| Other Abstract | The relationship between plant-plant interaction and group yield improvement of crops is a core issue in ecological studies. Unlike natural plants adapted to the adverse environment by improving individual fitness, crop plants optimize group yield in the field with relatively more resources. Up to now, many researchers argue Donald’s view of improving group yield through weakening competitive ability (or reducing individual fitness). Alternatively, the mechanism of group yield improvement in monoculture is unclear. Besides, it is not a common way to increase crop yield through mixture. We are not sure whether there is possible for yield improvement by mixture. In this study, six dryland spring wheat cultivars (Triticum aestivum L.) which released in different ears were used, including three old cultivars [Heshangtou (HST), Jinbaoyin (JBY) and Dingxi24 (DX24)) and three new cultivars (Longchun30 (LC30), Ganchun27(GC27) and NABUQ6). The field experiment with monoculture and mixture were conducted at three resources levels (irrigation and fertilization): (i) low water and fertilization condition;(ii) middle water and fertilization condition;and (iii) high water and fertilization condition. These plots irrigated with 0-, 50- and 100-mm tap water in low, middle and high water and fertilization conditions, respectively. We fertilized before sowing with nitrogen (N) 80, 160 and 240 kg ha-1, and phosphorus (P) of 12, 24 and 36 kg ha-1, respectively. This field experiment was carried out to answer three questions: (i) whether the relationship between individual fitness and population grain yield is field resource dependent;(ii) Is there possible the grain yield in mixture plot higher than the highest grain yield in monoculture;and (iii) whether the crop breeding and neighbor relationship can affect the reproductive allometric relationship of spring wheat? The main findings of this study as follows:
1. The grain yield of spring wheat is increased with more water and fertilizer applied. The grain yield of old cultivars was higher than that of new cultivars at low water and fertilization condition, which positively correlated with aboveground biomass (P <0.01), grain number per m2. (P <0.01), and plant height (P <0.05). At high water and fertilization condition, grain yield was higher in new cultivars than old cultivars;and positively correlated with aboveground biomass and harvest index (P <0.05). At middle water and fertilization condition, the grain yield varied among new and old cultivars.
2. The individual fitness was estimated by individual grain yield in mixture, while population yield is measured in monoculture. At low water and fertilization condition, there was positive relationship between population yield and individual fitness (P <0.01). In contrast, the negative relationship was found at high water and fertilization condition, as well as no relationship in middle water and fertilization condition. We can speculate here the population yield of wheat in low and high water and fertilization conditions can be improved by increasing and reducing individual fitness, respectively. However, there was no relationship between population yield and individual fitness. Thus, there is no consistent relationship of population yield and individual fitness at varied water and fertilization conditions.
3. The expected value in mixture was calculated by performance in monoculture the seedling ratio in mixture, while observed value in mixture is the measured value. At three water and fertilizer conditions, the observed values of grain yield and aboveground biomass of the new cultivars were significantly lower than expected values (P <0.05), indicating the new cultivars paid the cost in mixture. The observed values of aboveground biomass and grain yield of the old cultivars were higher than expected values (P <0.05) at middle and high water and fertilization conditions (rather than low condition), showing that old cultivars gained in mixture.
4. At three water and fertilizer conditions, the observed value of grain yield in mixture was significantly lower than the expected value (P <0.05), indicating that the gains of old cultivars in mixture were less than the costs paid by new cultivars. The observed value of thousand-grain weight in mixture was lower than the expected value (P <0.1), while the number of grains does not change in most cases (except for middle water and fertilization condition).
5. The coefficient of variation(CV) of aboveground biomass and grain yield of old cultivars were smaller than that of new cultivars;it can be seen that the stability of old cultivars is higher than that of new cultivars in the fluctuating environment of water and fertilizer resources. The CV of the aboveground biomass and grain yield of spring wheat in mixture were higher than that of the old cultivars in monoculture, but lower than that of the new cultivars in monoculture;showing that the stability of spring wheat in mixture is moderate.
6. Analyze the allometric growth relationship between spring wheat grain biomass (R) and vegetative biomass (V) in monoculture and mixture. The R-V allometric index (the slope of log R –log V) of HST, JBY, DX24, LC30, GC27, and NABUQ6 in monoculture are 1.10, 1.14, 1.18, 1.31, 1.27, and 1.36, respectively. It can be seen that the selection of cultivars increased the R-V allometric index of dryland spring wheat. The R-V allometric index of the six spring wheat cultivars in mixture did not differentiate between the new and old cultivars, ranging from 1.12 to 1.30, with the lowest in HST (1.12), and the highest in DX24 and LC30 (1.30).
7. The neighbor relationship among individual plants is different in monoculture and mixture. Compared with monoculture, the R-V allometric index of two old cultivars was increased in mixture (JBY and DX24) (P <0.05), but had no effect on other cultivars (P >0.05). It can be seen that with the increasing of individual size, JBY and DX24 will distribute more dry matter into the grains in mixture;and the effect of mixture on the R-V allometric index of the new cultivars is significantly less than that of the old cultivars.
In summary, the relationship between individual fitness of dryland spring wheat and group yield is field resource dependent. The group yield in mixture plot was lower than the highest group yield in monoculture. The main reason is that the gains of old cultivars are not as good as the cost of new cultivars. the selection process of cultivars improves the R-V allometric index of new cultivars, while mixture is more likely to increase the R-V allometric index of old cultivars. It can be seen that monoculture is an effective way to increase the yield of densely planted crops. It can adjust individual fitness through crop breeding to achieve the optimal group yield in different environments.
Key words: spring wheat, individual fitness, monoculture, mixture, group yield, allometric relationship |
