EXTERNAL BORON REQUIREMENTS FOR CANOLA (BRASSICA-NAPUS L) IN BORON BUFFERED SOLUTION CULTURE

Quantitative relationships between external boron concentrations that could elicit boron deficiency and plant growth, boron uptake rate, and plant boron concentrations have not been previously investigated due to the lack of water culture systems that provide satisfactory control over solution boron concentrations. Two approaches were tested to buf fer the boron concentrations in nutrient solutions: supplying differen t amounts of boron saturated resin Amberlite IRA 743 per unit solution volume; and loading the resin with boron at 1 to 100% of full saturat ion. Mean boron concentrations (mu M) ranged from 0.17 to 2.85 and fro m 0.047 to 27.03 in the two approaches, respectively, and in individua l pots with plants solutions remained constant in boron concentrations for at least 10 to 12 d. At solution boron concentrations from 0.04 t o 0.3 mu M, canola (Brassica napus L.) plants remained alive, but shoo t and root growth was stunted and showed classical boron deficiency sy mptoms. increasing solution boron concentrations progressively increas ed boron concentrations in shoots and roots. Boron concentrations in r oots were less than one-third of those in lower shoots, and less than those in upper shoots, except in boron deficient plants. In boron buff ered solutions, dry matter of canola at both 12 and 24 d increased wit h increasing solution boron concentration up to 0.54-0.87 mu M. With i ncreasing solution boron concentrations up to 26.5 mu M there was no f urther increase in dry matter, and no detrimental effects on plant gro wth. At 0.04 mu M boron, plants absorbed no boron from solution. Incre asing solution boron concentrations from 0.1 to 26.5 mu M increased re lative boron uptake rates from 0.005 to 0.1 mu mol g(-1) root f.wt d(- 1). Maximum root efficiency, defined as relative uptake rate divided b y the solution boron concentration, was achieved at 0.04 to 0.3 mu M b oron in solution. With increasing solution boron concentrations, relat ive uptake rates of calcium decreased from 248 to 10 mu mol g(-1) root f.wt. d(-1). The results suggest that boron specifically inhibits cal cium absorption. (C) 1997 Annals of Botany Company.