Effects of phosphate and nitrogen application on death of the root cortex in spring wheat

Programmed senescence and relative sink‐strength have both been proposed as mechanisms controlling cell death in the root cortex. To distinguish between the two, seedlings of spring wheat (Triticum aestivum L.) were grown for 23 d in a glasshouse, with five rates of phosphorus and three rates of nitrogen, and cortical cell death over the length of the first seminal root axis assessed after staining with acridine orange. The percentage of cortical cell death was greatest in plants moderately deficient in phosphorus and was less in plants receiving severely deficient or adequate‐to‐luxury supplies. Nitrogen supply did not affect the percentage of anucleate cells in the cortex. Both phosphorus and nitrogen supply influenced the distribution of cortical death with distance along the root except in regions close to the root tip. Nitrogen supply, however, had no effect on the extent of cortical cell death in regions of comparable age, except in plants receiving twice the level of phosphorus required for maximum plant growth. Phosphorus supply influenced the extent of cortical death through effects on both the ratio of root to shoot weight, and the length of the total root system. These findings cannot be completely explained by the process of natural programmed senescence, but are compatible with the theory that cortical cell death is related to the distribution or activity of assimilate sinks within the root.