Leaf Rust Resistance Gene Lr9 and Winter Wheat Yield Reduction: II. Leaf Gas Exchange and Root Activity

Abstract

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.