Effects of phosphorus nutrition on tiller emergence in wheat

Phosphorus (P) deficiency limits the yield of wheat, particularly by reduci ng the number of ears per unit of area because of a poor tiller emergence. The objectives of this work were to (i) determine whether tiller emergence under low phosphorus availability is a function of the availability of assi milates for growth or a direct result of low P availability, (ii) attempt t o establish a quantitative relation between an index of the availability of P in the plant and the effects of P deficiency on tiller emergence, and (i ii) to provide a better understanding of the mechanisms involved in tiller emergence in field-grown wheat. Wheat (Triticum aestivum L., cv. INTA Oasis ), was grown in the field under drip irrigation on a typic Argiudol, low in P (5.5 mu g P g(-1) soil Bray & Kurtz I) in Balcarce, Argentina. Treatment s consisted of the combination of three levels of P fertilization 0, 60 and 200 kg P2O5 ha(-1), and two levels of assimilate availability, a control ( non-shaded) and 65% of reduction in incident irradiance from seedling emerg ence until the end of tillering (shaded). Phosphorus treatments significant ly modified the pattern of growth and development of the plants. Shading re duced the growth and concentration of water-soluble carbohydrates in leaves and stems. Leaf photosynthetic rate at saturating irradiance was reduced b y P deficiency, but was not affected by shading. At shoot P concentrations less than 4.2 g P kg(-1) the heterogeneity in the plant population increase d with respect to the number of plants bearing a certain tiller. At a shoot P concentration of 1.7 g P kg(-1) tillering ceased completely. Phosphorus deficiency directly altered the normal pattern of tiller emergence by slowi ng the emergence of leaves on the main stem (i.e. increasing the phyllochro n), and by reducing the maximum rate of tiller emergence for each tiller.