Response of transplanted oilseed rape to zinc placement and root pruning

Transplanted oilseed rape (Brassica napus) appears to be sensitive to zinc (Zn) deficiency during its recovery after transplanting, suggesting that it has impaired root function and that seedlings would be responsive to placement of Zn fertilizer close to the roots. In a glasshouse study, the response of oilseed rape seedlings to post-transplanting Zn supply and to root pruning was examined. Zinc supply increased shoot dry matter from the first harvest at 10 days after transplanting whereas root pruning had no effect on shoot dry matter at 10 days but depressed it at 20 days and thereafter. The effects of root pruning on the growth response to Zn were additive to those of Zn supply. The lack of Zn uptake between 10 and 20 days after transplanting in oilseed rape with 75% of roots pruned suggests that this may be the critical period for Zn response in the field. By contrast with only 50% or none of the roots pruned, Zn uptake appeared to be unimpaired. In two field experiments, banding of Zn, Zn in irrigation water or root dipping of transplants in ZnO all increased final seed yield by an average of 10%. By contrast broadcasting ZnSO4 at transplanting had no significant effect on seed yield. All Zn fertilization treatments resulted in a large increase (>50%) in shoot dry matter at 10-leaf stage even though plants at that stage contained more than adequate Zn in their young leaves for growth. The strong growth responses to Zn at 10-leaf stage, and the lesser response at seed yield, suggests that oilseed rape experienced only a temporary Zn deficiency after transplanting. In conclusion, post-transplanting impairment of Zn uptake in oilseed rape seedlings was reduced by either placement of Zn fertilizer close to the roots or minimizing root damage during transplanting.