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
Authors: Akhtar Muhammad Shahbaz , Oki Yoko , Adachi Tadashi 
Published: November 2009

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.