfolding, after this stage there was a marked decrease in the proportion of cells proceeding to division. The duration of division in the leaf after unfolding was independent of radiation; although 70–98 per cent, of the final number of cells were formed after unfolding, the ultimate number was effectively determined by the rate of division prior to unfolding. Low nutrient supply restricted the duration of division in the leaf to a significant extent and the expansion of cells to a very considerable degree. The smaller leaves on plants receiving high rather than medium radiation were due to their cells being much smaller.