These sorry-looking individuals are darnel, a grass weed.  We grew them from seed in an inert potting medium and watered them with a solution containing all the nutrients needed for growth except phosphorus.

The seed germinated and produced a root and shoot.  The phosphorus required to make these tissues came from reserves stored in the seed.

When the stored phosphorus was used up, the plant continued to grow and make new leaves by initiating senescence in existing leaves and moving the phosphorus to where it was needed.

The high degree of stress developed by the plant hastened the onset of flowering.

Eventually the plant produced just one or two (viable) seeds, the development of which had been supported by senescence of all leaves.

So efficient is the recycling that virtually all of the phosphorus in the original seed made it through to the next generation seed in these plants.

Similar experiments have been done with other mobile nutrients like nitrogen.  More info in J L Dangl, RA Dietrich, H Thomas (2000) Senescence and programmed cell death. In: Biochemistry and Molecular Biology of Plants (eds B Buchanan, W Gruissem, R Jones) pp 1044-1100. Rockville: ASPP