The slow response of shoot dry matter to nutrient stress under many experimental conditions sometimes makes it an unsatisfactory criterion for defining critical concentrations of nutrients for deficiency diagnosis by plant analysis. This study was done to see if the relationship between the B concentration in young leaves and their rate of elongation could be used to define critical values for diagnosis of B deficiency in soybean [Glycine max (L.) Merr.] Soybean was grown in nonrenewed solution culture at two initial B concentrations chosen so that plants at the lower concentration became deficient during the growing period, while those at the higher B concentration remained healthy. Boron deficiency depressed dry matter (DM) of roots and young leaves but increased DM of older, expanded leaves, thus delaying the response of total shoot DM to B deficiency. As B supply became depleted, B concentrations in young leaves declined rapidly from 20 to 3 mg kg−1 DM, giving falsely low values for diagnosis of B deficiency when correlated with shoot DM. The rate of elongation of young expanding leaflet blades was depressed by B deficiency earlier than shoot DM. Correlation of B concentration in young leaves with their elongation rate suggests that they require 12 mg B kg−1 DM to achieve 90% of their maximal rate. It is suggested that this value represents the functional B requirement for leaf expansion and that it provides a more consistent and accurate critical value for diagnosis of B deficiency in soybean than that determined by correlation with shoot DM in the usual way.

No abstract