A Morphometric Study of Anatomical Changes During Sunflower Leaf Development under Low Light

Leaves of Helianthus annuus L. were divided into five compartments: palisade, spongy, vascular, and epidermal tissue, and intercellular space. The percentage of whole-leaf volume that each compartment occupied and the relative surface area of compartment cells in contact with intercellular space and adjacent cells for plants grown under low light were measured. All tissue compartments decreased in relative volume during development, while intercellular space increased. In mature leaves all compartments occupied nearly equal fractions of leaf volume. The palisade and spongy tissues occupied an equal volume of the leaf throughout development. Mesophyll cell surface area in contact with adjacent cells relative to total cell volume decreased in mature leaves. The effect was to reduce the width of the apoplastic pathway relative to the volume of tissue it supplied. There was a proportional decrease in the amount of contact between mesophyll and vascular tissue, while the surface area of mesophyll cells and vascular tissue in contact with intercellular space increased. On the basis of structural data, a potential shift in the major path of water movement occurred during development from apoplastic pathways in young leaves to intercellular space in mature leaves.