Date Published: October 10, 2018
Publisher: Public Library of Science
Author(s): Norman Philipp, Heiko Weichert, Utkarsh Bohra, Winfriede Weschke, Albert Wilhelm Schulthess, Hans Weber, Aimin Zhang.
Two winter wheat (Triticum aestivum L.) populations, i.e. 180 genetic resources and 210 elite varieties, were compared in a field trial to analyse how grain number and grain yield distribution along the spike changed during the breeding process and how this associates to yield-related traits. Elites showed in average 38% more yield compared to resources. This breeding improvement mainly derived from an increase in grains and yield per spike in addition to grains and yield per spikelet. These increments corresponded to 19, 23, 21 and 25%, respectively. Not much gain in thousand grain weight (4%) was observed in elites as compared to resources. The number of spikelets per spike was not, or even negatively, correlated with most traits, except of grains per spike, which suggests that this trait was not favoured during breeding. The grain number and grain yield distributions along the spike (GDAS and GYDAS) were measured and compared by using a novel mathematical tool. GDAS and GYDAS measure the deviation of a spike of interest from the architecture of a model spike with even grain and yield distribution along all spikelets, respectively. Both traits were positively correlated. Elites showed in average only a 1% improvement in GDAS and GYDAS values compared to resources. This comparison revealed that breeding increased grain number and yield uniformly along the spike without changing relative yield input of individual spikelets, thereby, maintaining the general spike architecture.
Wheat (Triticum spp.) accounts for 30% of global grain production and for 45% of cereal nutrition, thus representing a major food crop species . In recent years the actual rate of wheat production increased by only 0.5% per year, which is much less than the required 1.4% that would be necessary to cope with a still growing human population [2, 3]. Therefore, improved wheat production must be achieved by further increasing the grain yield per area. However, the current increase of wheat yield decelerates and the harvest index approaches a theoretical limit. Moreover, the available genetic pool seems to be widely exhausted and modern breeding has further led to reduced genetic variability . In this respect, exploiting wheat genetic resources could be promising to overcome this drawback .
Comparing yield-related traits between genetic resources and elite varieties can help to assess and understand breeding progress and selection. In this study, we focused on the development, description and validation of two new agronomic traits, GDAS and GYDAS, based on two populations of 180 genetic resources and 210 elite varieties of winter wheat. With the data collected in this context, we were further able to draw conclusions in which extent grain number and grain yield distributions along the spike were changed during the breeding process and how this is associated to other yield-related traits. Please consider that all found associations are limited to one environment and may modulate in other environments. Therefore, we were carefully with their interpretation and relied on supporting literature.
Breeding essentially increased grain numbers per area in the elites by enhancing grains per spike and spikelets without much gain in TGW. More spikelets per spike were obviously not preferably selected because it may increase grains per spike but not yield and with potential compensations in TGW and/or grains per spikelet. While sink limitation was alleviated by allocating more assimilates to spikes, the uneven yield contribution of the individual spikelets remained stable. Limited success to improve spike architecture suggests that either this trait was not targeted by breeding or its maintenance reflects characteristic features of spike physiology and development, which are not easy to alter with concomitant gain in yield. Manipulating the uneven grain number or grain yield distribution of spikelets along the spike could generate yield benefits possibly by addressing assimilate loading, unloading within the vascular system and short distance transport within spike, rachis and spikelets. These features control and constrain transport capacities and competition between spikelets and grains.