Branch roots of young maize seedlings, their production, growth, and phloem supply from the primary root

Branch root development on the primary root of maize (Zea mays L.) seedlings was followed for 9 d after planting. This period includes the shift from seedling heterotrophy to autotrophy. Linear density of branches in the basal region ranged from ∼38 cm^-1 at the base to ∼10 cm ^-1 beyond 10 cm. Branch roots in the first ∼8 cm were produced before assimilate was available. Branch length decreased from ∼26 mm at 1 cm along the primary root to ∼8 mm at 10 cm from the base. Without the cotyledon, branch root density in the basal region was ∼10 cm^-1 and roots were short (∼5 mm). Beyond 8-10 cm both measurements matched those of intact seedlings. Dark-grown seedlings had basal branch root densities higher than those without cotyledons but none beyond 10 cm. There were more and smaller diameter sieve tubes in the basal region of the primary root. These decreased distally in number but had larger diameters where branches formed after assimilate was available. Proliferation of basal branch roots in very young seedlings can have major advantages for successful seedling establishment in the field and could be screened for without difficulty.