A critical comparison of the external and internal boron requirements for contrasting species in boron-buffered solution culture

Despite reports that boron (B) requirements differ among plant species ther e is a shortage of critical evidence to demonstrate unequivocally whether s pecies differ in internal or external B requirements or both. The present r esearch was conducted to establish the external and internal B requirements of three contrasting species, a woody dicot (marri), an herbaceous dicot ( sunflower) and a monocot (wheat) using B-buffered solution culture. Boron-b uffered solution culture provided satisfactory control of external B concen trations ranging from 0.04 to 30 muM throughout the 20- (sunflower and whea t) or 40-day (marri) growth period. At low external B concentrations (less than or equal to 0.13 muM), the growth of marri and sunflower was severely depressed but by contrast the vegetative growth of wheat plants was satisfa ctory and free of B deficiency symptoms. Marri and sunflower plants achieve d total maximum shoot growth at greater than or equal to1.2 muM B in soluti ons while wheat plants did so at greater than or equal to 0.6 muM B. The cr itical B concentrations (mg kg(-1) dry matter) in the youngest open leaf bl ades of marri, sunflower and wheat plants were 17.9, 19.7 and 1.2 on 20, 10 and 10 days after transplanting (DAT), respectively. Lower internal and ex ternal B requirements of wheat were matched by a lower uptake rate of B com pared to marri and sunflower.