Comparative histological basis of sun and shade leaf dimorphism in Helianthus annuus

Abstract

The development processes which give rise to the histological differences between leaves expanding under full daylight and 25% daylight are described for Helianthus annuus. Generally the pattern of cell division and cell enlargement in shaded leaves lags behind that in unshaded leaves by about 2 days. There is no significant difference in the amount of cell enlargement in the paradermal plane in the epidermis and palisade layers of shaded leaves as compared with leaves expanding in full sunlight. However, cell division in all cell layers occurs at greater rates in unshaded leaves, resulting in greater final leaf area. Cell elongation in both palisade layers is of longer duration and occurs at a greater rate in unshaded leaves and is closely correlated with increase in leaf thickness. The volumes of palisade and spongy mesophyll are significantly greater in unshaded leaves than in shaded leaves, but the relative proportions of each tissue type does not change significantly. However, in shaded leaves there are fewer spongy mesophyll cells per unit area and a greater proportion of intercellular space than in unshaded leaves. Stomata are formed over the same time period in leaves grown at both intensities, but differentiate at greater rates in leaves grown in full sunlight, giving greater final stomatal density. These observations indicate that a sun plant such as Helianthus responds phenotypically to lowered light intensity primarily by a reduction in cell division (resulting in reduced leaf area), and secondarily by modifying cell expansion in a plane perpendicular to paradermal resulting in the characteristic anatomy of shaded leaves.