Plant Growth and Development — Class 11 Biology

This chapter deals with growth, differentiation and development in plants and the regulators that control them. Growth is an irreversible, permanent increase in size, measurable as increase in length, area, volume, weight or cell number. Plant growth is generally indeterminate and open because meristems retain the capacity to divide, giving primary growth from apical meristems and secondary growth from lateral meristems. Growth passes through meristematic, elongation and maturation phases, and its rate may be arithmetic (a linear curve) or geometric (an S-shaped sigmoid curve with lag, log and stationary phases). Differentiation is the maturation of meristematic cells into specialised cells; differentiated cells can dedifferentiate to regain division and then redifferentiate. Because differentiation and growth are open, development is flexible, and plants show plasticity such as heterophylly. Development, the sum of growth and differentiation, is controlled by intrinsic factors including plant growth regulators (PGRs) and extrinsic factors such as light and temperature. The five main PGRs are auxins (apical dominance, rooting, herbicides), gibberellins (stem elongation, bolting), cytokinins (cell division, delaying senescence), ethylene (fruit ripening, abscission) and abscisic acid (the stress hormone, inducing dormancy and stomatal closure). PGRs may act synergistically or antagonistically in events such as dormancy, senescence and abscission.