Effects of boron on leaf expansion and intercellular airspaces in mung bean in solution culture

Mung bean (Phaseolus aureus Roxb.) was grown in solution culture to study the effect of boron (B) deficiency on leaf expansion and intercellular airspaces. Different B concentrations were supplied just after initiation of the second trifoliate leaf because this is the first leaf whose growth depends on the exogenous B supply. The central leaflet of the second trifoliate was employed in this study. When plants were grown in 0.2 mumol L(-1) B, leaf expansion was limited compared with that of plants supplied with 50 mumol L(-1) B. During the progress of leaf development, plants grown in 0.2 mumol L(-1) B were transferred to medium with 50 mumol L(-1) B medium at days 2, 4, 6, and 8. The final leaf area of plants transferred on day 2 was half that of plants raised in 50 mumol L(-1) B solution. Plants transferred on days 6 and 8 produced a leaf area similar to that of plants grown in 0.2 mumol L(-1) B. Uptake experiments with (10)B-enriched boric acid showed that there was enough B present at the sites of cell expansion after 50 mumol L(-1) B was supplied for two days. The failure of deficient leaves to resume elongation may therefore have been due either to a lack of incorporation of B into the cell wall or boron or to a requirement for B early in leaf development to enable future cell wall expansion. In addition to the reduced leaf expansion resulting from B deficiency, the formation of intercellular airspaces was also depressed. The proportion of the spongy mesophyll region comprising airspaces in B-deficient leaves was less than 50% of that observed in leaves of plants supplied with sufficient B.