ROOT CARBON-DIOXIDE FIXATION BY PHOSPHORUS-DEFICIENT LUPINUS-ALBUS - CONTRIBUTION TO ORGANIC-ACID EXUDATION BY PROTEOID ROOTS

When white lupin (Lupinus albus L.) is subjected to P deficiency later al root development is altered and densely clustered, tertiary lateral roots (proteoid roots) are initiated. These proteoid roots exude larg e amounts of citrate, which increases P solubilization. In the current study plants were grown with either 1 mM P (+P-treated) or without P (-P-treated). Shoots or roots of intact plants from both P treatments were labeled independently with (CO2)-C-14 to compare the relative con tribution of C fixed in each with the C exuded from roots as citrate a nd other organic acids. About 25-fold more acid-stable C-14, primarily in citrate and malate, was recovered in exudates from the roots of -P -treated plants compared with +P-treated plants. The rate of in vivo C fixation in roots was about 4-fold higher in -P-treated plants than i n +P-treated plants. Evidence from labeling intact shoots or roots ind icates that synthesis of citrate exuded by -P-treated roots is directl y related to nonphotosynthetic C fixation in roots. C fixed in roots o f -P-treated plants contributed about 25 and 34% of the C exuded as ci trate 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 the P available to the plant.