Roots regulate ion transport in the rhizosphere to counteract reduced mobility in dry soil

Diffusion of ions in the soil depends on soil moisture content. In a dry soil, transport of nutrients towards the root and the concomitant uptake could be reduced. However, pot and field experiments showed that this is not always the case. The objective of this paper was to investigate possible mechanisms of plants to counteract reduced nutrient supply due to water shortage. A split root system was used to investigate P and K inflow of oat and sugar beet at different soil moisture contents (Θ) without water shortage for the plant. The measured average P and K inflows were compared to model calculations considering diffusion, mass-flow, sorption and uptake processes. In the calculations, soil dryness impeded diffusion and decreased nutrient inflow as expected. Measured K inflow was decreased in a similar way indicating that Θ influences K diffusion. In contrast to this, measured P inflow was not influenced by Θ and under-estimated by the model. Low and high molecular exudates were collected at different water supply levels showing that exudation rate of both compounds was increased at water shortage. Especially the high molecular exudates (i.e. mainly mucilage) from water-stressed plants increased P concentration in soil solution under dry conditions in an incubation experiment. Calculated inflow considering this increased P concentration agreed well with measured P inflow indicating that exudation of mucilage could be a mechanism to overcome nutrient transport problems due to soil dryness.     

Phosphorus efficiency of wheat and sugar beet seedlings grown in soils with mainly calcium,or iron and aluminium phosphate

Phosphorus is often limiting crop growth in soils low in P supplying capacity. The objective of this study was to investigate whether there are differences in P efficiency between sugar beet and wheat and to search for the plant properties responsible for different P efficiencies encountered and furthermore to see whether the kind of P binding in soil affects the P efficiency of crops. For this a pot experiment with an Oxisol with P mainly bound to Fe and Al (Fe/Al-P) and a Luvisol with P mainly bound to Ca (Ca-P) was run with increasing P fertilizer levels from 0 to 400 mg kg^–1 in a climate chamber. Shoot dry weights of wheat and sugar beet increased strongly with P application in both soils. Both crops, despite their large differences in plant properties, had the same P efficiency in both soils. Therefore none of the species was especially able to use either Fe/Al-P or Ca-P. Wheat relied on a somewhat lower internal requirement, a large root system (high root/shoot ratio) and a low shoot growth rate with a low influx while sugar beet with a small root system and a large shoot growth rate relied on a 5 to 10 times higher influx. A mechanistic mathematical model for calculation of uptake and transport of nutrients in the rhizosphere was used to assess the influence of morphological and physiological root properties on P influx. A comparison of calculated and measured P influx showed that prediction by the model is reasonably accurate for Luvisol. For Oxisol, the predicted P influx was much less than the observed one, even when P influx by root hairs was considered. A sensitivity analysis showed that physiological uptake parameters like I _max, K _m, and C_{L min} had no major influence on predicted influx. The greatest influence on influx had the P soil solution concentration C _{L i}. It is assumed that both species had used mechanisms to increase P availability in the rhizosphere similar to an increase of C _{L i}. Such mechanisms could be the exudation of organic acids, which are known as a sorption competitor to phosphate bound to Fe/Al-oxides or humic-Fe-(Al) complexes or to build soluble complexes with Fe and P. The close agreement between calculated and measured P influx in the Luvisol even at P deficiency indicates that root exudates were not able to mobilize Ca-bound P, whereas Fe/Al-P could be mobilized easily.     

Phosphorus uptake kinetics,size of root system and growth of maize and groundnut in solution culture

Phosphorus acquisition efficiency of maize (Zea mays L.) and groundnut (Arachis hypogaea L.) was investigated in a flowing nutrient solution culture at constant P concentrations of 0.2, 1 and 100 μM. To calculate the P influx and study changes in plant growth and P uptake in relation to plant age, four harvests were taken. Phosphorus uptake kinetics of the roots, i.e. maximum influx, I_\max, the Michaelis constant, K_m, and the minimum concentration, C_Lmin (the concentration at which no net uptake occurs) were estimated in a series of short-term experiments, based on the rate of depletion of P from solution over a range of concentrations. At 1 μM P, maize was more P efficient producing up to 90% of its maximum yield as compared to groundnut with only 20% of maximum yield. A 3 times faster P uptake rate was the reason for the maize P efficiency. In contrast for groundnut at 1 μM P, a net efflux was observed at some development stages of this crop indicating a much higher P requirement at the root surface for maximum growth. Maize had a 6 times higher I_\max value and a 2 times higher K_m value as compared to groundnut. The higher influx of maize was mainly because of the higher I_\max. Maize previously grown at low P concentrations had a C_Lmin of 0.1 μM, while groundnut had values of 0.2 and 0.6 μM. Furthermore groundnut previously grown at 100 μM, was not able to absorb P even at 40 μM. Acclimation to low P concentrations in solution by increasing I_\max or decreasing K_m was not evident in this study. Differences in P acquisition efficiency between maize and groundnut in solution culture were mainly because of differences in P-uptake kinetics, and to a lesser extent to the size of the root system.