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

In plants which have acclimatized to limiting supplies of nitrogen (st eady-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 photo synthetic rate per leaf area (measured at the growth climate) and, at limiting supplies, a carbon uptake which is in excess of immediate car bon usage in structural growth. For every increment of nitrogen absorb ed, root extension is greater at limited nitrate supply, but V-max val ues (per root dry weight) for nitrate absorption are typically less. H owever, the capacity of the whole root system for nitrate uptake at li mited supply is sufficient to allow for maximum growth, should nitrate supply be increased. It is concluded that a better understanding at t he cellular level of the mechanisms which result in a greater inhibiti on of the expansion of single leaves than of root extension would cont ribute to an understanding of differences in carbon sink strength amon g plant organs. This may be a crucial step towards a more physiologica lly-based appreciation of plant dry matter distribution among organs i n plants experiencing different nitrogen supplies.