Adaptation of growth rate of Populus euramericana to light and nitrate proceeds via the vascular system.

Shoots of Populus euramericana (Dode) Guinier cv. 'Robusta' were cultivated in subirrigated gravel culture at irradiances of 7.5 and 30.0 W m~2 and at 3 nitrate dosages in proportionality to irradiance. It is shown that (i) a poplar shoot adapts to a linear nitrate dosage in a similar way as to irradiance, viz. via adaptation of the volume of the growing shoot (GS), (ii) nitrate concentration of the organs, chlorophyll content, nitrate reductase activity of the leaves and shoot-root ratio are mainly a function of the ratio irradiance/nitrate-dosage (I/N-ratio) and (iii) the proposed morphogenic model provides a better basis for comprehension of these effects than the exponential model. It is suggested that the volume of GS is a reflection of the development of the vascular system. The constancy of the optimal I/N-ratio may point to a morphogenetic significance of protein synthesis for the enlargement of the vascular system.