The effect of different rates of potassium (K+) on shoot water potential, photosynthesis and carbon movement (using 14C) at the V3/4 growth stages was studied in mungbean (Vigna radiata L. Wilczek), a drought‐susceptible legume, and cowpea (Vigna unguiculata L. Walp), a drought‐tolerant legume, grown under low‐ and high‐irrigation regimes under controlled conditions. Soil moisture and K+ affected all measured parameters in the two species. The rate of photosynthesis was higher at reduced water stress when K+ was applied. The impact was greater in cowpea, which had an inherently high rate of carbon assimilation. Mungbean and to a lesser extent cowpea allocated greater quantities of carbon to roots under dry conditions, especially with added K+. The distribution of 14C into other plant parts was also increased at higher rates of K+ application under both soil moisture regimes. Thus, application of K+ seems to have a beneficial effect in overcoming soil moisture stress and increasing physiological parameters and carbon partitioning in these two important tropical food legumes.

Tropical food legumes are grown in a wide range of environments, and water stress is considered the principal environmental factor limiting growth and yield. Potassium fertilizer mitigates the impact of water stress in plants. However, the benefits of potassium in overcoming stress in tropical food legumes have not been investigated in comparative studies. The purpose of this study was to determine the benefits of potassium in overcoming water stress in mungbean and cowpea, two important tropical food legumes with different adaptabilities to soil moisture regimes. The experiment carried out under controlled conditions placed emphasis on vegetative growth and selected physiological parameters. The impact of potassium was different in the two legumes grown at optimal and suboptimal soil moisture. Potassium increased shoot growth of mungbean to a greater extent than in cowpea under suboptimal moisture conditions. The roots of cowpea showed a greater response to potassium fertilizer than in mungbean under suboptimal soil moisture. The plant water relations and photosynthetic rates of mungbean were improved to a greater extent by potassium under suboptimal soil moisture than those of cowpea. Although differences were observed in the responses of the vegetative growth of these species to moisture and potassium, in overall terms potassium promoted growth of both species when subject to suboptimal soil moisture. While field studies are required to validate the results, the application of potassium fertilizer can be considered a significant factor in overcoming soil moisture stress in these legumes commonly grown in tropical cropping systems.

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Crop establishment, determined by shoot and root growth during early growth is a vital component in procuring desired plant populations and high yields in rainfed tropical smallholder farming systems, where green manures help maintain soil fertility and sustainability. As green manure incorporation could affect early seedling growth, studies were conduced in a plant house using soils from fields, to evaluate the impact of incorporating two popular tropical green manures (Crotalaria juncea and Tithonia diversifolia) into soil under rainfed field conditions over 3 years, on selected physico-chemical properties of the soils and on establishment and early shoot and root growth of maize (Zea mays), with and without inorganic fertilizers. In a given year, green manure was grown in one season while maize was grown in the other. A soil that did not receive green manures but was left fallow during the period of green manure cultivation was used as the control treatment. The incorporation of green manures over the 3 years showed trends of increasing soil physical properties, and also the available nitrogen, phosphorus and potassium contents. Incorporation of Crotalaria, with its higher nitrogen content, promoted shoot growth, while Tithonia induced the development of an extended root system. The use of inorganic fertilizers stimulated the beneficial impact of the green manures in promoting shoot and root growth. The use of green manures, especially Tithonia, promoted the development of seminal and nodal roots of maize seedlings, both in the form of length and thickness (based on root surface area) and root length densities, which could assist in successful crop establishment and resource utilization, rather than primary roots. The benefits of using green manures, especially Tithonia, in promoting root growth and Crotalaria in shoot development of maize seedlings even without inorganic fertilizers is presented.

Extensive branching patterns of roots and the maintenance of adequate water within shoots enables plants to overcome water stress. However, information on the relationship between fertilizer potassium, root branching patterns and shoot water potentials of food legumes grown under different soil moisture regimes is scarce. Thus, an experiment was conducted in a phytotron to ascertain the effect of fertilizer potassium on root branching patterns and shoot water potentials of a popular tropical food legumes (Frenchbeans Phaseolus vulgaris L). The plants were grown in a sand medium with 0.1, 0.8 or 3.0 mM of potassium under a suboptimal and optima) soil moisture regime. Root lengths and dry weights were enhanced by potassium, especially under a suboptimal soil moisture regime. The branching patterns changed due to potassium, where the numbers of second and third order roots increased under both soil moisture regimes, although the impact was greater in plants grown with low soil moisture. Plant water contents measured in terms of shoot water potential, relative water contents, rurgid weight: dry weight ratio and water uptake capacity were also increased by potassium. A positive relationship was observed between root branching patterns and water potentials with increasing potassium levels especially in plants grown under suboptimal soil moisture conditions. Shoot growth and nodulation was also promoted by potassium. The ability of plants to develop a more extensive branching pattern of roots by inducing a greater number of second and third order roots and changing the root branching habit from a herringbone to a dichotomous type to maintain a greater shoot water potential especially under low soil moisture regimes is presented.

SU MMARYSmallholder cropping systems are a vital component in the agricultural sectors of tropical Asia, especially in the developing nations. These systems are important for producing food and providing a livelihood to a major proportion of the populations of Asia. While the rice systems, usually cultivated under puddled conditions are considered stable, the upland or highland units, which provide a range of food, fibre and feed commodities, are generally situated on marginal lands. Low soil fertility, erosion, sub-optimal crop management and subsistence farming conditions characterize these units. Hence, these units require improved but low cost strategies to reverse the trend of lower productivity, loss of sustainability and most importantly, the loss of livelihood for the farming populations. Among the many strategies available, the regeneration of soil fertility through agronomic measures would be a useful and easily adoptable method of enhancing productivity of tropical Asian upland smallholding farming units. The methods available include the revival of age-old traditions such as adding organic matter directly to soils, green manuring and mimicking forest ecosystems through agroforestry. Agronomic measures using plants to enrich the soil and regenerate its supporting capacity are presented in the present paper. Relevant examples are cited to highlight the potential of these methods for regenerating productivity of the upland smallholder farming units and more importantly the agro-based livelihood of a large majority of the populations of tropical Asia.

Potassium (K) is reported to improve plant's resistance against environmental stress. A frequently experienced stress for plants in the tropics is water shortage. It is not known if sufficient K supply would help plants to partially overcome the effects of water stress, especially 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 m M K) on symbiotic nitrogen fixation was evaluated with two legumes under high (field capacity to 25% depletion) and low (less than 50% of field capacity) water regimes. Plants were grown in single pots in silica sand under controlled conditions with 1.5 mM N (15N enriched NH4NO3). The species were faba bean (Viciafaba L.), a temperate, amide producing legume and common bean (Phaseolus vulgaris L.), a tropical, ureide producing species. In both species, 0.1 m M K was insufficient for nodulation at both moisture regimes, although plant growth was observed. The supply of 0.8 or 3.0 m M K allowed nodulation and subsequent nitrogen fixation which appeared to be adequate for respective plant growth. High potassium supply had a positive effect on nitrogen fixation, on shoot and root growth and on water potential in both water regimes. Where nodulation occurred, variations caused by either K or water supply had no consequences on the percentage of nitrogen derived from the symbiosis. The present data indicate that K can apparently alleviate water shortage 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 supply than plants themselves. However, as long as nodulation occurs, N assimilation from the symbiotic source is not selectively affected by K as opposed to N assimilation from fertilizer.

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