Phosphorus (P) found in rice grains originates from two possible sources that are exogenous (post-flowering P uptake from soil) or endogenous (P remobilisation from vegetative parts). Julia et al. investigate P partitioning and P remobilisation throughout rice grain filling.

Proposed model of upward P fluxes in rice plants before (A) and after (B) flowering.
Proposed model of upward P fluxes in rice plants before (A) and after (B) flowering. Model variations B1 and B2 represent the two possible reproductive fluxes enabling P to be loaded into grains, that is either using roots (B1) or vegetative tissues (B2) as the main secondary P source. Size of arrows is proportional to the amount of P mobilized by fluxes from sowing to flowering (A) and during grain filling (B) measured in Opt-P treatment of the hydroponic study. Values in italic depict the percentage of 33P activity in plants 24 h after spiking in Opt-P.

They demonstrate that post-flowering P uptake from soil is a critical contributor to the grains’ P content in irrigated rice. In the light of a P tracer study, and propose a model of P loading into grains that should be considered in any attempt to breed rice cultivars with lower grains P content.