Effects of temperature on the development of metaxylem in primary wheat roots and its hydraulic consequence

The effects of different temperatures on the development of metaxylem were studied in the primary seminal root of winter wheat (Triticum aestivum L.) seedlings. Xylem development was studied microscopically at different distances behind the root apex after safranin staining to reveal lignification.Diameter of the central late metaxylem (LMX) and its proportion to the stele cross-sectional area increased in the acropetal direction. Diameter of the LMX and stele decreased with an increase in growing temperature. Numbers of early metaxylem (EMX) were seven, seven and six at 10, 20 and 30 °C, respectively. EMX was lignified much more rapidly than the LMX along the seminal root axes. Lignification of xylem elements commenced further towards the root apex at the higher temperatures. The LMX vessels of the roots grown at the higher temperature had thicker secondary walls.The relative conductivity of seminal roots, calculated from Poiseuille's equation, decreased as growing temperature increased. In a drought-prone environment where wheat plants rely heavily on stored soil water, a lowered axial conductivity in the roots would be advantageous. The plants would tend to conserve water during vegetative growth for use during the critical periods of flowering and grain-filling.Breeders selecting wheat plants for altered LMX diameters should control temperatures during primary root development, section the roots at the same distance from the tip and be aware that cross walls may exist in the LMX for up to 30 cm from the tip.