PHOSPHORUS-NUTRITION OF SPRING WHEAT (TRITICUM-AESTIVUM L) .1. EFFECTS OF PHOSPHORUS SUPPLY ON PLANT SYMPTOMS, YIELD, COMPONENTS OF YIELD, AND PLANT PHOSPHORUS UPTAKE

The effects of phosphorus (P) deficiency on plant symptoms, yield, and components of yield of wheat (Triticum aestivum L. cv. Halberd), P up take, and the distribution of dry weight within plants of variable P s tatus were examined in 2 glasshouse and 5 field experiments. Apart fro m stunted growth and depressed tillering, the symptoms of acute P defi ciency, most noticeable on older leaf blades, were equivocal; they wer e not always observed on acutely deficient plants and were absent on m oderately deficient plants. In glasshouse experiments, the leaves of a cutely deficient plants were spindly, erect: and dark green, whereas i n field experiments, the leaves were pale green. In acutely P-stressed plants, leaf senescence, phasic development, and anthesis were delaye d. The disorder restricted tiller development and therefore the rate o f appearance and the number of leaves per plant. It depressed grain yi eld principally by reducing the number of fertile tillers. Severe P de ficiency depressed shoot growth within 15 days of sowing and ultimatel y reduced plant height, root mass, and grain yield. In all experiments , shoot yield responses to applied P increased progressively until ste m elongation (Zadoks Scale 30) and changed little thereafter. As a res ult, the external requirement for P (i.e. P level required for 90% max imum growth) increased with time during vegetative development in most experiments. Severe P deficiency also affected the distribution of dr y matter between the roots and shoots and between the leaf blades and conducting tissues (sheaths and stems). Both of these responses intens ified with advancing plant age. Treatment differences in P uptake in s hoots also occurred early in growth and persisted until grain maturity . The partitioning of P between roots and shoots favoured P uptake or retention in the roots of P-deficient plants. Under conditions of acut e and moderate P stress, the resources of the wheat plant appear to be directed towards maintaining root growth (at least initially), limiti ng and delaying shoot proliferation, and maximising the leaf:stem rati o. These regulations appear circumstantially to be adaptive mechanisms for conserving sufficient P to ensure the survival of at least I weak , but fertile, tiller on each plant.