Differences in cluster-root formation and carboxylate exudation in Lupinus albus L. under different nutrient deficiencies

In contrast with the well document role of proteoid root formation and carboxylate exudation in acclimation to P deficiency in white lupin (Lupinus albus L.), their role under other nutrient deficiencies and their ecological significance are still poorly understood. In the present work, differences in proteoid root formation, exudation of carboxylates by root clusters, non-proteoid and proteoid root tips by using a non-destructive method, and concentrations of organic acids in the tissues of plants grown in the absence of P, Fe or K were studied. Proton release from roots increased soon after withdrawing Fe from the medium; within three days the solution pH decreased from 6 to about 4, and this increased release in protons continued until the end of the experiment. Acidification appeared much later, on the 10th day and the 14th day after withdrawal of P and K, respectively; the extent of the acidification was also weaker than under -Fe (5.2 for -P and 5.7 for control on the 10th day; 6.0 for -K and 6.1 for control on the 14th day). Root clusters formed when plants were grown under -P and -Fe, but not under -K conditions. The root clusters developed sooner under -Fe conditions, but the number of clusters was far less than under -P. Under P deficiency, root clusters released mainly citrate, but also some malate; while the major organic acid released by root tips of both non-proteoid and proteoid roots was malate. However, under Fe deficiency, the majority of the organic acids exuded both by the root clusters and root tips was malate, whereas only a small amount of citrate was detected. The release rate of citrate by -P root clusters was greater than that by -Fe root clusters. Moreover, the release rate of malate was greater in -Fe root clusters than in -P root clusters, but the opposite was found in proteoid root tips, i.e. faster in -P than in -Fe proteoid root tips. The significances of proteoid root formation and release of organic acids in acclimation to different nutrient deficiencies for white lupin plants are discussed.