Root pruning and transplanting increase zinc requirements of canola (Brassica napus)

Previous field and glasshouse studies suggested that oilseed rape (Brassica napus L.) was especially sensitive to zinc (Zn) deficiency in the recovery period following transplanting. However, it is not clear whether transplanting, per se, or root damage during transplanting was primarily responsible. Three glasshouse experiments were carried out to test the hypothesis that transplanting increases external Zn requirement of canola cv. Hyola 42 during its post-transplanting recovery. Canola was either directly sown into Zn-treated soils or transplanted at four-leaf stage, and grown until harvest at 7- and 10-leaf stages. In a second experiment with chelate-buffered solution culture, direct-sown and transplanted plants were treated with three concentrations of Zn. In the third experiment, plants were given three levels of Zn supply, and either direct-sown into soils or transplanted at four-leaf stage with pruned (50% of roots removed) or unpruned root systems. Transplanted plants required higher soil Zn supply for maximum root length and root dry weight than direct-sown plants. By contrast, shoots required similarly low external Zn for maximum dry weight in both direct-sown and transplanted plants in soil. Direct-sown plants were more efficient in utilizing soil supplied Zn than transplanted plants particularly compared to those transplanted with a pruned root system, and achieved maximum growth at 100 μg Zn kg^?1 soil compared to 500 μg Zn kg^?1 required by transplanted plants. Since the higher external Zn requirement for the growth of transplanted plants was also obtained in well-stirred solution culture, it was concluded that it was related to the time required for transplanted plants to recover from root injury and re-establish a favourable shoot: root ratio rather than to rhizosphere modification processes. Both transplanting, per se, and root damage during transplanting appeared to contribute to higher external Zn requirements for canola growth compared to direct-sown plants.