Effects of Shading the First Leaf of Barley Plants on Growth and Carbon Nutrition of the Stem Apex

Effects of shading the first leaf on development of the apical region were investigated by examining the growth of leaf primordia and the apical dome in the early seedling stages. Shade treatment affects the size of the dome; it was shown that values for height, width, and volume of the dome of 12-day control plants were always higher than for shaded plants. Primordial growth, in terms of length and dry weight, was reduced by shade and growth in dry weight of the second, third, and fourth leaves was shown to be dependent on photosynthetic production by the first leaf. Incorporation of 14C in the apical region was detected by autoradiography on day 6 and increased with age. Transfer of assimilated carbon from the first leaf to the apex occurred during the first 3 h after exposure to 14CO2. On a unit dry-weight basis it is shown that the third and fourth leaves and apex incorporated proportionately more labelled carbon than the larger older organs such as the, second leaf. Shade treatment reduced incorporation into the apical region and this is associated with the failure of the apex to grow over the period up to day 15. Evidence is provided to show that in control plants the second leaf supplies carbon to the apex from about day 12. The crucial importance of the contribution of the first leaf to plant development is discussed.