The introduction of resistance genes from wild relatives into wheat (Triticum aestivum L.) may have detrimental effects on yield. In earlier experiments, we observed a 12% yield reduction in leaf ...rust (Puccinia recondita Rob. ex Desm. f. sp. tritici) resistant near isogenic lines (NIL) carrying the resistance gene Lr9 compared with the susceptible recurrent parent Arina. The aim of this study was to find physiological mechanisms for this yield reduction. Two NIL and Arina were investigated in field experiments. In a first experiment, CO2 exchange rate (CER), transpiration, stomatal conductance (gs), and leaf intercellular CO2 concentration (Ci) of the flag leaf were determined after ear emergence. In a second experiment, root activity was estimated from the beginning of stem elongation to early dough stage by the application of Rb and Sr at two soil depths. The NIL and Arina had a similar CER in the morning, but in the afternoon, CER was 7 to 20% lower for the NIL compared with Arina. Differences between the NIL and Arina were larger under dry soil conditions. The CER was closely correlated with gs and Ci indicating that CO2 uptake was restricted by gs. Roots of the NIL took up 15 to 20% less Sr at the 25- to 40-cm soil depth and 12% less Rb at 10-cm soil depth compared with Arina. The reduced Sr and Rb uptake indicated less root activity for the NIL. It is concluded that lower root activity in the resistant NIL led to water stress and reduced CO2 assimilation. Such stress periods may reduce grain number and grain size of the rust resistant NIL.
Leaf rust (Puccinia recondita Rob. ex Desm. f. sp. tritici) resistant near isogenic lines (NIL) of wheat (Triticum aestivum L.) have generally lower yields than the susceptible recurrent parent under ...disease free conditions. Analysis of growth and yield should increase our understanding of the reduced yielding ability of NIL. In a 3-yr field study, the yield components of the leaf rust susceptible cultivar Arina were compared with six resistant NIL carrying the resistance gene Lr9 introduced from Aegilops umbellulata. The NIL were derived from two independently developed backcross populations with Arina as the recurrent parent. The NIL were phenotypically very similar to Arina. At anthesis and at medium milk stage, the total aboveground dry matter was similar for the NIL and Arina. Differences in the dry matter accumulation appeared only after the medium milk stage. At maturity, the NIL had a 12% lower grain yield than Arina (5.88 t ha-1) with a range from 5 to 14%. The lower grain yield resulted from a 3 to 11% reduction in grain number per square meter and a 2 to 7% reduction in mean grain weight. The smaller grain number per square meter was the result of a 6% smaller tiller number per plant and a 2% smaller grain number per ear. Artificially reducing grain number per ear suggested that both a reduced supply of assimilates and a diminished capacity to incorporate assimilates in the grain caused the reduced grain yield of the NIL. Deleterious effects of alien genes linked with the Lr9 gene from Aegilops umbellulata cannot be ruled out. However, the small differences within and between the NIL of the two Lr9 populations for the parameters tested suggest a direct relation between the yield depression and the leaf rust resistance conferred by the Lr9 gene
