Abstract. Maintenance of realistically low solution P concentrations under controlled conditions is a major difficulty in studies of P nutrition. In this report, we describe a relatively simple and economical sand culture system capable of sustaining plant growth to maturity under controlled yet realistic P regimes. The system uses Al2O3 as a solid‐phase P buffer, and modern process control technology to control irrigation and addition of other mineral nutrients. Aspects of the design, use and potential applications of automated solid‐phase systems are discussed. The system was used to grow Phaseolus vulgaris to matarity at 0.4 mmol m3, 1.0 mmol m3 and 27 mmol m3 P with and without mycorrhizal inoculation. At flowering, low solution P concentrations were associated with reduced leaf concentrations of P in nonmycorrhizal plants, and reduced leaf concentrations of Ca in both mycorrhizal and nonmycorrhizal plants. Mycorrhizal inoculation increased leaf P, K, Mg and Mn concentrations, but reduced leaf N concentration. Low P regimes reduced final seed yield by diminishing both the number of pods per plant and the number of seeds per pod. Mycorrhizal inoculation significantly enhanced seed yield under low P regimes by increasing seed weight, the number of pods per plant, and the number of seeds per pod.

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ABSTRACIRates ofP influx and effiux were determined in whole plants at ambient P co tions compamble to those found in soil solutions. Maize (Zea mays L. var NC+59) seedings were trimnmed (endosperm and adventitions roots removed) and grown in a greenhouse in solution cultures at P concentrtions of approximately.0A and 1.8 micromolar. Roots of intact phlts previously exposed to NP-labeled solutions at 0.2 and 2.0 micromolar P for 48 hours were riosed 10 minutes in P-free solution and exposed to 33P solutions at 0.2 and 2.0 micromolar for 10 minutes. Net depletion of 3P from and appance of 32p in the ambient solution were used to measure influx and efflux. The ration of32P efflux to 3P influx was about 0.68 at 0.2 micromolar and 0.08 at 2.0 micromolar. When plants were allowed to deplete P from solutions, the P concentration in the medium dropped to about 0.15 micromolar within 24 hours and 0.05 micromolar within 60 hours. Results indicate that P efflux is a substantial component ofnet P accumulation at P concentrations normally found in soil solutions.Loss of P from plant roots to the ambient medium has been well documented (9,11,16,26). Conflicting opinions have been advanced, however, on the significance of P efflux in the P economy of plants. Mazel' and Fokin (16) argue that P efflux is strictly a passive exchange phenomenon, and of little consequence for net P accumulation. Bieleski and Ferguson (4) suggest that P efflux is at least partially under metabolic control and is a component of the mechanism whereby P balance is maintained in plants. There is very little information on the magnitude of P efflux in relation to P influx. The limited data available come from studies with aquatic plants (4, 15), in which both roots and shoots absorb ions from the medium. In Spirodela oligorrhiza, the ratio of efflux to influx was approximately 0.08 in plants maintained in 1.0 mm P solutions. Both influx and efflux decreased as the ambient P concentration was lowered, but the decrease in influx was proportionally greater than the decrease in effiux. In plants transferred from 1.0 mM P to 1.0 AM P, the ratio of efflux to influx was 0.70.Indirect evidence for the importance of efflux in P accumulation comes from studies of P depletion from nutrient solutions. If decreasing ambient P concentration affects influx to a greater extent than efflux, then efflux and influx should be equal at some limiting ambient P concentration, i.e. there would be no net accumulation. Experiments with several maize genotypes have indicated that net P accumulation ceases at concentrations ranging from 0.04 to 3.7 ,uM (1, 2 tration range of bulk soil solutions (20). Thus, it appears that efflux may be nearly equal to influx at ambient P concentrations comparable to those experienced by roots of terrestrial plants.In the work with Spirodela (4, 15) influx and efflux were measured in separate experiments. Nitrate influx and efflux, however, were measured simultaneously using 14NO3 and 15NO- (12,14,17). The availability of two radioisotopes of P, 3...

The relationship of incident photosynthetically active radiation (PAR) and mineral nutrient allocation was evaluated in canopies of Borojoa patinoi (Cuatr.) growing in the Chocó rainforest of Colombia, South America. Allocation of P in the canopy was positively correlated with incident PAR, principally because of increased leaf frequency (number of leaves per unit volume of canopy) brought about by local branching, with a smaller contribution from increased specific leaf weight (SLW, leaf dry weight per unit leaf area). The chemical fractionation of P within leaves did not respond to incident PAR. Canopy N allocation also was positively correlated with incident PAR because of increased leaf frequency and SLW. The N partitioning to soluble protein rather than chlorophyll was positively correlated with incident PAR. The allocation of K, Ca, Mg, S, Mn, and Cu also was positively correlated with incident PAR primarily because of increased leaf frequency and secondarily because of increased SLW. The area of individual leaves and the concentration of nutrients in leaf dry weight were not important in determining nutrient allocation responses to incident PAR. Our observations suggest that leaf frequency caused by local branching, followed by changes in SLW, are the primary determinants of canopy nutrient allocation in this tropical fruit tree.