Yoshitaka Nakashima

Microcosm investigation on differential potential of free floating Azolla macrophytes for phytoremediation of P-controlled water eutrophication.

Eutrophication is caused by an over enrichment of aquatic ecosystems with nutrients, principally with chronic release of phosphorus (P) from point and non-point sources, leading to nuisance algal blooms, anoxic events, impaired water quality and stubborn environmental issues. Aquatic macrophytes display an efficient phytosequestration of inorganics in plant parts due to their non-degradable nature. Serial microcosm experiments were conducted to estimate differential phytoremdiation ability of A. japonica, A. pinnata, and A. hybrid to remove P from different P-eutrophicated solutions under different incubation periods. Azolla plants showed substantial P-removal efficiency from P-eutrophicated solutions, and removed P-amounts were significantly correlated with P-accumulated in plant biomass. About 1-fold decrease in solution pH might be ascribed to Hefflux. Plants without P-hunger showed lower P-removal rates compared to P-hunger plants. A. japonica displayed highest Premoval efficiency in different experiments. From these kinetic experiments, it is plausible to conclude that phytoaccumulation was the possible mechanism for P-removal, and due to fast growth, high tolerance and accumulation ability, free floating Azolla might be the best candidate among macrophytes to combat P-driven eutrophication. Results obtained will not only provide information to environmental managers to mitigate P-eutrophication but will also provide data base to scientists for their future ventures.

Genotypic Variations in Growth Response and P-acquisition Efficiency by Spring Wheat Cultivars Exposed to Sparingly Soluble P-sources

To estimate genetic variability in growth characteristics and phosphorus acquisition efficiency from sparingly soluble P-sources; four genetically diverse spring wheat cultivars, categorized as ‘P-efficient and low P-tolerant (class-I), and P-inefficient and low P-sensitive (class-II) cultivars’ based on P-efficiency characteristics in a hydroponic study, were grown in a sand culture in order to evaluate their relative performance in a glass house experiment. Pre-treated, cleaned uniform sized seeds of cultivars were sown in 4-kg capacity pots containing yamazuna sand with negligible available P. P-treatments were (i) (LP)-control treatment without any applied P (ii) (AP)-NH4H2PO4 @ 200 μM P kg-1 as an adequate P-supply, (iii) (TCP)-tri-calcium phosphate (Ca3(PO4)2 @ 0.5 g kg-1, and (iv) (RP)-rock phosphate @ 0.5 g kg-1 of sand. Tri-calcium phosphate and rock phosphate were sparingly soluble P-sources containing negligible P soluble in water. Biomass accumulation by plants, P-concentration and uptake in roots and shoots of plants, phosphorus stress factor and P-efficiency characteristics were differed in tested cultivars indicating sufficient genetic diversity in wheat cultivars. Biomass and growth parameters were significantly correlated with plant P-parameters indicating that P taken up by the plants from sparingly soluble P-sources was accumulated into the plant biomass. Class-I cultivars exhibited better performance than class-II cultivars indicating their better ability to scavenge P at all P levels.

Phosphorus Stress-Induced Differential Growth, and Phosphorus Acquisition and Use Efficiency by Spring Wheat Cultivars

ABSTRACT Phosphorus (P) is a finite, non-renewable, and natural resource and a vital major nutrient for plant metabolic and developmental processes. However, adverse soil biogeochemical characteristics of alkaline-calcareous soils (especially Aridisols) and highly weathered acid soils (i.e., Ultisols and Oxisols) render orthophosphate (Pi) as the least available major nutrient due to P complexation, sorption, and/or fixation. In such soil environments, plant bioavailable P is only a small fraction of total soil P, seriously limiting crop growth and production. Different plant species, and even cultivars of the same species, may display a suite of growth responses that enable them to solubilize and scavenge soil P either by enhancing external Pi acquisition or reprioritizing internal Pi use under P-stress soil environments. This paper reports relative growth responses, P acquisition and P-use efficiency characteristics by 14 cultivars of spring wheat (Triticum aestivum L.) grown in solution culture with high/low P supply induced by applying soluble NH4H2PO4, sparingly soluble rock phosphate, and Ca3(PO4)2. The wheat cultivars exhibited considerable genetic diversity in biomass accumulation, P concentrations, P contents, factor (PSF) and P efficiency characteristics [i.e., P utilization efficiency (PUE), P efficiency (PE), and PE ratio (PER)]. Plant growth and PE parameters were significantly correlated, while P uptake was linearly related with biomass increase and solution pH decrease. The wheat cultivars with high PUE, PER and P uptake, and low PSF, and plant P concentration were more efficient in utilizing P and, hence, more tolerant under P-stress environment. Biomass and P contents of “P efficient/low-P tolerant” wheat cultivars were superior to “P inefficient/low-P sensitive” cultivars at all P-stress levels. Hence, “P efficient/low-P tolerant” cultivars are the most desirable wheat genotypes for P-stress environments because they are able to scavenge more P from sparingly soluble P sources or soil-bound P forms.

Solubilization and Acquisition of Phosphorus from Sparingly Soluble Phosphorus Sources and Differential Growth Response of Brassica Cultivars Exposed to Phosphorus-Stress Environment

Phosphate (Pi), the fully oxidized and assimilated form of phosphorus (P), influences virtually all developmental and biochemical processes in plants; however, its availability and distribution are widely heterogeneous. Paradoxically, although total P is abundant in lithosphere, elusive soil chemistry of Pi renders the element the most dilute and the least mobile in natural and agricultural ecosystems, resulting in P deprivation due to its low mobility and high fixation capacity in the soil. Nonmycorrhizal Brassica does not produce specialized cluster/dauciform roots but is an effective P user compared to other crops. Using a soil low in P (Mehlich 3–extractable P) with or without P fertilization, Brassica cultivars showed substantial genetic diversity in P-utilization efficiency (PUE), P efficiency (PE), P-efficiency ratio (PER), and P-stress factor (PSF). Cultivars producing greater root biomass accumulated greater total P contents, which in turn was related negatively to PSF and positively to shoot and total biomass. Plant survival and reproduction rely on efficient strategies in exploring culture media for P. Acquisition of orthophosphate from extracellular sparse P sources may be enhanced by biochemical rescue strategies such as copious H+ efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H+-ATPase and anion channels triggered by P starvation. The P-starvation-induced solution pH changes due to H+ efflux, and carboxylates exudations were estimated by low-P-tolerant and low-P-sensitive cultivars in solution culture experiments. Low-P-tolerant cultivars showed more decrease in pH compared to low-P-sensitive cultivars when cultivars were grown under a P-stress environment induced by using sparingly soluble P sources (rock phosphate and tricalcium phosphate). The P contents of cultivars were inversely related to decrease in culture media pH. Low P-tolerant cultivars presented enhanced H+-efflux and total carboxylates exudations compared to low-P-sensitive cultivars, resulting in more rhizosphere acidification to scavenge Pi, evidencing their adaptability to P starvation. These elegant P-stress-induced rescue strategies by tested cultivars provided the basis of enhanced P solubilization and acquisition of P from sparingly soluble P sources to combat P-starved environments.