DIFFERENCES BETWEEN WHEAT CULTIVARS IN ACQUISITION AND UTILIZATION OFPHOSPHORUS

In an attempt to evaluate whether breeding and selection for high yiel ding capacity changed the P requirement of modern wheat cultivars, the response of two wheat cultivars to different levels of P supply was i nvestigated. A traditional cultivar ('Peragis') and a modern spring-wh eat cultivar ('Cosir') were cultivated in a C-loess low in available P and high in CaCO3 in 120 cm high PVC tubes. In addition and for compa rison, nutrient solution experiments were also conducted. Shoot growth , root growth, P uptake, P translocation and P distribution within the shoot at different developmental stages were compared. The grain yiel d of the modern cultivar 'Cosir' was higher at limiting and non-limiti ng P supply and, therefore, this cultivar can be considered as more P- efficient than the traditional cultivar. Grain yield reduction at low P supply was mainly due to an inhibition of tillering and thus lower n umber of ears per plant, whereas the number of grains per ear was hard ly affected. Reduced tillering at low P supply could not be related to P concentrations in the shoot meristematic tissues which were general ly much higher than in other plant tissues and kept at an elevated lev el even at limiting P supply. Root branching (1(st) order laterals) wa s reduced at limiting P supply in 'Cosir' but not in 'Peragis' which, generally, had lower numbers of laterals at the beginning of tillering . From the results it can be concluded that the main factors contribut ing to the higher P efficiency of the modern cultivar 'Cosir' are (i) efficient use of assimilates for root-growth characteristics which enh ance P acquisition; enhanced root branching and thus smaller mean root diameter and longer root hairs, (ii) an efficient P uptake system, (i ii) efficient remobilization of P from vegetative plant organs to the grains, and most importantly (iv) lower P requirement for grain yield formation because of lower ear number per plant but higher grain numbe r per ear.