Previous work has shown that daily temperature maxima of 38 °C eliminate N 2 ‐fixing activity in common bean ( Phaseolus vulgaris L.) in less than 3 d. The purpose of this work was to (i) determine if this response is related to diminished supply of C to nodules and (ii) investigate the recovery of nodule function following short‐term heat stress. ‘Jutiapa’ was grown in sand culture at 26/22 °C (day/night) until 16 d after planting, then transferred to 38/22 °C for 6 d. Acetylene reduction activity (ARA) was eliminated at 38 °C, but nodule concentrations of sucrose and glucose at that temperature were equal or higher than at 26 °C throughout the 6‐d period. Stressed nodules and controls had 127 ± 12 and 90 ± 10 mg of starch per g of dry weight, respectively, after 3 d of treatment, which was the largest difference observed. Treated plants tended to have higher levels of sucrose, glucose, and starch in shoots, particularly in stems, than did controls. In a similar experiment, cells of stressed nodules showed loss of cytoplasm and rupture of peribacteroid membranes after 6 d of heat. Recovery of ARA started 1 wk after the removal of the stress; at that point, treated plants and controls had shoot N contents of 30 and 118 mg plant −1 , respectively. This N deficiency caused long‐lasting alterations of plant growth and development. The inhibition of nodule function by high temperature was not related to availability of carbohydrate, but to the breakdown of the bacteria‐infected cells.

The upper temperature limit for nodulation and N2 fixation by common bean (Phaseolus vulgaris L.) is not well defined. Controlled‐environment experiments were conducted to address this issue and to determine the relative importance of shoot and root temperatures. Bean cultivars Redkloud and Jutiapa were grown in sand culture and exposed to day temperatures (13 h d−1) of 26, 32, and 38 °C from sowing or 26, 32, 34, 36, and 38 °C for 5 to 10 days during vegetative growth. In other experiments, day shoot‐root temperatures of 26‐26, 26‐38, 38‐26, and 38‐38 °C were compared. Night temperature was 22 °C in all instances. Transferring plants from a 26 °C to a 38 °C environment resulted in elimination of acetylene reduction activity (ARA) (controls reached over 12 nmol plant−1 s−1), severe N deficiency (55 ± 3 mg shoot‐N plant−1 after 10 d compared with 177 ± 7 mg in controls), and virtual cessation of leaf expansion. Ten days at 32 °C decreased (P ≤ 0.05) shoot‐N content relative to controls. Nodulation was inhibited at 38 °C when treatment was started immediately after inoculation of established plants or seeds but not when it was applied after nodules were initiated. At root temperature of 38 °C, N2 fixation was inhibited regardless of shoot temperature. Conversely, at root temperature of 26 °C, nodulation and shoot‐N accumulation were unaffected even when shoot temperature was 38 °C. It appears that only high soil temperature would constrain N2 fixation by bean; the critical temperature was 32 °C for nodule function and 38 °C for nodulation.