Growth adaptation of leaves and internodes of poplar to irradiance, day length and temperature

The adaptation of absolute growth rate of leaves and internodes of Populus euramericana (Dode) Guinier cv. 'Robusta' to irradiance and day length proc eeds by increases in the volume of the apex. Diameter and height of the ape x increase linearly with time, resulting in linear increases in rates of le af initiation and stem height growth that can be described by an accelerati on factor. The acceleration factor is proportional to day length. The relat ionship between the acceleration factor and irradiance is curvilinear and s aturates at irradiances above about 300 W m(-2). Absolute stem height growt h rate is the product of mean relative growth rate in stem height and the l ength of the growing part of the stem. The temperature-dependent growth pat tern of each individual leaf or internode reflects a specific relationship between its relative growth rate and organ age that is independent of irrad iance and plant age; however, it is dependent on day length during the prim ordial phase. The constancy of the growth patterns and the correlation betw een leaf length and leaf initiation rate indicate that growth of primordia is predetermined in the apex, presumably by the precisely structured vascul ar system.