Saturday, 15 July 2006

Phosphorus Foraging Root Growth of Brassica Plants in Humus-Rich P-Deficient Soils.

Masami Nanzyo, Hitoshi Kanno, and Tadashi Takahashi. Graduate School of Agricultural Science, Tohoku Univ, 1-1, Tsutsumidori-Amamiyamachi, Aobaku, Sendai, Japan

Enhanced plant-root growth in the N and/or P rich patches in soil is well known. This type of root growth is rather common to many plants. In particular, we found an extreme case that Brassica plants and buckwheat roots show very intensive development around P fertilizer particles in humus-rich P-deficient soils. We named this root development as “P foraging root growth”. We at first used alginate gel beads in which powdery DiCalcium Phosphate Dihydrate (DCPD) and alpha-cyclodextrin were suspended (DCPD gel bead). The DCPD gel bead was originally developed by Takada (Azabu University, Japan) et al. in 1997. We increased the concentration of DCPD and alpha-cyclodextrin from originally reported 4 mM to 0.1 M and enlarged the size to reduce the number of application to soil. The modified DCPD gel beads enabled us to find the characteristic root growth around them because of their larger diameter (1.5 cm) than that of ordinary P fertilizer particles. The thin Brassica lateral roots completely covered the DCPD gel beads in a P-deficient nonallophanic Andisol and no soil area remained between the roots and the DCPD gel beads as in an example shown in the figure below. This morphological plasticity of Brassica roots is effective for improving the P recovery rate because even a few mm of soil with a high P retention capacity between a plant root and P fertilizer reduces the P recovery rate. The preferential root proliferation for the DCPD gel beads was also observed in Fagopyrum esculentum Moench as well as Brassica plants among the 20 tested plants. Then, we identified the component in the DCPD gel beads that acounts for this extreme root proliferation. The DCPD gel beads includes P, Ca, alpha-cyclodextrin and alginate. The P-foraging root growth was observed in plots applied with either polyolefin-coated NH4H2PO4 (POC-MAP) or DCPD powder instead of the DCPD gel beads. The DCPD powder was applied in soil as several spots with a diameter of about 1 cm at the depth of 5 cm from the soil surface before seeding. The POC-MAP neither contains Ca, alginate nor alpha-cyclodextrin. Thus, the essential component in the DCPD gel beads for the P foraging root growth around them was only P. Further study was to examine the effect of various inorganic P sources on the P uptake of B. rapa nothovar. These P sources were applied in soil as mentioned above for the DCPD powder application. While significant P uptake was obtained in the plot applied with apatite from Florida, USA, sediment origin (F-Ap), little P uptake was obtained in that with apatite from Quebec, Canada, igneous origin, in a P-deficient nonallophanic Andisol. Hence, a P-release level from F-Ap was near the lower limit for the P uptake by the B. rapa nothovar. under the present experimental condtions. Thus, these results show the P foraging growth of the Brassica roots is effective to improve the P recovery rate in the agricultural fields with localized application of moderately-soluble P fertilizers without additional materials. It may also be possible to include other fertilizers, plant growth regulators and so on in the P fertilizer spots to supply these materials effectively to the plants. Figure shows cross section of a DCPD gel bead (a) covered with B. pekinensis, Rupr. lateral roots (b). The dark material outside the root layer around the DCPD gel bead is a P-deficient nonallophanic Andisol. Scale = 2 mm.

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