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Cluster roots are adaptations for nutrient acquisition, found throughout the world in many different plant families and habitats. They arise from changes in root initiation, meristem maintenance and physiology. In Lupinus albus cluster roots form under low internal plant phosphate and low internal plant iron levels. In this study, we compare morphology, structure and physiology of cluster roots formed under -P and -Fe conditions. -Fe cluster roots had a lower density of shorter rootlets than -P roots, and were yellow in colour, probably because of increased phenolics due to down-regulation of peroxidase. Rootlet length and width was reduced in -Fe conditions. The change in exudation of citrate, over time, of -P and -Fe cluster roots shared identical temporal dynamics, with an exudative burst occurring in day 3. However, the -Fe cluster roots displayed much higher rates of exudation than the -P cluster roots. Results are discussed within the context of structural and functional control.

In cluster roots of P-deficient white lupin, intense exudation of citrate, and the contemporaneous acidification of the rhizosphere, are an adaptation to mobilise sparingly soluble soil P sources. Transport processes involved in citrate exudation were investigated through a membrane physiological approach, using isolated root plasma membrane (PM) vesicles. Exogenous application of citrate [100–250 μM] to root PM vesicle preparations stimulated H+-ATPase, activity. However, a strong inhibition occurred at concentrations >2 mM, independently of Mg supply. No such effects were observed when malate was applied. Accordingly, application of 5 mM citrate to inside-out PM vesicles decreased ATP-dependent intravesicular H+ accumulation, which was increased after the application of 5 mM malate. In contrast, lower citrate concentrations [2 mM] stimulated vesicle acidification even in the absence of Mg-ATP. Acidification was further enhanced by the addition of Mg-ATP, and was most evident in vesicles isolated from cluster roots of P-deficient plants. Moreover, the uptake of 14C citrate in the presence of ATP and PM H+-ATPase activity were particularly enhanced in PM vesicles obtained from cluster roots. The results provide evidence for a P deficiency-induced transport mechanism for citrate anions at the PM of cluster root cells, coupled with the activity of the PM H+-ATPase, On the other hand, PM H+-ATPase, activity may be modulated by concentration levels of citrate characteristic for the cytosol.

Hakea prostrata R.Br. plants were grown in nutrient solution in a split-root design, to address the question if the external or the internal P concentration affects initiation, growth and carboxylate exudation of cluster roots in this species. To this end, one root half was exposed to 1 μM P, whereas the other was exposed to either 1, 25 or 75 μM P. The P concentration increased in the shoots of plants exposed to a higher P concentration, but P was not readily translocated from the roots grown at a high P supply to the other root half grown with 1 μM P. In the roots exposed to 1 μM P, cluster root initiation was not suppressed by the high P supply to the other root half. In contrast, in the roots exposed to 1 μM P, cluster root growth and the rate of carboxylate exudation were suppressed by the high P supply to the other root half.