A field experiment was conducted on a tropical soil (Humoxic Tropohumult) with a high P sorption capacity to compare the critical external and internal P requirements of soybean (Glycine max (L) Merr.) and cowpea (Vigna unguiculata () Walp.) as affected by the predominant mode of N nutrition during crop growth. The experiment had a split-plot design with two N-level subplots established within each of six Plevel mainplots. Phosphorus treatment ranged from 0.0015 (unamended soil) to 0.08 (1,880 kg P/ha) µg P/ml in 0.01M CaCl 2 solutions equilibrated with soil for 6 days. Nitrogen levels were either deficient (plants primarily dependent on N fixation) or sufficient (N fertilizer supplied at rates sufficient to satisfy the crop N requirement). Nitrogen-fixing soybeans required 750 kg P/ha to obtain a 900/. relative yield which was 320 kg P/ha more than that required by N-supplied plants to obtain a comparable relative yield. The P concentration of N-fixing soybean plants was significantly lower than that of Nsupplied plants at all levels of applied P fertilizer. The external P requirement and tissue P concentration of cowpea were unaffected by soil N level. The data show that cowpea was more tolerant of P stress than soybean, especially when dependent on N fixation. The cowpea cultivar grown without P or N fertilizer yielded 72% of the maximum yield obtained at optimum P levels while the comparable relative yield for the soybean cultivar was 28%. We conclude that (i) some N-fixing grain legumes can make respectable yields with little or no P fertilizer while others might not and, (ii) screening N-fixing grain legumes for tolerance to nutrient stress should be conducted on N-deficient soil to insure that nutritional requirements are assessed for the N-fixing plant, especially on the highly weathered soils of the tropics.

gy reducing nitrate when NO 3 -N is the source of available N. Thus, for legumes able to form symbiotic associations with Rh izobium, the pattern of dry matter distribution (DMD) within the plant will differ depending upon its mode of N nutrition.The partitioning of dry matter between root and shoot is a heritable characteristic determined by the genotype of the plant (Andrews, 1939;Shank, 1943). Root morphology likewise is considered to be genetically determined (Smith, 1934; Zobel, 1975;Street, 1969). The expression of these characteristics can be altered by environmental conditions. Deficiencies of essential mineral nutrients have been shown to affect both the DMD within the plant and lateral root development. Plants deficient in N or P tend to accumulate relatively more dry matter in their roots than do plants which are adequately supplied (Turner, 1922;Brouwer, 1962). Weisum (1958) demonstrated that root branching in pea (Ptsum sattvum L.) was stimulated by nutrients as follows: NO 3 -N>P>K> Mg>Ca. Nitrate applied to a discrete root segment increased both the rate of lateral root extension and number of lateral roots per unit length of root (Hackett, 1972; Mdntyre and Raju, 1967;Drew, 1975).The establishment of an active N-fixing nodule system on the roots of a legume complicates these relationships. During the vegetative stage of growth, active root nodules utilize significant quantities of photosynthate for nodule growth and for N fixation (Minchin and Pate, 1972;Herridge and Pate, 1977). Summerfield et al. (1977) found that the root:shoot dry weight ratio in cowpea (Yigna unguiculata L. Walp.) was larger in nonnodulated plants than in nodulated plants grown at equivalent levels of applied N. Experiments with red clover (Trifolium pratense L.) and barrel medic (Medicago tribuloides Desr.) indicate that there is an inverse relationship between nodule number and lateral root formation (Nutman, 1948;Dart and Pate, 1959). Also, there are qualitative observations concerning differences between the root morphology of grain legumes provided combined N and those which are effectively nodulated (Weber, 1966;Wych and Rains, 1978).The purpose of this study was to define the changes in DMD and root development in the soybean plant as influenced by the mode of N nutrition, the magnitude of root nodulation, and P deficiency. ABSTRACTIn the field, plant root development is of primary importance under P deficient conditions. Two sand culture experiments were conducted to examine the effects of P stress, nodulation, and N source on the growth, dry matter distribution, and root development of "Clark 63" soybean (Glycine max L. Merr.). In both experiments two levels of N (0 and 5.0 mM N) were employed: plants were either solely dependent upon symbiotic N fixation (N-fixing), or primarily dependent upon uptake of combined N from the nutrient solution (N supplied). Nodule dry weight of N-fixing plants grown at the highest P level (2.0 µ/ml) comprised 9%, of total plant dry weight and 61% of root dry weight of 35-dayold soyb...

In order to determine the response of some tropical grass species to low light situations such as under plantation crops, seasonal cloud cover, etc., six tropical forage grasses were evaluated over a 20 month period on an Oxic Haplustoll in Hawaii (100 m above sea level) under four light regimes (100, 70, 45, and 27% daylight using polypropylene netting) in the field. The forage grasses evaluated were: Brachiaria brizantha, B. miliifirmis, Digitaria decumbent, Panicum maximum, Pennisetum clandestinum, and P. purpureum. Dry matter yields of N‐fertilized (365 kg N ha−1 yr−1) grasses were highest at 100 and 70% daylight (16 to 40 metric tons of dry matter (DM) ha−1 yr−1), with P. maximum and P. purpureum having highest yields. Under 27% daylight, yields were 8 to 15 tons, with P. maximum, B. brizantha, and B. miliiformis having highest yields. When no N was added maximum yields in tons ha−1 yr−1 were B. miliiformis, 9.2 at 27% daylight; D. decumbens, P. maximum, and B. brizantha, 13.5 to 15.0 at 45% daylight; and P. clandestinum, 9.2 at 70% daylight. P. purpureum without N yielded 30 tons at full daylight, apparently because its very extensive root system invaded adjacent N‐fertilized plots. Percentage of DM in the forage decreased with shading and N fertilization. Percent N increased with decreasing light intensity (from 1.0 to 1.6% hi the minus‐N and from 1.2 to 1.9% hi the plus‐N treatments). Slight acetylene reduction activity was found in soil cores beneath all species, except B. brizantha, indicating that the plants were almost entirely dependent on soil and fertilizer N for growth. Sward height increased significantly with decreasing light intensity and N fertilization. Concentrations of P, K, Ca, Mg, S, Cu, and Zn tended to be higher in shaded forage, higher in N‐fertilized forage (except for P and Zn), and generally higher during the cool season. Thus under N‐deficient conditions, most yield and forage quality parameters were enhanced under moderate shade. Conversely, the tropical grasses studied generally responded to N fertilization only under conditions of moderate to high solar radiation. Root weight data and observations on the rate of recovery after clipping indicate the shaded pastures would require careful management to avoid excessive depletion of root reserves, either by lenient grazing to maintain high leaf areas or by allowing an extended recovery period.

Decreased solar radiation due to cloud cover or shading by plantation crops or associatedass can severely limit the production of tropical forage legumes. We therefore evaluated the response of six legumes (three replicated and three not replicated) to four radiation regimes (100, 70, 45, and 27% of unshaded solar radiation, hereafter termed "full sun") with polypropylene netting in the field. The three replicated legumes had significant yield reductions at 27% full sun, with intermediate reductions at 70 and 45%. Dry matter (DM) yields at full sun [metric tons (mt) ha-'yr-' and proportional yields at 27%] were: Desmodium intortum cv. Greenleaf (20.0, 46%); Centrosema pubescens 'centro' (13.7, 44%); and Macroptilium atropurpureum cv. Siratro (12.9, 20%). Greenleaf was relatively tolerant of moderate shading; proportional yields at 70 and 45% of full sun were 93 and 75%, respectively. Centro and Siratro yields declined linearly as shortwave radiation (hereafter termed "radiation") decreased, but Siratro yields declined significantly more than Centro. Marked seasonal differences were noted in the response of the legumes to shade, and this was attributed to differences in the ability of the legumes to utilize solar radiation during periods of cool temperatures. The three non-replicated legumes were evaluated similarly, except that yields were adjusted for replication effects. Dry matter yields and proportional yields at 27% full sun were: Leucaena leucocephala cv. Hawaiian Giant (23.5, 40 %); Stylosanthes guianensis cv gr. Schofield (17.0, 17%); and Desmodium canum `kaimi clover' (12.2, 32%). Hawaiian Giant and Greenleaf yielded similarly at 45 and 27% full sun. Kaimi clover DM yield tended to be slightly higher (13.8) at 70% full sun, but thereafter yields declined linearly with reduced radiation. Schofield stylo was the most sensitive to shading. Dry matter and N concentrations were not significantly elevated by reduced radiation or cool weather except that N increased during the cool season. Concentrations of N differed among species being highest for centro and Hawaiian Giant (3.4%) and lowest for kaimi clover (2.6%). Total N yields were associated with DM yields. Nitrogen yields (kg N ha-'yr-') of replicated legumes at full sun and proportional yields at 27% full sun were: Greenleaf (540, 45%), centro (461, 44%), and Siratro (362, 27%); and for the non-replicated legumes, Hawaiian Giant (751, 38%), Schofield (4%,18%), and kaimi clover [340 (361 at 70% full sun), 38%]. Acetylene reduction rates by nodules in soil cores were highly correlated with radiation regimes (r = 0.92-0.995; except for centro, r = 0.71.) Correlation of acetylene reduction rates with DM yields ranged from 0.80 to 0.996. The higher yielding legumes, Greenleaf, centro, and Hawaiian Giant (plus kaimi clover for ≈70% full sun only) appear well-suited for areas of low solar radiation because they have relatively constant concentrations of DM and N and fixed significant quantities of N even under dense shade.