Practical solutions are described to problems incurred in growing cultivars of cowpea, soyabean, lima bean and pigeon pea under simulated tropical conditions in controlled environments, namely (a) 'Saxcil' growth cabinets in which daylength, day and night temperatures, light intensity and quality, relative humidity and CO 2 concentration are precisely controlled and can be varied over time; (b) compartmentalized glasshouses and adjoining dark compartments where daylength, and day and night temperatures are automatically controlled and night-break illumination can be provided, and (c) heated plastic film houses in which daylength-insensitive tropical legumes can be grown to maturity during the UK summer months since the environment of the wet tropics can be simulated in all respects other than daylength.

S U M M A R YTwo experiments were done in Saxcil growth cabinets in order to investigate the effects of climatic factors and nitrogen nutrition on the growth, reproductive development and seed yield of soyabean cv. TKg. In the first, plants were grown to maturity in eight environments comprising all combinations of two short daylengths (I I h 40 min and 13 h 20 min), two day (27 and 33 "C) and two night (19 and 24 "C) temperatures. In the second, day temperature was kept at 33 "C but the night temperature was varied (19 and 24 "C) as was the mineral nitrogen supply (20 and 197 ppm N) to plants which were either inoculated or not with an effective single strain of Rhizobium.Taller, more branched, later flowering plants were produced in the longer daylength but seed yield was hardly affected because the components of yield did not all respond similarly. I n the higher day temperature treatments seed yield per plant was reduced by half because all yield components were adversely affectedpods per plant by 34 %, mean seed dry weight by 24 % and seeds per pod just slightly. There was a marked effect of the higher night temperature which promoted early vegetative growth, induced early flowering and although the number of pods per plant was, overall, reduced by 48 %, seed yield per plant was little affected as mean seed dry weight was increased by 37 yo and the number of seeds per pod was also increased slightly.Prior to flowering, nodulated plants obtained about two thirds of their total nitrogen requirement via direct uptake and one third through the symbiotic system. Vegetative dry weight and plant nitrogen content were increased by the higher mineral nitrogen level and, although height was slightly diminished, more branches were produced. Seed yield, however, was only slightly increased.These experiments have shown that night temperature is an environmental factor of major importance for the growth of this soyabean cultivar. They have provided, also, a more rational basis for interpreting seasonal variations in growth and seed yield of soyabean in the tropics where, clearly, day and night temperature effects can override those of daylength and nitrogen nutrition.

Effectively nodulated cowpea plants, grown in pots without applied nitrogen, were vegetatively equal to non-nodulated plants supplied with 60 ppm N throughout growth (88 days) and produced significantly greater seed yields. Supplying non-nodulated plants with 120 or 240 ppm N improved seed yields (but not significantly) compared with plants completely dependent on symbiotic fixation. Nodulation promoted branching, and improved pod set and/or retention compared with plants relying on applied N. * One of a series of papers resulting from a collaborative project with the International Institute of Tropical Agriculture, Nigeria; sponsored by the UK Ministry of Overseas Development.Present addresses: 1 ICRISAT, I-I 1-256,

Separate experiments examined nodulation and seed yield of cowpea cv. Prima after (a) changes in the level of combined nitrogen from 25 to 0 or 60 ppm N, (b) cycles of wilting and rehydration, and (c) shading to ca 50% full daylight. Plants were grown ill the simulated tropical environment of a plastics 'bubble' house and experienced these changes over the growth stages: emergence to first flower, first flower to mid pod-fill or mid pod-fill to maturity.Seed yields of nodulated plants were unaffected by combined nitrogen supply and almost double those of non-nodulated plants (100 g cf 56 g per plant) -due mainly to increases in pod number per plant and mean seed weight. Reducing the nitrogen level from 25 to 0 ppm N, especially between mid pod-fill and maturity, reduced seed yields of non-nodulated plants to 36 g per plant. At the first flowering stage, plants grown without combined nitrogen had nitrogenase activities less than 10% of those supplied with 25 ppm N ; 60 ppm N at any stage of development more than halved nitrogenase activity when compared with plants supplied with 25 ppm.Repeated wilting prior to flowering markedly reduced seed yields compared with the unstressed controls (41 g c[ 76 g per plant) -mainly by decreasing subsequent pod production. Nodule weight and nitrogenase activity per plant were also much reduced. Wilting after flowering did not reduce yield, and nitrogenase activity was less affected.Shading throughout, or from first flower onwards, reduced seed yield by about 25% because fewer pods were produced. All shading treatments significantly increased mean seed weight compared with unshaded controls (116-121 mg cf 105 rag).

Assessment of the relative advantages or otherwise of inter-cropping compared with sole cropping needs to be done over a wide range of plant populations. Systematic spacing designs lend themselves to this, and the paper discusses the interpretation of the yield/plant population curves which can be so derived.