SOIL-MOISTURE AND POTASSIUM AFFECT THE PERFORMANCE OF SYMBIOTIC NITROGEN-FIXATION IN FABA BEAN AND COMMON BEAN

Potassium (K) is reported to improve plant's resistance against enviro nmental stress. A frequently experienced stress for plants in the trop ics is water shortage. It is not known if sufficient K supply would he lp plants to partially overcome the effects of water stress, especiall y that of symbiotic nitrogen fixation which is often rather low in the tropics when compared to that of temperate regions. Thus, the impact of three levels of fertilizer potassium (0.1, 0.8 and 3.0 mM K) on sym biotic nitrogen fixation was evaluated with two legumes under high (fi eld capacity to 25% depletion) and low (less than 50% of field capacit y) water regimes. Plants were grown in single pots in silica sand unde r controlled conditions with 1.5 mM N (N-15 enriched NH4NO3). The spec ies were faba bean (Vicia faba L.), a temperate, amide producing legum e and common bean (Phaseolus vulgaris L.), a tropical, ureide producin g species. In both species, 0.1 mM K was insufficient for nodulation a t both moisture regimes, although plant growth was observed. The suppl y of 0.8 or 3.0 mM K allowed nodulation and subsequent nitrogen fixati on which appeared to be adequate for respective plant growth. High pot assium supply had a positive effect on nitrogen fixation, on shoot and root growth and on water potential in both water regimes. Where nodul ation occurred, variations caused by either K or water supply had no c onsequences on the percentage of nitrogen derived from the symbiosis. The present data indicate that K can apparently alleviate water shorta ge to a certain extent. Moreover it is shown that the symbiotic system in both faba bean and common bean is less tolerant to limiting K supp ly than plants themselves. However, as long as nodulation occurs, N as similation from the symbiotic source is not selectively affected by K as opposed to N assimilation from fertilizer.