Effects of phosphorus on nutrient uptake and rhizosphere acidification of soybean (G/ycine max L.)

Pot experiment was conducted to examine how application of KH2PO4 （0-165 mg·kg^-1 P） to affect nutrient ion uptake and rhizosphere acidification of soybean （Glycine max L.） grown in greenhouse for 90 days. When supplied of 82 and 165 mg·kg^-1 P, soybeans showed excessive poison. Under all kinds of P levels, the K, Ca, Na and Mg concents in plant tissues were as below order K was nodules 〉 roots 〉 pods 〉 shoots; Ca was shoots 〉 roots 〉 nodules 〉 pods; Na was roots 〉 nodules 〉 pods 〉 shoots and Mg was shoots 〉 nodules 〉 roots 〉 pods. K concent in plant tisssues had greater effect on rhizosphere acidification than other cations in this experiment irrespective of P supply, and was significantly negative to pH. Na concentration was significantly positive to pH. Excessive P supply induced rhizosphere acidification, pH decreased as P supply increased from 82 to 165 mg·kg^-1. Ash alkalinity in shoots and roots was significantly positively correlated with rhizosphere pH irrespective of P supply. All these results suggested that P supply affected nutrient uptake, induced ash alkalinity to increase and rhizosphere pH to decrease in soybean.

Mobilization and Acquisition of Sparingly Soluble P-Sources by Brassica Cultivars under P-Starved Environment II. Rhizospheric pH changes,Redesigned Root Architecture and Pi-Uptake Kinetics

Non-mycorrhizal Brassica does not produce specialized root structures such as cluster or dauciform roots but is an effective user of P compared with other crops. In addition to P-uptake, utilization and remobilization activity, acquisition of orthophosphate （Pi） from extracellular sparingly P-sources or unavailable bound P-forms can be enhanced by biochemical rescue mechanisms such copious H＋-efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H＋ ATPase and anion channels triggered by P-starvation. To visualize the dissolution of sparingly soluble Ca-phosphate （Ca-P）, newly formed Ca-P was suspended in agar containing other essential nutrients. With NH4＋ applied as the N source, the precipitate dissolved in the root vicinity can be ascribed to rhizosphere acidification, whereas no dissolution occurred with nitrate nutrition. To observe in situ rhizospheric pH changes, images were recorded after embedding the roots in agar containing bromocresol purple as a pH indicator. P-tolerant cultivar showed a greater decrease in pH than the sensitive cultivar in the culture media （the appearance of typical patterns of various colors of pH indicator in the root vicinity）, and at stress P-level this acidification was more prominent. In experiment 2, low P-tolerant class-I cultivars （Oscar and Con-II） showed a greater decrease in solution media pH than low P-sensitive class-II （Gold Rush and RL-18） cultivars, and P-contents of the cultivars was inversely related to decrease in culture media pH. To elucidate P-stress- induced remodeling and redesigning in a root architectural system, cultivars were grown in rhizoboxes in experiment 3. The elongation rates of primary roots increased as P-supply increased, but the elongation rates of the branched zones of primary roots decreased. The length of the lateral roots and topological index values increased when cultivars were exposed to a P-stress environment. To elucidate Pi-uptake kinetics, parameters related to P influx： maximal transport rate （Vmax）, the Michaelis-Menten constant （Km）, and the external concentration when net uptake is zero （Cmin） were tested in experiment 4. Lower Km and Cmin values were better indicative of the P-uptake ability of the class-I cultivars, evidencing their adaptability to P.starved environmental cues. In experiment 5, class-I cultivars exuded two- to threefold more carboxylates than class-II cultivars under the P-stress environment. The amount and types of carboxylates exuded from the roots of P-starved plants differed from those of plants grown under P-sufficient conditions. Nevertheless, the exudation rate of both class-I and class-II cultivars decreased with time, and the highest exudation rate was found after the first 4 h of carboxylates collection. Higher P uptake by class-I cultivars was significantly related to the drop in root medium pH, which can be ascribed to H＋-efflux from the roots supplied with sparingly soluble rock-P and Ca3（PO4）2. These classical rescue strategies provided the basis of P-solubilization and acquisition from sparingly soluble P-sources by Brassica cultivars to thrive in a typically stressful environment.

Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity

Ammonium(NH_(4)^(+))and nitrate(NO_(3)^(-))are major inorganic nitrogen(N)sources for plants.When serving as the sole or dominant N supply,NH_(4)^(+)often causes root inhibition and shoot chlorosis in plants,known as ammonium toxicity.NO_(3)^(-) usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations,referred to as nitrate-dependent alleviation of ammonium toxicity.Our previous studies indicated a NO_(3)^(-) efflux channel SLAH3 is involved in this process.However,whether additional components contribute to NO_(3)^(-)-mediated NH_(4)^(+)detoxification is unknown.Previously,mutations in NO_(3)^(-) transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH_(4)^(+).Whereas,in this study,we found when the high-NH_(4)^(+) medium was supplemented with low concentrations of NO_(3)^(-),nrt1.1 mutant plants showed hyper-sensitive phenotype instead.Furthermore,mutation in NRT1.1 caused enhanced medium acidification under high-NH_(4)^(+)/Iow-NO_(3)^(-) condition,suggesting NRT1.1 regulates ammonium toxicity by facilitating H+uptake.Moreover,NRT1.1 was shown to interact with SLAH3 to form a transporter-channel complex.Interestingly,SLAH3 appeared to affect NO_(3)^(-) influx while NRT1.1 influenced NO_(3)^(-) efflux,suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels.Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity through regulating NO_(3)^(-) transport and balancing rhizosphere acidification.