External and internal boron requirements of plants using boron buffered solution culture

In soils and in conventional nutrient solutions, establishing the internal and external boron (B) requirements of plants is often confounded because the external B concentration changes during plant growth. On the other hand, the major problem of working with low B concentrations in conventional nutrient solutions is that these concentrations are quickly depleted as B is taken up by the plant roots. Hence in most conventional solution cultures, the initial B concentration is often very high to ensure adequate supply of B throughout an experiment. This research was carried out to develop a solution culture system in which the free B concentration would be buffered at constant concentrations, ranging from low to high. This developed B-buffered solution culture system was then tested as a means of determining the internal and external B requirements of different plant species. Further experiments were conducted to study the external and internal B requirements of plants at different growth stages, and to study distribution of B in different plant parts by using the above B-buffered solution culture system. The B specific resin, Amberlite IRA 743, which complexes H3BO3 on its Nmethyl glucamine functional groups, was chosen for this study because the B saturated resin maintained a realistic equilibrium B concentration in solution and there was no evidence that the resin had significant effects on plant growth other than in releasing and equilibrating B in the nutrient solution. The resin released nitrogen (N) into solution but, provided an adequate solution N supply was maintained, there were no indirect effects of the resin on plant growth apart from its control of B solution concentration. The B sorption capacity of the resin varied from 2.2 to 5.0 mg B/g resin. Boron saturated resin maintained an equilibrium solution B concentration of 46 μM when added at the rate of 2 g of resin to 1L of B-free triple deionised water. Plants grown in complete nutrient solution with B-saturated resin added at 1 g per litre of nutrient solution grew as well as plants grown in conventional nutrient solution containing 9.2 μM B and their shoots contained adequate B concentrations for growth. Glasshouse experiments were undertaken to establish buffered B concentrations ranging from deficient to adequate in nutrient solutions. Supplying different amounts of B saturated resin, Amberlite IRA 743, per unit solution volume; and loading the resin with B at 1 to 100 % of full saturation resulted in solution B concentrations (μM) ranging from 0.17 to 2.9 and from 0.05 to 27.0, respectively. The latter method was more effective in producing a wide range of B supply, from deficiency to adequacy. By this method critical external and internal B concentrations at vegetative growth of canola were 0.6 fjM B and 6-8 mg B kg dry weight, respectively. At solution B concentrations from 0.04 to 0.3 μm, canola (Brassica napus L.) plants remained alive but both shoot and root growth were stunted with classical B deficiency symptoms. Increasing solution B concentrations progressively increased B concentrations in shoots and roots. In roots, B concentrations were less than one-third of those in lower shoots and less than those in upper shoots, except in B-deficient plants. At 0.04 μM B, plants absorbed no B from solution. Increasing solution B concentrations from 0.1 to 26.5 μM increased relative B uptake rates from 0.005 to 0.1 μ mole g root fresh weight day . Maximum root efficiency, defined as relative uptake rate divided by the solution B concentration, was achieved at 0.04 to 0.3 μM B in solution. With increasing solution B concentrations, relative uptake rates of calcium decreased from 248 to 10 μ mole g-1 root fresh weight day-1. The results suggest that B specifically inhibited calcium absorption or accelerated calcium efflux. Canola was grown to flowering in a subsequent experiment to study the effect of external B concentration, established with B-loaded resin, on the distribution of B in plants and to investigate the external B concentrations for near maximum vegetative and reproductive growth. Mean B concentrations in B-buffered nutrient solutions ranging from 0.36 to 46.6 μM were achieved by loading B-specific resin at 4 to 100 % of full saturation. At low levels of B, the resin maintained constant B in nutrient solutions from Day 0 to 55. When relative growth rates for the periods 0-10, 10-22 and 22-55 DAT were related to mean external B concentration, the critical external B concentration for the growth period of 0-55 DAT remained unchanged with time and plant growth stage. Boron concentrations in shoots and roots increased strongly with increasing solution B concentrations up to 1-2 μM B and then more weakly with increases in solution B above 2 μM B. At deficient to marginal external B concentrations, stems had higher B concentrations than leaf blades on Days 10 and 22 but not at Day 55. Although the plants at low external B (< 0.45 μM) concentrations had some vegetative growth they did not produce reproductive parts compared to the plants with adequate B. At the flowering stage, maximum B concentration was found in florets and growth of these plant parts was more sensitively depressed by low B than vegetative plant parts. At 0.86 μM B in nutrient solutions, plants achieved maximum vegetative dry matter and flowered normally. Plants supplied with < 0.45 μM B produced no flowers or flowers were abnormal with aborted stamens and pistils. At 0.35 μMB, plants, whilst stunted, continued to produce vegetative dry matter though reproductive growth was completely suppressed. At flowering higher B concentrations occurred in the flowers (50.3 mg/kg dry weight) than in leaf (19.9 mg/kg dry weight), stem (19.0 mg/kg dry weight) or root (14.7 mg/kg dry weight). This study suggested that external B requirements for canola at the reproductive stage is 0.86 μM for maximum or near to maximum growth. External and internal B requirements of three plant species were studied using the B-buffered solution culture technique. In a glasshouse solution culture experiment, B concentrations were buffered with B saturated resin (Amberlite IRA 743); and loading the resin with B at 1 to 100 % of full saturation. Average B concentrations (μM) in nutrient solutions ranged from 0.04-28.3. The external and internal B requirements of a monocot (wheat, Triticum aestivum), a herbaceous dicot (sunflower) and a woody dicot (marri Corymbia calophylla) were examined using the buffered culture system. Plants were harvested after 10 and 20 days (wheat and sunflower) or 20 and 40 days (marri). At low external B (< 0.13 μM), growth of marri and sunflower was severely depressed, whereas the growth of wheat plants was only weakly depressed at 0.04 pM B in solutions. Where maximum dry weight of shoots was obtained, B concentrations (mg/kg) in leaf blades at the first harvest were 17.9, 19.7 and 1.2, for marri, sunflower and wheat, respectively. Results of this experiment suggested that the two dicotyledons marri and sunflower have higher external and internal B requirements than wheat. Increasing solution B from 0.05 to 28.5 μM increased relative B uptake rates from 0.43-1.02, 0.64-0.94 and 0.02-0.07 μ mole g-1 root dry weight day -1 in the case of marri, sunflower and wheat, respectively. Thus, the higher internal and external B requirements of sunflower and marri compared to wheat were supported by higher rates of B absorption. In conclusion, the B-buffered solution culture system developed in this study has considerable potential for B nutrition studies of plants. This system is robust and easy to establish. Whilst B-specific resin supplied B to canola till flowering stage at higher B loadings (> 16% of full B saturation of resin) it did not maintain constant B concentrations for more than 22 days growth. The length of time for which effective buffering of solution B was achieved appeared to vary with rate of biomass accumulation which, in turn, was a function of time of year and plant species. It is suggested that buffered B concentrations in nutrient solutions may be achieved for experiments of long duration either by increasing the amounts of resin per unit nutrient solution or replacing the B-specific resin after every 8-10 days