Early effects of boron deficiency on membrane function in higher plants
The transfer of plants to boron-free solutions induces rapid responses in membrane functions without necessarily affecting root growth and anatomy. In sunflowers (Helianthus annuus), root growth slows within 3-6 h. However in maize (Zea mays), no growth effects are apparent after more than 30 h without boron (-B). In both species early disturbances in ion uptake and cell wall deposition are seen. Ultrastructural studies on sunflower root tips after 5.5 h or 3 d -B are reported. Detailed studies on the absorption of Pj and K+ by root tips were complemented by studies on protoplasts isolated from the root tips of +B and -B plants. There were no significant differences in the protoplast yield or viability according to their B status. Ion absorption by protoplasts isolated from roots of +B and -B plants generally resembled that by intact roots of the corresponding B status. Altering the B status of the protoplasts was expected to initiate earlier responses than in the roots where cell wall binding and diffusion times buffer the system against change; but the greater variability inherent in measuring the protoplast responses prevented the detection of subtle changes. The activities of two+ membrane bound arjzymes were investigated; β-glucan synthetase and a K+-stimulated, Mg2+ -dependent ATPase. UDPG incorporation by protoplasts continued for over 18 h and was consistently higher in +B protoplasts and root membranes than -B. However SEM revealed no significant differences in fibre deposition around sunflower and maize protoplasts according to their boron status. (K++Mg2+)-ATPase from sunflower roots was found to be reversibly impaired by the loss of B; and preliminary investigations implied that restoration of activity when B was resupplied to the intact roots was correlated with the B content of the membrane fraction, as determined by the (n,α) method.