On the correlation of primary root growth and tracheary element size and distance from the tip in cotton seedlings grown under salinity

Abstract

Cotton seedlings ( Gossypium hirsutum L. cv. Acala SJ-2) were germinated and grown in vermiculite for 7 days, at 1–150 mM NaCl salinity. Primary root samples were harvested daily, root lengths measured and, after clearing and staining, the distance from the root tip to the closest protoxylem tracheary element (PTE) secondary cell wall depositions was determined. The length of control roots correlated with the PTE distance and the root elongation rate. Initially, as roots became longer, elongation rates increased and so did the distance for PTE secondary wall depositions. The highest elongation rates and the farthest PTE distances were reached for 75–125 mm long roots. Thereafter, as root length increased, the PTE distance and growth rates dropped gradually. A high linear correlation between the distance for PTE secondary wall depositions and the root growth rate was obtained for control plants (up to 200 mm long) over the experimental period. This correlation was weakened by rising salinity. At 150 mM NaCl no such correlation was found. Salinity resulted in PTE distances closer to the tip in roots of intermediate length (50–150 mm). In longer and in shorter roots, salinity did not affect the distance from the root tip for PTE secondary wall depositions. Vermiculite-grown seedlings, 4–11 days old, were used to assess the effects of 200 mM NaCl salinity on the development of metaxylem vessel members (MVMs). In macerated stele samples from regions close to the root base, the dimensions of MVMs were measured. High salinity resulted in reductions of both width and length of MVMs, and this inhibition increased with plant age. Large, central MVMs matured mostly earlier in control plants than in salt-stressed plants.