Physical Factors, Hormones, and Differentiation

Abstract

Physical factors may alter cytodifferentiation by regulating hormone activity or availability, by influencing the carbon source required for differentiation, or by modifying some metabolic pathway associated with this developmental process (Roberts 1983). Physical conditions involved in modifying the expression of vascular element formation include temperature, water, light, gases, mechanical stress, and acidity. In most cases the observed modifications result from an environmental stress. The roles of physical factors on xylogenesis in vitro have been summarized (Roberts 1983). Denne and Dodd (1981) reviewed the environmental control of xylem formation in trees. The latter publication emphasizes the effects of climatic conditions on wood fiber diameter and wall thickness. During tree growth a change of environment has a direct effect on the rate of cell expansion, wall thickening in differentiating xylem elements, duration of cytodifferentiation phases, and indirect effects on substrate availability and growth regulator levels (Denne and Dodd 1981).