Diversification of quantitative morphological traits in wheat

Abstract

Background and aims: The development and morphology of crop plants have been profoundly altered by evolution under cultivation, initially through unconscious selection, without deliberate foresight, and later by directed breeding. Wild wheats remain an important potential source of variation for modern breeders; however, the sequence and timing of morphological changes during domestication are not fully resolved.

Methods: We grew and measured 142 wheat accessions representing different stages in wheat evolution, including three independent domestication events, and compared their morphological traits to define the morphospace of each group.

Key results: The results show that wild and domesticated wheats have overlapping morphospaces, but each also occupies a distinct area of morphospace from one another. Polyploid formation in wheat increased leaf biomass and seed weight but had its largest effects on tiller loss. Domestication continued to increase the sizes of wheat leaves and seeds and made wheat grow taller, with more erect architecture. Associated changes to the biomass of domesticated wheats generated more grains and achieved higher yields. Landrace improvement subsequently decreased the numbers of tillers and spikes, to focus resource allocation to the main stem, accompanied by a thicker main stem and larger flag leaves. During the Green Revolution, wheat height was reduced to increase the harvest index and therefore yield. Modern wheats also have more erect leaves and larger flower biomass proportions than landraces.

Conclusions: Quantitative trait history in wheat differs by trait. Some trait values show progressive changes in the same direction (e.g. leaf size, grain weight), whereas others change in a punctuated way at particular stages (e.g. canopy architecture), and other trait values switch directions during wheat evolution (e.g. plant height, flower biomass proportion). Agronomically valued domestication traits arose during different stages of wheat history, such that modern wheats are the product of >10 000 years of morphological evolution.

Keywords: Green Revolution; Wheats; domestication; evolution; morphology; polyploidy; selective breeding; wheat (Triticum aestivum L.).

Fig. 1.

The morphospace occupied by wild and domesticated wheats, presented as a principal components analysis (PCA) for morphological traits during vegetative and reproductive phases. Smaller points correspond to individual plants, while larger points represent species means. (A) The colour coding distinguishes wild from domesticated and modern wheats. (B) The colour coding shows species, as indicated in the key. The black routes track the histories of three domesticated wheat lineages.

Fig. 2.

Diversity in morphology and architecture among wheat species. (A) Initial plant height. (B) Plant height at the end of vegetative growth. (C) Main stem diameter at the end of vegetative growth. (D) Leaf insertion angle on the main stem. Different letters above points indicate significant differences at P < 0.05 using Tukey’s multiple comparison test.

Fig. 3.

Diversity in the number of tillers and spikes among wheat species. For each species, the solid colour shows the final number of spikes at maturity, and the paler colour shows the maximum number of tillers that we observed during development. The difference between these values gives tiller loss, highlighted in the coloured boxes. The numbers correspond to trait values. Different letters above points indicate significant differences at P < 0.05 using Tukey’s multiple comparison test on the maximum number of tillers.

Fig. 4.

Diversity in flower proportion and biomass allocation at anthesis among wheat species. (A) Flower biomass relative to the whole above-ground biomass. (B) Biomass allocation to the flower, leaf and shoot (stem and leaf sheaths). The numbers show the biomass values for each tissue. Different letters above points indicate significant differences at P < 0.05 using Tukey’s multiple comparison test.

Fig. 5.

Diversity of leaf traits among wheat species. (A) Maximum leaf length. (B) Flag leaf width. (C) Individual leaf area (ELA), which is calculated using the maximum leaf length and width. (D) Individual leaf biomass. Different letters above points indicate significant differences at P < 0.05 using Tukey’s multiple comparison test.

Fig. 6.

Diversity of yield-related traits among wheat species. (A) Number of spikelets on the largest spike. (B) Number of grains on the largest spike. (C) Weight of grain on the largest spike. (D) Mean individual grain weight. (E) Expected final yield for one plant, considering tiller loss. (F) Expected harvest index. Different letters above points indicate significant differences at P < 0.05 using Tukey’s multiple comparison test.

Fig. 7.

Conceptual diagram of wheat evolution, showing trait changes at four evolutionary stages (polyploid formation, domestication, landrace improvement and the Green Revolution). The traits coloured orange have increased values, whereas those coloured purple have decreased values.