Ca: a new concept in potato tuber calcium nutrition

doi: 10.1111/j.1399-3054.2006.00741.x Calcium is believed to be transported with water in the xylem. Consistent with this proposal, low-transpiring organs such as potato Solanum tuberosum tubers are known to suffer from calcium deficiency. Although roots on tubers and stolons have been shown to supply water to tubers, there is no direct evidence for the calcium transport pathway to tubers. Both a xylem and a phloem transport pathway have been suggested. We investigated in vivo calcium transport to developing potato, cv. Dark Red Norland and cv. Russet Burbank, tubers using Ca in a controlled environment facility. Whole plant split pot experiments allowed the placement of Ca either in the main (basal) root or the tuber and stolon areas of the pot. The results showed that Ca was transported to the shoot with the transpiration stream from both areas but was not re-translocated to tubers or the main (basal) root system even 57 days after Ca application. Radioactivity could only be detected in the tuber when Ca was fed to the stolon and tuber area. When Ca was fed to specific tubers, radioactivity was detected in the aerial shoot; however, no activity was detected in other tubers or themain (basal) roots. In another set of experiments, roots on a stolon near a tuber were precisely fed Ca and Safranin O. The radioactive signal exactly overlapped the water transport pathway in the tuber marked with Safranin O dye, suggesting that water and calcium can be simultaneously transported from stolon roots to the tuber. No transport of Ca across the tuber periderm was detected 8 days after Ca was applied to the tuber periderm. This indicated that no significant transport of calcium occurs from the soil across the periderm. Our results provide evidence that: (1) calcium is not re-translocated via the phloem from the aerial shoot tubers and main (basal) roots; (2) the main root system does not supply calcium to the tuber; (3) calcium is not transported across the periderm to the interior tuber tissue; (4) calcium is transported to the tuber via the xylem along with water, and the roots on the stolon associated with the tuber supply water and calcium to the developing tuber; and (5) transpirational demand is a significant determinant of calcium distribution within the plant.