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 NH(4)(+) 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 (V(max)), the Michaelis-Menten constant (K(m)), and the external concentration when net uptake is zero (C(min)) were tested in experiment 4. Lower K(m) and C(min) 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 Ca(3)(PO(4))(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.

Mobilization and acquisition of sparingly soluble P-sources by Brassica cultivars under P-starved environment I. Differential growth response, P-efficiency characteristics and P-remobilization.

Phosphorus (P) starvation is highly notorious for limiting plant growth around the globe. To combat P-starvation, plants constantly sense the changes in their environment, and elicit an elegant myriad of plastic responses and rescue strategies to enhance P-solublization and acquisition from bound soil P-forms. Relative growth responses, P-solublization and P-acquisition ability of 14 diverse Brassica cultivars grown with sparingly soluble P-sources (Rock-P (RP) and Ca(3)(PO(4))(2) (TCP)) were evaluated in a solution culture experiment. Cultivars showed considerable genetic diversity in terms of biomass accumulation, concentration and contents of P and Ca in shoots and roots, P-stress factor (PSF) and P use efficiency. Cultivars showed variable P-stress tolerance, and cultivars depicting low PSF and high P-efficiency values were better adaptable to P-starvation. In experiment 2, after initial feeding on optimum nutrition for 12 d after transplanting (DAT), class-I (low P-tolerant (Oscar and Con-II)) and class-II (low P-sensitive (Gold Rush and RL-18)) cultivars were exposed to P-free environment for 25 d. All of the cultivars remobilized P from above ground parts to their roots during growth in P-free environment, the magnitude of which was variable in tested cultivars. P-concentrations ([P]s) at 37 DAT were higher in developing compared with developed leaves. Translocation of absorbed P from metabolically inactive to active sites in P-stressed plants may have helped class-I cultivars to establish a better rooting system, which provided a basis for enhanced P-utilization efficiency (PUE) and tolerance against P-stress. By supplying TCP and RP spatially separated from other nutrients in split root study, class-I cultivars were still able to mobilize RP and TCP more efficiently compared with class-II cultivars. To compare the growth behavior under P-stress, cultivars were grown in pots for 41 d after sowing, using a soil low in P (NaHCO(3)-extractable P = 3.97 mg/kg, Mehlich-III-extractable P = 6.13 mg/kg) with (+P = 60 mg P/kg soil) or without P addition (0P) in study 4. Tested cultivars showed genetic diversity in PUE, P-efficiency (PE), P-efficiency ratio (PER) and PSF. P-stress markedly reduced biomass and plant P contents. Cultivars that produced higher root biomass accumulated higher total P-contents (r= 0.98**), which in turn was related negatively to PSF (r=-0.95**) and positively to shoot and total biomass. PER and PE showed significant correlations with shoot P-contents and biomass. Cultivars depicting high PUE and PE, and low PSF values showed better growth behavior under low soil P-environment. Systematic analysis and deployment of the plant rescue traits underlying the nutrient acquisition, assimilation, utilization and remobilization under P-starvation will bring more sparingly soluble P into cropping systems and will help to scavenge more P from plant unavailable bound P reserves.

Intraspecific variations of phosphorus absorption and remobilization, P forms, and their internal buffering in Brassica cultivars exposed to a P-stressed environment.

Translocation of absorbed phosphorus (P) from metabolically inactive sites to active sites in plants growing under P deprivation may increase its P utilization efficiency (PUE). Acclimation to phosphate (Pi) starvation may be caused by a differential storage pool of vacuolar P, its release, and the intensity of re-translocation of absorbed P as P starvation inducible environmental cues (PSIEC) from ambient environment. Biomass assay and three P forms, namely inorganic (Pi), organic (Po), and acid-soluble total (Ptas) were estimated in Brassica cultivars exposed to 10 d P deprivation in the culture media. Considering that -delta Pi/delta t denotes the rate of Pi release, Pi release velocity (RSPi) was determined as the tangent to the equations obtained for Pi f(t) at the mean point in the period of greatest Pi decrease, whereas the inverse of the RSPi was an estimate of the internal Pi buffering capacity (IBCPi). Inter cultivar variations in size of the non-metabolic Pi pool, RSPi, re-translocation of Pi from less to more active metabolic sites, and preferential Pi source and sink compartments were evaluated under P starvation. The cultivar 'Brown Raya' showed the highest Pi storage ability under adequate external P supply, and a more intensive release than 'Rain Bow' and 'Dunkled' under P stress. Cultivar 'B.S.A' was inferior to 'Con-1' in its ability to store and use Pi. Roots and upper leaves were the main sink of Pi stored in the lower and middle leaves of all cultivars and showed lower IBCPi and larger RSPi values than lower and middle leaves. In another trial, six cultivars were exposed to P-free nutrition for 29 d after initial feeding on optimum nutrition for 15 d. With variable magnitude, all of the cultivars re-translocated P from the above ground parts to their roots under P starvation, and [P] at 44 d after transplanting was higher in developing leaves compared with developed leaves. Under P deprivation, translocation of absorbed P from metabolically inactive to active sites may have helped the tolerant cultivars to establish a better rooting system, which provided a basis for tolerance against P starvation and increased PUE. A better understanding of the extent to which changes in the flux of P absorption and re-translocation under PSIEC will help to scavenge Pi from bound P reserves and will bring more sparingly soluble P into cropping systems and obtain capitalization of P reserves.