DOES SHADE PROLONG JUVENILE DEVELOPMENT? A MORPHOLOGICAL ANALYSIS OF LEAF SHAPE CHANGES IN CUCURBIT A ARGYROSPERMA SUBSP. SORORIA (CUCURBITACEAE)

Several studies have concluded that shade extends the juvenile phase of plant development based on the prolonged production of juvenile‐looking leaves along the shoot. Until now, the alternative hypothesis that leaves produced in shade converge in shape with more juvenile leaves through plastic responses of individual leaves has not been investigated. The literature has shown that differences in shape among leaves in a heteroblastic series are manifest very early in development, often at or near inception, whereas divergence in development between sun and shade leaves does not become apparent until considerably later. This study is the first to distinguish between these alternatives by comparing the developmental morphology of young leaves of the heteroblastic plant Cucurbita argyrosperma subsp. sororia. Differences in shapes of mature leaves along the shoot in sun and shade were quantified in terms of leaf area, perimeter, and shape using truss analysis. Developmental morphology from initiation through expansion was examined for representative transition and for later (adult) leaves using scanning electron microscopy and allometry. Determinants of shape established very early in development were the same for leaves at the same position grown in sun and shade. Differences in morphology between sun and shade leaves at the same position did not arise until these leaves reached lamina lengths greater than 1,000 μm. Thus, the less‐lobed, more juvenile looking leaf produced at later positions in the shade arose through later developmental responses of individual leaves to shade, rather than through a prolonged phase of juvenile development.