Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

7. Plant Growth and Development
7.5 Leaf Growth and Development

During its development, a leaf goes through four phases: Formation of the primordium; ESTABLISHMENT OF THE leaf axis (petiole); initiation of the leaf blade driven by the marginal meristem; and blade growth via Cell elongation.

Each leaf primordium originates as a small protuberance in the peripheral meristem of the SHOOT apex through localized cell divisions (with the division plane parallel to the apical surface). The primordium of a lateral (axillary) bud appears slightly later. Eventually, it develops an apical meristem homologous to that of the main shoot apex. Leaf primordia exert a signaling influence on the underlying Tissues, inducing the differentiation of vascular bundles. This process is driven by the influx of auxin synthesized within the primordia. The apical Cells of the leaf primordium cone divide intensively, transforming the initial protuberance into a finger-like projection (about 1 mm long). This projection largely consists of cells that will form the midrib and the petiole. At the margins of the midrib zone, the marginal meristem initiates activity, laying the foundation for the leaf blade. Simultaneously, apical leaf growth ceases. After 8–9 rounds of division, the cells of the marginal meristem begin to elongate. Epidermal cells complete division first, yet they undergo the longest elongation phase. Spongy parenchyma cells finish division and growth earlier than other cell types. The division and elongation of the palisade parenchyma, combined with epidermal elongation, cause the spongy parenchyma cells to pull apart, creating large intercellular spaces. Palisade Cell Division and growth halt slightly before epidermal elongation finishes, resulting in minor separations between palisade cells that form small intercellular spaces as well.

The pith meristem of the apex and the procambium—whose formation is induced by growing leaf primordia—constitute the fundamental Tissues of the stem. Emerging from the meristematic zone, cells begin to elongate, driving rapid shoot elongation. This elongation zone spans several centimeters. Growth via elongation is stimulated by Gibberellins and auxin. In dicots, stem thickening results from The activity of the cambium (activated by indole-3-acetic acid derived from the shoot apex) and the cork cambium (phellogen), which originates from various outer layers of the stem. The growth of lateral buds is controlled by the apical bud and the leaves in whose axils the lateral buds reside.

In higher plants, the ROOT apical meristem has a relatively simple Structure, spanning a zone of 1–2 mm. Unlike the shoot apical meristem, it does not produce lateral Organs. The root meristem generates the tissues of both the root proper and the root cap. Along with actively dividing cells, the root meristem contains a population of cells characterized by a low rate of DNA Synthesis AND very slow division. Located between the root cap and the active meristematic zone, this group is known as the quiescent center. It is believed to act as a promeristem that replenishes specialized initial cells as they naturally wear out or become damaged.

One group of initial cells at the distal end of the apex gives rise to the rhizodermis and root cap cells. A second initial Lineage is responsible for generating the cells of the primary cortex. A third lineage sustains the meristematic activity of cells differentiating into vascular bundle tissues. In roots, files of specialized cells can be clearly traced back to their initial cells. Thus, the apical Meristems continue the tissue- and organ-forming activities that were originally established during Embryogenesis.

The hormonal interplay between the dominant centers of the shoot (indole-3-acetic acid) and the root (Cytokinins) appears to serve as a critical endogenous mechanism regulating growth and morphogenesis throughout the entire plant. This is further modulated by the interaction of these dominant centers with leaves, which produce gibberellins and ABA.

The Emergence of quantitative gradients and specific ratios of phytohormones, coupled with the appearance or loss of tissue competence to respond to these Hormones, provides the foundation for the Spatial Organization of Plant Growth and morphogenesis.