N2 FIXATION AND NITRATE UPTAKE BY WHITE CLOVER SWARDS IN RESPONSE TO ROOT TEMPERATURE IN FLOWING SOLUTION CULTURE

Nodulated white clover plants (Trifolium repens L. cv. Huia) were grown as simulated swards for 71 d in flowing nutrient solutions with roots at 11 °C and shoots at 20/15 °C, day/night, under natural illumination. Root temperatures were then changed to 3, 5, 7, 11, 13, 17 or 25 °C and the total N2, fixation over 21 d was measured in the absence of a supply mineral N. All treatments were subsequently supplied with 10 mmol m-2 NO2 - in the flowing solutions for 14 d, and the relative uptake of N by N2, fixation and NO3 - uptake was compared. Net uptake of K+ was measured on a daily basis.Root temperature had little effect on root d. wt over the 35-d experimental period, but shoot d. wt increased by a factor of 3.5 between 3 and 25 °C, with the sharpest increase occurring at 7-11 °C. Shoot: root d. wt ratios increased from 2·5 to 6·8 with increasing temperature at 7-25 °C. N2-fixation per plant (in the absence of NO2 -) increased with root temperature at 3-13°C, but showed little change above 13 °C. The ratios of N2 fixation: NO2 - uptake over 14 d (mol N: mol N) were 0.47-0.77 at 3-7 °C, 0·92-1·54 at 11-17 °C, and 0·46 at 25 °C, reflecting the dominance of NO3 - uptake over N2 fixation at extremes of high and low root temperature. The total uptake of N varied only slightly at 11-25 -C (0·95-1·10 mmol N plant-1), the decline in N2 fixation as root temperature increased above 11 °C was compensated for by the increase in NO- 3 uptake. The % N in shoot dry matter declined with decreasing root temperature, from 3·2% at 13 °C to 1·5% at 3 °C. In contrast, concentrations of N expressed on a shoot water content basis showed a modest decrease with increasing temperature, from 345 mol m-3 at 3 °C to 290 mol m-3 at 25 °C. © 1990 Annals of Botany Company.