Bridging the gap between plant physiology and breeding : identifying traits to increase wheat yield potential using systems approaches

Part of the required increase in wheat production in currently high yielding wheat producing regions, like in the (sub-)tropics (ME1), will have to be obtained primarily by increased genetic wheat yield potential. Progress obtained by breeding to increase yield potential was substantial in the past, but is currently declining. Support from other disciplines to improve breeding efficiency is urgently needed. A systems approach was applied to identify processes that determine wheat growth and yield, and traits that need to be adapted to increase genetic yield potential for ME1 conditions. Increasing biomass through leaf nitrogen affords little opportunity to obtain gains under optimum management conditions. Growth analysis suggested the source capacity of wheat to exceed yield at higher yield levels, while remobilization of stem reserves was needed to supplement grain demand at lower yield levels. The latter aspect was confirmed in an analysis of stem reserve dynamics. Despite sufficient assimilate production, remobilization also occurred at higher yield levels, suggesting other processes, like wasteful respiration and negative feedback on photosynthesis, to become important. Also, higher amounts of residual reserves were found under higher than under lower yield levels. These results indicate that wheat yields under optimum conditions in ME1 are limited by sink capacity, as source capacity is abundant. Sink capacity can be increased by increasing kernel number or weight. Kernel number is determined between early booting and final anthesis, and depends on the duration and the crop growth rate of this period and on the allocation to the spike. Required adjustments to increase kernel number depend on enviromnental conditions. Competition among kernels for carbohydrates limited their growth, in particular those positioned further away from the rachis. This may be due to an inadequate transport system. Without competition, yields were assessed to increase at least one ton per hectare over a wide range of enviromnental conditions. It is concluded that increase in wheat yield potential under ME1 conditions should be obtained by increased sink capacity, which may be realized through the adaptation of the spike morphology. Sufficient genetic variation of this trait is at hand to be incorporated in a breeding program.