Examination of morphological changes in the first formed protoxylem in Arabidopsis seedlings

We examined morphological changes in the first-formed protoxylem vessels in Arabidopsis seedlings. Between 2.5 and 8 days after imbibition, mean hypocotyl and root length increased 1.52 and 23.3 times, respectively. In the 2.5-day-old seedlings, two continuous protoxylem vessels were present in the hypocotyl-root axis. In the 8-day-old upper hypocotyls, six protoxylem vessels were observed, and in the lower hypocotyls, four protoxylem vessels and one or two metaxylem vessels were observed. In the 8-day-old roots, there were two protoxylem vessels and two or three metaxylem vessels. Two protoxylem vessels in the hypocotyls connected to two metaxylem vessels in the roots of 8-day-old seedlings. At the 0.3-mm part below the hypocotyl-root boundary, the mean intervals of neighboring annular secondary wall thickenings in protoxylem vessels in 8-day-old roots were 12.9% larger than those in 2.5-day-old roots. In more apical parts of 8-day-old roots, the mean intervals fluctuated between 1.71 and 2.29 μm. In 8-day-old seedlings, metaxylem vessels were formed between 0.4 mm above the hypocotyl-root boundary and 17 mm below the boundary. The intervals in these regions were not extended so much as protoxylem vessels were collapsed. The first-formed protoxylem vessels presumably retain their water-conductive function after metaxylem formation.