High-affinity potassium uptake in seedlings of two citrus rootstocks Carrizo citrange (Citrus sinensis [L.] Osb. × Poncirus trifoliata [L.] Raf.) and Cleopatra mandarin (Citrus reshni Hort. ex Tanaka)

Plant growth and K influx responses to K supply were studied in two citrus rootstocks (Carrizo citrange, Citrus sinensis [L.] Osb. × Poncirus trifoliata [L.] Raf.; Cleopatra mandarin, Citrus reshni Hort. ex Tanaka) to determine mechanisms of K acquisition. Seedlings of Carrizo citrange (CC) and Cleopatra mandarin (Cleo) were grown in aerated nutrient solutions for 40 days with either no K added (2–5 μM K), or with additional 25, 50 or 100 μM K. Growth parameters, mineral composition and K influx by K depletion from solutions were determined. The rootstock CC showed greater growth than Cleo, but the effect of K treatments was similar in both, reaching a similar growth at 25, 50 and 100 μM K. There was a decrease in total biomass and leaf K concentration (21% and 38%, respectively) in non-K seedlings when compared to rest of the K treatments. Seedlings treated with low K having root K concentrations below 29 mg g root dry weight induced the high-affinity K uptake that depleted K from a 50-μM K solution within 7 h. In this experiment, genes encoding putative high-affinity K transporters were isolated and characterised. The study reported that these genes have a high homology with the Arabidopsis AtKUP3, which encodes a putative K transporter of the phylogenetic group II of the KT/HAK/KUP family of K transporters. Therefore, the data suggest that citrus seedlings possess efficient high-affinity K uptake systems that supply K for growth even at very low K concentrations and that genes of KT/HAK/KUP family of K transporters are apparently present in these citrus genotypes.