No abstract

No abstract

Mobilization of soil P as the result of plant-induced changes of soil pH in the vicinity of plant roots was studied. Seedlings of ryegrass were grown in small containers separating roots from soil by a 30-~m meshed nylon screen which root hairs could penetrate but not roots. Two soils were used, a luvisol containing P mainly bound to calcium and an oxisol containing P mainly bound (adsorbed) to iron and aluminum. Plant-induced changes of soil pH were brought about by application of ammonium-or nitrate-nitrogen. After plants had grown for 10 d the soil was sliced in thin layers parallel to the root mat which had developed on the screen, and both soil pH and residual P determined. Mobilization of P was assessed by P-depletion profiles of the rhizosphere soil.Soil pH at the root surface decreased by up to 1.6 units as the result of ammonium N nutrition and it increased by up to 0.6 units as the result of nitrate N nutrition. These changes extended to a distance between 1 and 4 mm from the root surface depending on the type of soil and the source and level of nitrogen applied. In the luvisol, compared to zero-N treatment, P mobilization increased with the NH4-induced decrease in pH, whereas the NO3-induced pH increase had no effect. In contrast, in the oxisol a similar pH decrease caused by NH 4 nutrition had no effect, whereas the pH increase caused by NO3 increased markedly the mobilization of soil P. It is concluded that in the luvisol calcium phosphates were dissolved by acidification, whereas in the oxisol adsorbed phosphate was mobilized by ligand exchange.

The recently isolated root-hairless mutant of barley ( Hordeum vulgare L), bald root barley, brb offers a unique possibility to quantify the importance of root hairs in phosphorus (P) uptake from soil. In the present study the ability of brb and the wild-type, barley genotype Pallas producing normal root hairs to deplete P in the rhizosphere soil was investigated and the theory of diffusion and mass flow applied to compare the predicted and measured depletion profiles of diffusible P. Pallas depleted twice as much P from the rhizosphere soil as brb . The P depletion profile of Pallas uniformly extended to 0.8 mm from the root surface, which was equal to the root hair length (RHL). The model based on the theory of diffusion and mass flow explained the observed P-depletion profile of brb, and the P depletion outside the root-hair zone of Pallas, suggesting that the model is valid only for P movement in rhizosphere soil outside the root-hair zone. In low-P soil (P in soil solution 3 m m m m M ) brb did not survive after 30 d, whereas Pallas continued to grow, confirming the importance of root hairs in plant growth in a P-limiting environment. In high-P soil (P in soil solution 10 m m m m M ) both brb and Pallas maintained their growth, and they were able to produce seeds. At the high-P concentration, RHL of the Pallas was reduced from 0.80 ± ± ± ± 0.2 to 0.68 ± ± ± ± 0.14 mm. In low-P soil, P-uptake rate into the roots of Pallas was 4.0 ¥ ¥ ¥ ¥ 10-1 and that of brb was 1.9 ¥ ¥ ¥ ¥ 10, which agreed well with the double amount of P depleted from the rhizosphere soil of Pallas in comparison with that of brb. In high-P soil, the P uptake rates into the roots of brb and Pallas were 3.3 and 5.5 ¥ ¥ ¥ ¥ 10, respectively. The results unequivocally confirmed that in a low-P environment, root hairs are of immense importance in P acquisition and plants survival, but under high-P conditions they may be dispensable. The characterization of phenotypes brb and Pallas and the ability to reproduce seeds offers a unique possibility of molecular mapping of QTLs and candidate genes conferring root-hair formation and growth of barley.

A new method allowing control of rhizosphere pH and mineral nutrition was applied to study depletion of various organic and inorganic phosphorus fractions extractable sequentially with 0.5M KHCO 3 (pH 8.5), 0.1M NaOH and residual P extractable with 6M H2SO 4 from the rhizosphere soil.Soil pH was affected about 2 mm from the root mat. Depletion zones of inorganic P (KHCO3-P~) extractable with 0.5M KHCO 3 extended up to about 4 mm but the depletion zones of all other P fractions were about 1 mm only. The root-induced decrease of soil pH from 6.7 to 5.5 increased the depletion of total P from all fractions by 20% and depletion of KHCO3-P i and residual P by 34% and 43%, respectively. Depletion of organic P (KHCO3-Po) extractable with 0.5M KHCO 3 was not affected by a change in rhizosphere pH. With constant or increased pH, dePletion of inorganic P (NaOH-Pi) was 17% and organic P (NaOH-Po) was 22% higher than with decreased pH. Only 54-60% of total P withdrawn from all fractions was from KHCO3-P ~. Substantial amounts of KHCO3-P o and NaOH-Po were mineralized and withdrawn from the rhizosphere within 1 mm from the root mat, as 11-15% of total P withdrawn originated from the organic P fractions. A remaining 11-16% was derived from NaOH-P~, and 15-18% from residual P fractions likely to be rather immobile. Thus, 40-46% of the P withdrawn near the root mat of rape originated from non-mobile P fractions normally not included in 0.5M NaHCO 3 extraction used to obtain an index of plant-available soil P.