Root Competition Of Two Spring Wheat Cultivars In Dry-farming Area Of The Loess Plateau

Barren soil and scarcity of moisture are limiting factors for wheat productivity in semi-arid area of the Loess plateau. We can offer a new thought of wheat production by means of consider and discuss about the origin of ecology for breeding at the same time of resolve this problem by irrigation. For origin of breeding, the target to breeding is grain yield (Reproductive output), and there have two key points:the first is production of group, it required containing the maximum of number of individual plants, the variety congruence sowing by high density will be paid close attention in general; the second point is grain yield (Reproductive output), and it required the variety has high harvest index (HI).All of individual plants constituted a population yield are selfish individuals, this phenomenon be called "tragedy of the commons" as a result of selfish individuals jostling for common resource (nutrient, moisture) acquisition. Water resources are scarce in this region and nutrient acquisition by water as an instrumentality. This will be an important research objective how reduce the tragedy of the commons generated by the root competition that effect on population production in above mentioned new thought of breeding. In this paper, we chose two spring wheat cultivars (Triticum aestivum) released in different years on semi-arid area in the capacity of test material, and quantify effect of root competition on spring wheat production in this region through interplant and intraplant root competition experiments, Pot experiment carry out under same light resources, same root growth space and soil nutrients, and set two water gradients (HW:90%FWC (Field water capacity), LW:40%FWC), four density gradients(1,4,7,10per half pot), planting methods(Owner:no interplant root competition, sharing individuals:with interplant root competition) and cultivars collocation (He Shangtou(HST); Long Chun8275(LC), HST&HST; HST&LC; LC&LC). Field experiment carry out in homogeneous farmland, we only set three treatments (monoculture and mixture, HST; HST&LC; LC), the result and conclusion of experiments as follows:1. Trend of growth of wheat total biomass is a sigmoid ("S") curve. The owner and sharing of planting methods can’t effect on the total biomass significantly in same cultivar and water gradient, but the water level can do it. In the same water gradient and growth period (excluding seedling period, the root structure is not completely in seeding period, the root competition has not appeared), the aboveground biomass is not effected significantly from different planting methods in each cultivar, however, the aboveground biomass in Sharing have a decreasing tendency compere with that in Owner. The underground biomass in Sharing significantly greater than in Owner in each cultivar (same water gradient and growth period). The effect of water gradient on plant root-shoot ratio isn’t significant, in same water gradient and growth period (excluding seedling period, the root structure is not completely in seeding period), the plant root-root ratio in Sharing is significantly greater than in Owner. The water factor is the main factor for limited the total biomass of target plant, in the situation of fixed resources, plant root competition effect on allocation of plant biomass.2. The single plant biomass of each cultivar express a characteristic of decreasing function in density gradients of same water level. As the increased in densities, the single plant biomass and aboveground biomass are decreased significantly. It is find that the water and plant density significant effect on the single plant biomass, however, in the condition of low water level and high planting density, LC8275in Owner have higher single plant aboveground biomass than in Sharing. Even though "the tragedy of the common" generated by root competition in Sharing, it can get higher single plant aboveground biomass than HST when it in low resources condition. The plant in Sharing have higher root-shoot ratio than in Owner, and the root-shoot ratio of LC8275smaller than HST in same condition.3. The interplant competition of spring wheat cultivars in dry-farming area appeared characteristic of decreasing function as same as the intraplant competition (each cultivar in same water level and planting method), the difference is HST in Sharing with low densities can get more total biomass though hold partner’s resources, and can reach significant levels. Those can explain that in sharing individuals, root competition generate "the tragedy of the common". This tragedy can’t remove absolutely in farm ecosystem, we just can relatively reduce the degree of tragedy and the cause of tragedy.4. The tragedy of the commons generate by root competition is same between interplant and intraplant root competition experiments, but inconsistent on degree. Pot experiments and field experiments shows the total biomass size relations is HST-S-HST> HST-S-LC> LC-S-LC in low-density treatments. In contrast, the total biomass size relations is HST-S-HST<HST-S-LC<LC-S-LC in high-density. The size relations of field production as same as that in high density, HST-S-HST(198.7g))<(HST-S-LC(240.3g))<(LC-S-LC(318.3g). On account of main result above mentioned, the variety which have strong competitive will generate more huge tragedy of the common than the variety which have weak competitive. And smaller root system is a powerful way to reduce the tragedy of the commons.Wheat breeding process is the process of weakening the individual competition. From the aspect of phenotypic plasticity, artificial selection is a process of phenotypic plasticity becomes high. When yield achieve the maximum threshold provided from the field by wheat amelioration and breeding to target plant, we only make better the relationship of among selfish individual plants to bring about a high harvest index.Roots research is very difficult, the phenomenon of root competition, root architecture, rhizosphere interactions and root recognition, root growth regulation and root stress response are not clearly, and these aspects require further study.