The effect of temperature and nitrogen interactions on growth and nitrogen assimilation of white clover

Abstract

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot−1) of ammonium (NH 4 + ) or nitrate (NO 3 − ) nitrogen, or received no mineral N and relied on N2 fixation. Greatest growth and total leaf area of clover plants occurred in N2 fixing and NO 3 − -fed plants grown at 20/15°C and poorest growth occurred in NH 4 + -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N2-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO 3 − -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO 3 − -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.