NITROGEN SUPPLY AND THE CONTROL OF EXPANSIVE GROWTH AND FUNCTION IN LEAVES AND ROOTS

In plants which have acclimatized to limiting supplies of nitrogen (steady-state nutrition), leaf expansion (numbers and sizes of leaves and cells) is under tight control. Over a wide range of nitrogen supplies, the control of leaf growth is associated with a narrow band of photosynthetic rate per leaf area (measured at the growth climate) and, at limiting supplies, a carbon uptake which is in excess of immediate carbon usage in structural growth. For every increment of nitrogen absorbed, root extension is greater at limited nitrate supply, but V-max values (per root dry weight) for nitrate absorption are typically less. However, the capacity of the whole root system for nitrate uptake at limited supply is sufficient to allow for maximum growth, should nitrate supply be increased. It is concluded that a better understanding at the cellular level of the mechanisms which result in a greater inhibition of the expansion of single leaves than of root extension would contribute to an understanding of differences in carbon sink strength among plant organs. This may be a crucial step towards a more physiologically-based appreciation of plant dry matter distribution among organs in plants experiencing different nitrogen supplies.