Alfalfa Root Nodule Distribution and Inhibition of Nitrogen Fixation by Heat¹

Abstract

Greenhouse and field trials were done to test the likelihood that impairment of N fixation contributes to late summer decline in productivity of Medicago sativa L. In greenhouse solution cultures, acetylene reduction and N fixation by nodules were impaired by daily heating of the culture solutions to 32 C, and eliminated by repeated exposures to 36 C or a single exposure to 40 C. However, in a field trial conducted on a Yolo silt loam (Typic Xerorthent, fine silty mixed non‐acid thermic), alfalfa showed no signs of N deficiency and responded little to application of 500 kg NH4NO3/ha although soil temperatures above 32 C were recorded for 5 to 6 hours each day during a 6‐day hot spell following a late summer cut, with peak soil temperatures above 40 C at 2 cm depth. Lack of response in the field trial is explained by observations of temperature profiles and nodule distribution. Only the topmost 5 cm of soil heated above 30 C, and this contained less than 10% of the nodules. Most of the nodules were at depth 10 to 30 cm, and remained at nearly optimal temperatures of 22 C to 27 C. Heat inhibition of nitrogen fixation should not significantly limit growth of alfalfa in the field unless nodule‐formation is constrained to shallow depths.

Selection of thermotolerant bradyrhizobial strains for nodulation of soybean (Glycine max L.) in semi-arid regions of Iran

“…High soil temperatures may affect different aspects of the rhizobium-legume symbiosis including the survival of rhizobia in the rhizosphere (Day et al 1978), the formation of root hairs (Giller and Wilson 1993), binding of the rhizobia to the root hairs (Munevar and Wollum 1981b), the formation of infection threads (Rao 1977), the structure, growth and development of nodules (Hashem et al 1998), leghemoglobin content and nitrogenase activity (Dart and Day 1971;Munns et al 1977;Rainbird et al 1983) and dry matter production (Day et al 1978). The effect of root temperature is very complex and varies with the host plant as well as with the rhizobium (Pate 1961;Lie 1971;Munevar and Wollum 1981b;Hungria and Franco 1993).…”

Soil Anxiety Effects Influencing Symbiotic Microbial Activity in the Soil

“…Soil temperature also greatly influences competition for nodulation. This effect due to a temperature-induced delay in nodulation or the restriction of nodules to the sub-surface region (Munns et al, 1977).…”

Using Machine Learning to Develop a Fully Automated Soybean Nodule Acquisition Pipeline (SNAP)

“…Due to the importance of nodulation in legume crops in terms of crop health and yield, numerous studies have investigated nodule distributions on roots [7][8][9][10][11]. To evaluate these nodule plant relationships, nodule evaluations have traditionally been done with qualitative ratings taken on the entire root, just the tap root, secondary roots, or combinations thereof.…”