Boron supply into wheat (Triticum aestivum L. cv. Wilgoyne) ears whilst still enclosed within leaf sheaths

The present study investigates whether there is significant remobilization of B-10 previously loaded in the flag and penultimate leaves into the young, actively growing ear enclosed within the sheaths of flag and penultimate leaves. It also explores whether B transport into the enclosed ear declines when air humidity in the shoot canopy increases. After 5 d B-10 labelling during the period from early to full emergence of the flag leaf, the plants were transferred into nutrient solutions containing either 10 muM B-11 or no added B for 3 d. Regardless of the subsequent B supply levels to the roots, B-10 contents in the ear continued to increase by up to 5-fold 3 d after the end of B-10 supply in the nutrient solution. During these 3 d, the ear experienced a rapid increase in biomass. However, the majority of B in the ear during the 3 d treatment period was from the newly acquired B-11 from root uptake, rather than retranslocation of B-10 previously deposited in the leaves. By comparing the relative distribution of B-10, Rb (xylem-to-phloem transfer marker) and Sr (xylem-marker) in the ear and the flag leaf, the distribution of B-10 resembled that of Rb more than Sr. Canopy cover treatment greatly suppressed leaf transpiration and decreased the amount of newly acquired B-10 in the flag leaf and the ear, but not in the upper stem segments. The results suggest that whilst the young ear was still fully enclosed within the leaf sheaths without any significant transpiration activity, B transport into the ear is predominantly dependent on the long-distance B transport in the xylem driven by leaf transpiration and, therefore, on concurrent B uptake from the roots.