N2Fixation Response to Drought in Common Bean (Phaseolus vulgarisL.)

Nitrogen fixation activity in common bean is generally thought to be low and sensitive to soil drying and, consequently, drought can have important negative effects on N accumulation and yield potential. The objectives of this research were to examine the response of N 2 fixation to drought stress in common bean, and to test the hypothesis that drought sensitivity of N 2 fixation in common bean is linked to ureide levels in the plants. Two glasshouse experiments were conducted to compare the responses of leaf transpiration and acetylene reduction activity (ARA) to soil water contents. ARA decrease during soil dehydration was found to lag behind the decline in transpiration. This indicates that ARA is relatively less sensitive to soil dehydration compared to leaf gas exchange. Further, in comparing two cultivars there was no consistent difference in the relative response of ARA and transpiration to soil drying. The ureide concentrations measured in common bean plants were low, ranging from 0.1 to 1.0 mmol l -1 in xylem sap exudates. Ureide concentrations in the sap exudate varied significantly among the two genotypes even though there was no difference in ARA response to drought. It was concluded that in common bean, the lower sensitivity of N 2 fixation to drought compared to leaf gas exchange could be related to low ureide concentrations in petioles and xylem sap.