Plant Architecture Plasticity And Its Response To Adverse Stresses In Dryland Triticeae Crops Under Artificial Selection Pressure

Evolvement of plant type in dryland Triticeae crops and its adaptation to adversity is one of the most important fundamental scientific issues. Domestication of Triticeae crops is mostly due to the result of artificial selection, and it is briefly categorized into two types including strong domestication type (mainly refers to as wheat and barley) and weak domestication type (mainly refers to as oat and triticale). In this paper, a group of studies were conducted on the basis of Darwin’s theory of evolution, Vavilov’s homologous variation law and biological metabolism theory using wheats and oats as representative crops. The test materials were chosen from Triticeae crops with genetically evolutionary relationship and various domestication gradients in the combination of experimental verification and meta-analysis methods. A series of eco-physiological and agronomic data were measured and recorded including the parameters of the variation in plant type, water use, yield formation, osmotic adjustment and gas exchange and so on, under adverse stresses including drought, high temperature&low light and high population density. The objectives of this study are to reveal the evolutionary characteristics of crop plant type, reconstruction direction of plant architecture, and the strategies of adaptation to adversity under the pressure of artificial selection. The results would provide new theoretical potential for cultivation management and breeding practice of dryland Triticeae crops. Major results were achieved as follows:1. The results from field experiments showed that plant architecture was evolved towards the tendency from a compact type in diploid wheats to an incompact type in tetraploid wheats, and further to more compact type in hexaploid wheats. Leaf morphological traits tend to become wider and shorter. Field experiment was designed to determine morphological changes in the upper three leaves in three different ploidy wheat species (two diploid MO1and MO4, two tetraploid DM22and DM31, and two hexaploid L8275and Monkhead). The results showed that basal angle, opening angle and cambering angle of three ploidy wheats ranged from22.1°to34.6°,30.0°to66.0°and19.0°to36.4°, respectively. In three ploidy wheats, tetraploid wheats had the highest values among these parameters. The opening angles of hexaploid wheats are greater than diploid wheats, but their basal angles and the cambering angles are lower than those of diploid wheats. Moreover, leaf length-width ratios of leaf, ranged from13.7to26.5, and they increased along with the rise in chromosome sets.2. Field experiments further suggested that source-sink relationship in the evolvement of dryland wheats from wild to modern varieties has undergone two important phases. The first phase was to enhance source dimension, and the second one was to strengthen sink dimension. Yet, leaf net photosynthetic rate has not been improved. With the similarity as the above results, the biomass of leaf, stem, sheath and aboveground biomass all increased significantly from diploid to tetraploid, but there were not significant difference in the transition from tetraploid to hexaploid. From diploid to tetraploid and hexaploid, population yield and harvest index increased, although root biomass decreased gradually. In addition, diploid wheats had the largest net photosynthetic rate, tetraploid wheat was at second place, and hexaploid had the lowest net photosynthetic rate.3. Pot-culture experiment illustrated that biomass allocation pattern experienced an upward shift "from down to up". Upward shift of biomass allocation was observed in three wheat species, in which the dry weight percentages of root, stem, leaf and ear in total biomass were43%,20%,10%and27%in diploid species,21%,34%,16%and29%in tetraploid species and14%,34%,14%and38%in hexaploid species respectively. The number of tiller was significantly greater in diploid species than that of either tetraploid or hexaploid species. Yield was increased from diploid to hexaploid (0.48to0.96g/plant); thousand seed weight was also increased from diploid to hexaploid. Allometric analysis of biomass allocation and water use with other parameters indicated that body-size scaling exponents of total biomass vs ear weight, stem weight, leaf weight, sheath weight and root weight were generally greater in wild relatives than those of hexaploidy wheats. The results verified that this allometry was weakened by artificial selection while characteristics of artificial populations had been enhanced.4. Results of meta-analysis indicated that plant type evolvement of dryland wheats was endowed with high-yield and water-saving characteristics, in which drought tolerance strategy was enhanced during the process of domestication of dryland wheat. A meta-analysis was made through collecting about800papers of drought adaptability in wheats which had been published over past decades. In meta-analysis, indexes of plant type including leaf area, plant height, tiller number and root length, decreased significantly for wild, old, and modern wheat genotypes under drought stress. In comparison with wild and old cultivars, modern wheat cultivars had the lowest reduction. Root biomass of modern wheats decreased under drought stress, but increased in wild and old wheats. Root to shoot ratio increased for all three species, and the old cultivars showed the highest value. Osmotic potential decreased significantly in three species, with the largest decline in modern wheats. WUE were both decreased for wild and old genotypes. In contrast, WUE for modern wheat cultivars increased under drought stress. Results illustrated that drought avoidance strategy become weakened during the process of domestication.5. Pot-culture experiment using different ploidy wheats was conducted to verify the results of meta-analysis. The results indicated that diploid and tetraploid wheats tended to take an adaptive strategy to avoid drought using increasing their root to shoot ratio (RSR), changing their leaf orientation (leaf angle) and decreasing leaf area. In contrast, modern hexaploid cultivars tended to improve their drought tolerance mainly through increasing osmotic adjustments and maintaining higher WUE and lower RSR, as well as higher photosynthetic capacity. Grain yield of modern cultivars has been improved remarkably during the domestication of dryland wheat. Under the pressure of artificial selection, the character of drought avoidance was gradually weakened, while drought tolerance ability was strengthened considerably.6. Tolerance ability was gradually enhanced during the process of artificial selection. In this study, we selected eight Triticeae varieties of different evolutionary gradients, and the plant material was imposed by high temperature and low light intensity. The results suggested that diploid and tetraploid crops had worse adaptability for abiotic stresses and also lower final yield. Yields of two diploid wheats MO1and MO4were0g/plant, and tetraploid wheats DM22and DM31were0.09and0.04g/plant respectively. Diploid and tetraploid cultivars were observed to reduce tiller number, and allocate more biomass to vegetative organs, so maintained individual survival through leaf rolling and leaf glaucous to avoid high temperature and low light intensity stresses. As a result of artificial selection, modern hexaploid crops and oats acquired better tolerance abilities to adapt high temperature and low intensity and keep higher maintenance ratio of final yield. Proline and soluble sugar contents of leaf were greater in these cultivars than those of other cultivars under stress conditions. Overall, highly-domesticated Triticeae cultivars displayed better adaptability to high temperature and low light conditions and maintained larger green leaf area and better water conservation status.7. Oats cultivars with different plant type responded to density stress in different adaptive mechanism. Two oat genotypes, Manotick with erect leaf type and Oa1316-1with prostrate leaf, were applied in four planting densities in a factorial experimental design with4replicates. Our data showed that yield, leaf area, chlorophyll content, plant height and stem diameter of both genotypes decreased significantly with increasing plant density. Biomass allocated to aboveground organs decreased while the biomass allocated to roots increased at higher density. Under high density stress (8and16plants per pots) conditions, compared to solid treatment, Manotick allocated 3-10%more biomass to the root system, produced50%more tillers, leading to higher number of non-productive tillers, and resulted in lower harvest index under the alternative arrangement. In contrast, the prostrate type Oa1316-1allocated proportionally more biomass to the panicles and stems, and less to roots. Consequently, fewer tillers produced in Oa1361-1. Our data indicates that yield differences between the two types of oats resulted from diverse life history strategies. With increasing plant density and strengthening plant-to-plant competition, Manotick reduced aboveground biomass allocation, which led to lower yield, while Oa1316-1decreased biomass allocation to the root, but increased biomass allocation to the stems and panicles under increasingly competitive environment. These adjustments in prostrate type genotypes maintained high and strong stems, ensured biomass allocation to reproductive components and achieved high final yield.To sum up, plant type domestication of Triticeae crop is evolved towards the high-yielding and high-efficiency direction with enhancing stress tolerance ability but weakening stress avoidance ability, and being apt to close planting circumstances. Future efforts would be focused on spatial reconstruction of leaf type, exertion of leaf photosynthetic rate and reducing growth redundancy. Due to high complexity of plant type evolvement and its adaptive mechanism for Triticeae crop, further work will be needed to improve stress avoidance ability and strengthen mutual advantages of plant type, finally to satisfy the pursuit of high yield and high population superiority in the future.