Yield components and compensation in wheat: Opportunities for further increasing yield potential

There are some indications that wheat yields may be leveling off It may be necessary to analyze physiological processes to help identify opportunities for future breeding aimed at breaking the apparent barriers. There are various yield determining characteristics that can be considered, including those related to the crop's ability to intercept and use available radiation, to partition its dry matter into competing organs, and to absorb and use water and nutrients. We will concentrate on yield components and compensation, and discuss opportunities for manipulating phenological development as a tool for increasing yield potential in wheat. To analyze grain yield in terms of number of grains m- 2 (and its components) and average individual grain weight has been traditional. However, this approach was lately recognized as ineffective because these components are negatively correlated to each other, and the magnitude of the parameters of these relationships are variable. Thus, only by understanding the basis of these negative relationships would it be possible to manipulate numerical components aiming to obtain a particular yield target. An initial stop could be understanding phasic development, as there are relationships between the development of the apex and formation of yield components and stages that are most important in building yield potential. In this paper, we describe the initiation (and survival) of different yield components throughout wheat development, concluding that the formation of sub-components of number of grains m-2 overlaps somewhat and the negative relationships between these yield components could be attributed to some form of feedback processes, determining a sort of compensation between them. In contrast, the final weight of individual grains appears to be determined with only minimal overlapping with the determination of the number of grains m-2; thus negative relationships between major yield components may be different in nature. It is generally accepted that wheat yield is better related to grain number m-2 than to individual grain weight. Thus, a critical phase for yield potential, once the number of grains m-2 is established, has been recognized during the period of ca. 20-30 days immediately before anthesis; this indicates that only the narrow window of time coinciding with mortality of tillers and florets and with active growth of stems and s pikes appears to be of critical importance for the determination of final number of grains m-2