Phosphorus Speciation in Plant Parts of Wheat and Canola Is Affected by Phosphorus Supply.

As plants start to senesce, the primary sink for P is the seed but it is unclear how plant P status affects the resulting P concentration and speciation in the seed and remaining plant parts of the mature residue. Wheat and canola were grown in the glasshouse at three different P rates (5, 30 and 60 kg P/ha equivalent). At maturity plants were harvested and separated into root, stem, leaf, chaff and seed. Phosphorus speciation was determined for all plant parts and P rates, using solution ³¹P nuclear magnetic resonance (NMR) spectroscopy. The dominant storage form of P was either orthophosphate or phytate depending on the plant organ. All root samples contained orthophosphate, phospholipid, RNA and phytate with the exception of wheat root which also contained pyrophosphate, likely from the confirmed presence of mycorrhizal associations. Stem and leaf material contained high concentrations of orthophosphate (average 75%) over the three P rates. Orthophosphate is the form of P most readily translocated during grain fill, and both wheat and canola showed very efficient translocation with only 1-4% of total plant P remaining in stems at maturity. Phytate was detected in all plant parts except the leaves and was the dominant P form in seeds. The presence of varying phytate concentrations in root, stem and chaff plant parts indicates that plants can regulate P storage as phytate in response to plant P status. Wheat chaff increased in phytate concentration from 10-45% as the P rate increased and proportionally less P was translocated to filling grain. Canola chaff did not show a similar trend with majority of the P in the form of orthophosphate.