White lupin (Lupinus albus) is able to adapt to phosphorus deficiency by producing proteoid roots that release a huge amount of organic acids, resulting in mobilization of sparingly soluble soil phosphate in rhizosphere. The mechanisms responsible for the release of organic acids by proteoid root cells, especially the trans-membrane transport processes, have not been elucidated. Because of high cytosolic pH, the release of undissociated organic acids is not probable. In the present study, we focused on H ϩ export by plasma membrane H ϩ ATPase in active proteoid roots. In vivo, rhizosphere acidification of active proteoid roots was vanadate sensitive. Plasma membranes were isolated from proteoid roots and lateral roots from P-deficient and -sufficient plants. In vitro, in comparison with two types of lateral roots and proteoid roots of P-sufficient plants, the following increase of the various parameters was induced in active proteoid roots of P-deficient plants: (a) hydrolytic ATPase activity, (b) V max and K m , (c) H ϩ ATPase enzyme concentration of plasma membrane, (d) H ϩ -pumping activity, (e) pH gradient across the membrane of plasmalemma vesicles, and (f) passive H ϩ permeability of plasma membrane. In addition, lower vanadate sensitivity and more acidic pH optimum were determined for plasma membrane ATPase of active proteoid roots. Our data support the hypothesis that in active proteoid root cells, H ϩ and organic anions are exported separately, and that modification of plasma membrane H ϩ ATPase is essential for enhanced rhizosphere acidification by active proteoid roots.P is one of the most important plant nutrients that significantly affect growth and metabolism. Although the total amount of P in soil may be high, it is often present in unavailable forms such as phytic acid (Richardson, 1994), or Ca, Fe, and Al phosphates (Holford, 1997). Low availability of P is a major constraint for crop production in many low-input systems of agriculture worldwide, especially in the highly weathered soils of the humid tropics and subtropics, in many sandy soils of the semiarid tropics, and in calcareous soils of the temperate regions, where crop productivity is severely compromised through lack of available P (Raghothama, 1999). Also, after application of P to the soil the recovery of applied P by crop plants in a growing season is very low, because in the soil more than 80% of the P becomes immobile and unavailable for plant uptake due to adsorption on Al or Fe oxides/hydroxides, precipitation with Ca, or conversion to organic forms (Holford, 1997).Higher plants have developed various strategies of acquiring sparingly soluble nutrients from soil. In response to P deficiency, various species from different families develop so-called proteoid roots. These are bottlebrush-like clusters of rootlets of limited growth with an average length of 0.5 to 1 cm. The rootlets are closely arranged along lateral roots and are usually covered with long and dense root hairs (Purnell, 1960; Dinkelaker et al., 1995;Watt and Evans...

Vegetable cultivation during winter season is economically profitable, but the impact of the intensive production on soil and water quality remains to be studied. The objectives of this study were to investigate the seasonal dynamics of soil nutrients, acidification and salt accumulation in vegetable fields in South-Eastern China. Various vegetables were grown either under open-field conditions or under two different alternating open-field and greenhouse conditions with three replications. Soil samples were collected periodically and analyzed for pH, plant available nitrogen (N), phosphorous (P), potassium (K), electrical conductivity (EC), and urease activity. Water samples from wells located in or near the plots were collected and analyzed for nitrate. Soil nitrate, available phosphate and salt concentrations declined in summer under open-field conditions and significantly increased from December to May under greenhouse conditions. Exchangeable K also decreased in summer season, but did not increase in the spring. Under alternating open-field and greenhouse conditions, nutrient accumulation, soil salinity and acidification were significantly higher for soil used for vegetable cultivation for 2 years (2-y-plot) than that for only half year (0.5-y-plot). The accumulation of nitrate significantly correlated with soil EC and soil acidification. Thirty-two percent of groundwater samples from the 2-y-plot showed a nitrate concentration higher than 50 mg NO 3 l -1 . Conversely, no groundwater sample of 0.5-y-plot showed such high nitrate concentration. It can be concluded that the nitrate accumulation in soil used for vegetable cultivation under alternating openfield and greenhouse conditions not only causes soil salinization and soil acidification but also presents a high pollution potential for groundwater.

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Corn (Zea mays L.) root adaptation to pH 3.5 in comparison with pH 6.0 (control) was investigated in long-term nutrient solution experiments. When pH was gradually reduced, comparable root growth was observed irrespective of whether the pH was 3.5 or 6.0. After low-pH adaptation, H ؉ release of corn roots in vivo at pH 5.6 was about 3 times higher than that of control. Plasmalemma of corn roots was isolated for investigation in vitro. At optimum assay pH, in comparison with control, the following increases of the various parameters were caused by low-pH treatment: (a) hydrolytic ATPase activity, (b) maximum initial velocity and Michaelis constant (c) activation energy of H ؉ -ATPase, (d) H ؉ -pumping activity, (e) H ؉ permeability of plasmalemma, and (f) pH gradient across the membranes of plasmalemma vesicles. In addition, vanadate sensitivity remained unchanged. It is concluded that plasmalemma H ؉ -ATPase contributes significantly to the adaptation of corn roots to low pH. A restricted net H ؉ release at low pH in vivo may be attributed to the steeper pH gradient and enhanced H ؉ permeability of plasmalemma but not to deactivation of H ؉ -ATPase. Possible mechanisms responsible for adaptation of plasmalemma H ؉ -ATPase to low solution pH during plant cultivation are discussed.Acid soils make up to 40% of the worldЈs arable land (Kochian, 1995). Plant growth and development on acid soils may be affected by high levels of Al and Mn, as well as by limited availability of various nutrients (Adams, 1981). On the other hand, low pH (high H ϩ activity) in root medium (pH e ) may directly inhibit plant growth (Islam et al., 1980; Schubert et al., 1990). Mechanisms of Al toxicity have been studied extensively during the last decade (Kochian, 1995), whereas the understanding of H ϩ toxicity in plants remains poor. It has been observed that root growth rate was related to net H ϩ release, which may be restricted at low pH e . Therefore, it has been suggested that H ϩ homeostasis of plant root cells may be influenced by low pH e , resulting in the reduction of root growth rate (Yan et al., 1992). Net H ϩ release results from H ϩ efflux driven by plasmalemma H ϩ -ATPase activity and from H ϩ influx following the plasmalemma H ϩ gradient. Reduced net H ϩ release may be attributed to a decrease in H ϩ pump activity, an increase in plasmalemma H ϩ permeability, or both. Because of its overall importance in physiological processes, the plasmalemma H ϩ -ATPase has been investigated extensively during the last two decades. This enzyme has been found to respond to a number of environmental factors, such as saline stress (Braun et al., 1986; Ayala et al., 1996), nutrient supply (Kuiper et al., 1991; Santi et al., 1995; Schubert and Yan, 1997), high-O 2 treatment (Pinton et al., 1996; Xia and Roberts, 1996), mechanical stimulation (Bourgeade and Boyer, 1994), and fusicoccin, a fungal toxin (Marré, 1979). Although there are reports in the literature describing a response of yeast H ϩ -ATPase to low pH e (Eraso and Gancedo, 198...

The effect of low pH on net H+ release and root growth of corn (Zea mays L.) and broad bean (Vicia faba L.) seedlings was investigated in short-term experiments at constant pH. Broad bean was more sensitive to low pH than corn: the critical values (pH values below which net H+ release and root growth ceased) were pH 4.00 (broad bean) and pH 3.50 (com) magnesium (1, 9, 22). On the other hand, low pH (high H+ activity) may directly inhibit plant growth (2,15,25,26,28,30), probably by adverse effects at the root plasmalemma level. An increase of plasmalemma permeability at high H+ activity in the medium has been shown to be alleviated by the addition of calcium (15,17,19,20). Likewise, plant growth at low pH was improved at higher calcium concentrations (9,20). Calcium is believed to have a specific function at the external side of the plasmalemma for membrane integrity (5,13).It has recently been suggested that at low pH net H+ release by H+ ATPase activity is restricted, thus limiting dry matter production during vegetative plant growth (26,28). With respect to net H+ release and growth at high H+ activity in the root medium, broad bean seems to be particularly sensitive to low pH (22,26). Although there are various reports in the literature describing species differences in acidity tolerance (2,12,21), nothing is known about the role of net H' release in acidity tolerance.'Supported by the Deutsche Forschungsgemeinschaft.The aim of our present investigation was to test for species differences with respect to net H+ release at low pH and to test the effect of these differences in net H+ release on root growth rate. From the results by Islam et al. ( 12), it appeared that corn was more tolerant of low pH than broad bean. Therefore, we chose corn and broad bean to evaluate the effect of high H+ activity in the root medium on net H+ release and root growth under various experimental conditions. Furthermore, we investigated whether at low pH in the root medium proton pumping by H+ ATPase is limited by ATP energy supply. MATERIALS AND METHODS Plant CultivationCorn (Zea mays L. cv Blizzard) and broad bean (Viciafaba L. cv Alfred) seeds were soaked in 0.5 mM CaSO4 for 1 d and then germinated at 25C in the dark on filter paper moistened with 0.5 mm CaSO4. After 3 to 4 d, seedlings were transferred to a stainless steel mesh suspended over an 0.5 mM CaSO4 solution in the dark. This procedure was chosen to obtain (corn) plants with one