Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/1996
Publication Date: N/A
Citation: Interpretive Summary: Phosphorus (P), one of the primary nutrients required for plant growth, is a costly farm input. Current projections indicated that world P supply will be critically low by 2050. It is, therefore, important to know how to improve the efficiency of plant P acquisition and how crop plants adapt to low soil P. White lupin plants, when grown under low P conditions, form densely clustered tertiary roots called proteoid roots. These proteoid roots excrete large amounts of organic acids into the soil to increase P solubilization. We designed experiments to: 1) determine how much and what type of organic acid were excreted by P stressed lupin roots; and 2) evaluate whether proteoid roots were important sites for synthesis of organic acids. Plants were grown either without or with P. Shoots or roots of intact plants from both P treatments were independently given the tracer radioactive carbon dioxide (14CO2). We found that proteoid roots of fP stressed white lupin produced large amounts of two different organic acids not found in normal roots. This study shows that the shape of and chemical constituents in lupin roots can change in order to adapt to nutrient stress. This study is important because it shows that white lupin has a unique adaptive mechanism to acquire unavailable soil P, thus increasing P uptake efficiency. Information gained in this study may be used to improve the P uptake efficiency of other crop plants.
Technical Abstract: When white lupin (Lupinus albus L.) is subjected to phosphorus deficiency, lateral root development is altered. Densely clustered, tertiary lateral roots (proteoid roots) are initiated, and these proteoid roots exude large amounts of citrate which increases P solubilization. Plants were grown with either 1 mM P (+P treated) or zero P (-P treated). Shoots or roots of fintact plants from both P treatments were labeled independently with 14CO2 to assess the relative contribution of C fixed in each to C exuded from roots as citrate and other organic acids. About 25 fold more acid stable 14C, primarily in citrate and malate, was recovered in exudates from the roots of -P treated plants compared to +P treated plants. The rate of in vivo C fixation in roots was about 4 fold higher in -P treated plants compared to +P treated plants. Evidence from labeling intact shoots or roots indicates that synthesis of citrate exuded by -P treated roots is related directly to nonphotosynthetic C fixation in roots. Carbon fixed i roots of -P treated plants contributed about 25% and 34% of the C exuded as citrate and malate, respectively. Nonphotosynthetic C fixation in white lupin roots is an integral component in the exudation of large amounts of citrate and malate thus increasing plant available P.