Relationship of Phloem Size to Leaf Size and Position

Prior studies have shown that phloem cross-sectional area in leaf bases or petioles may influence the rate of export of photosynthetic products from leaves. Since information could not be found on the variability in phloem size due to leaf size and position, an experiment was conducted to study phloem area in the leaf bases of two grass species and in petioles of two dicots. Leaves were removed from several nodes on each stem when the leaf at each node became fully expanded. Tall fescue (Festttca arundinacea Schreb.), sugarcane (Saccharum officinmrum L.), soybean (Glycine max Merr.), and peanut (Arachis hypogaea L.) were grown outside during the summer in full sunlight. Soybean and peanut were also grown under 50% shade. Lamina base or petiole sections were embedded in paraffin, sectioned, and stained for measurements of phloem cross-sectional area under the light microscope. Leaf area increased with leaf position from the stem base toward the top. In peanut, soybean, and tall fescue, however, the distal one or two leaves were smaller than the one just below. Phloem area in the leaf bases or petioles followed a pattern similar to that for leaf area and the two parameters were closely correlated. The increased phloem area in larger leaves was due mainly to an increase in the number of vascular bundles per leaf in tall fescue and sugarcane. In soybean larger leaves had more and larger bundles per petiole and in peanut increased phloem in larger leaves was due to more phloem per bundle, since bundle number was constant at 5. Shading reduced phloem size in soybean even though leaf area was increased. Leaf area was barely affected by shading in peanut, but phloem size was reduced. Leaf weight was reduced to a greater extent than phloem size by shading, so that phloem per unit of leaf weight was increased in peanut and soybean. It is concluded that leaf position, size, and environment have an effect on phloem size that may complicate comparisons of species in studies of photosynthate translocation and anatomy.