N-2 FIXATION RESPONSE TO DROUGHT IN COMMON BEAN (PHASEOLUS-VULGARIS L.)

Nitrogen fixation activity in common bean is generally thought to be l ow and sensitive to soil drying and, consequently, drought can have im portant negative effects on N accumulation and yield potential. The ob jectives of this research were to examine the response of N-2 fixation to drought stress in common bean, and to test the hypothesis that dro ught sensitivity of N-2 fixation in common bean is linked to ureide le vels in the plants. Two glasshouse experiments were conducted to compa re the responses of leaf transpiration and acetylene reduction activit y (ARA) to soil water contents. ARA decrease during soil dehydration w as found to lag behind the decline in transpiration. This indicates th at ARA is relatively less sensitive to soil dehydration compared to le af gas exchange. Further, in comparing two cultivars there was no cons istent 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. U reide 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 o f N-2 fixation to drought compared to leaf gas exchange could be relat ed to low ureide concentrations in petioles and xylem sap. (C) 1998 An nals of Botany Company.