EFFECTS OF INCREASED CO2 CONCENTRATION AND TEMPERATURE ON GROWTH AND YIELD OF WINTER-WHEAT AT 2 LEVELS OF NITROGEN APPLICATION

Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environ ment for the 1990/1991 growing season at two levels each of atmospheri c CO2 concentration (seasonal means: 361 and 692 mumol mol-1), tempera ture (tracking ambient and ambient +4-degrees-C) and nitrogen applicat ion (equivalent to 87 and 489 kg ha-1 total N applied). Total dry matt er productivity through the season, the maximum number of shoots and f inal ear number were stimulated by CO2 enrichment at both levels of th e temperature and N treatments. At high N, there was a CO2-induced sti mulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. Th is contrasts with other studies, where positive interactions between t he effects of increases in temperature and CO2 have been found. Temper ature had a direct, negative effect on yield at both levels of the N a nd CO2 treatments. This could be explained by the temperature-dependen t shortening of the phenological stages, and therefore, the time avail able for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4-degrees-C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (-37%) than on yield (-18%), due to a compensatin g increase in average grain mass. At low N, despite increasing total c rop dry mass and the number of ears, elevated CO2 did not increase gra in yield and caused a significant decrease under ambient temperature c onditions. This can be explained in terms of a stimulation of early ve getative growth by CO2 enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.