The Mechanisms Underlying The Effect Of ABA Sensitivity On The Regulation Of Population Density In Arabidopsis

Population density regulation is central to ecology. As ensential links between individual-and population-level traits of species and ecosystem structure, dynamics and functions, the allometric mass-density relationships have drawn extensive attentions in recent years. Although the constant theories of scaling relationship, just as represented by the classic "-3/2" theory based on the simple geometry and recent "-4/3" theory based on the fractal branching distribution model, may meet the people’s innate propensity to persuit the generality of patterns and processes in nature, the flaws of the assumptions of these theory and emerging experience evidence for heterogenous scaling exponents have challenged the existence of constant laws in describing population dynamics. The competition for resources between individual is the driving force of system-level properties, therefore environmental characters are always invovled in population density regulations. Previous studies have demonstrated that the mass-density relationships varied across moisture gradients. However, the mechanisms underlying the variation were rarely touched. Abscisic acid (ABA) plays a critical role in regulating plant metabolism, growth, development and adaptive plasticity in response to stress. ABA sensitivity has been proved to change mass-density relationships. Whether the effects of ABA sensitivity on plant density regulation are in charge of the variable mass-density relationships across moisture gradient remains unsolved. Based on the studies on the regulation roles of ABA in plant growth and metabolism in response to environmental chagnges, which consequently affected population density regulations, we hypothesized that "ABA sensitivity may affect population mass-density relationships at different soil water level by regulating individual resource use strategy, biomass allocation pattern and nutrient absorption and use strategy in response to environmental changes".The availability of certain mutants with different ABA sensitivity, i.e. abil-1mutant (insensitive to ABA), eral-2mutant (hypersensitive to ABA) and their WTs (Ler-0and Col-0, respectively) provide us useful tools to test our hypothesis. The variety of traits in mutants at both individual-and population-levels under benign and drought scenarios will illustrate the effects of ABA sensitivity on individual physiological and morphological plasticity as well as population dynamics and construction in response to heterogeneous environments, providing physiological regulation mechanism underlying variable mass-density relationships in plant populations with increasing drought stress. The main results were as followed:1. The individual resource use efficiency (indicated by water use efficiency and leaf mass per area) of eral-2mutants were higher than that of abil-1mutants, leading to higher environmental capacity for individuals in eral-2mutant population under both benign and drought growth conditions. The self-thinning exponents of eral-2mutants were therefore higher than abi1-1mutants, and the self-thinning line were flatter in eral-2mutant population. The results indicated that ABA sensitivity could influence population dynamics by adjusting resource use efficiency under both benign and drought environment. Under drought treatment, the individual resource use efficiency of all mutants and WTs are higher than under benign growth conditions, leading to flatter self-thinning lines. However, the resource use efficiency of abil-1mutant was still in a relative lower level than eral-2mutant and WTs. The resource use efficiency and self-thinning exponent were not increased as high as eral-2mutant and WTs in response to drought stress, demonstrating that ABA sensitivity was determinant in resource use efficiency and self-thinning exponent in plant populations at different soil water level. It is consistent with function of ABA as a stress response hormone. ABA sensitivity regulated the scaling relation of plant biomass and metabolic rate in mutants and WTs in the same way, being the main cause of heterogeneous metabolic scaling exponents across moisture gradients. However, the variability of both self-thinning exponent and metabolic scaling exponent was not contradictory with the energy equivalence rule. In fact, the energy equivalence rule was established on the co-variation of the two exponents.2. The above-and belowground biomass patterns were different in abil-1mutants and eral-2mutant, eral-2mutant tended to allocate more biomass to the belowground part. The pattern of above-and belowground is generally different. The aboveground competition is size-asymmetric while the belowground competition is size-symmetric. The competition intensity of belowground competition was relatively lower than aboveground competition, leading to higher self-thinning exponent and flatter self-thinning lines. The scaling exponent of above-and belowground biomass allocation showed a strong negative linear relationship with self-thinning exponent, indicating the effect of ABA sensitivity on above-and belowground biomass allocation pattern and the consequent mass-density relationships. However, the above-and belowground biomass allocation patterns were more complex under drought scenarios, with no statistical relations with mass-density relationships. It evokes a further argument whether ABA sensitivity affected the mass-density relationship through mediating stomata behavior or biomass allocation pattern under drought.3. Nitrogen and phosphorus are important elements for plant growth and metabolism. The leaf N and P concentrations varied from abil-1mutants and eral-2mutant under both benign and drought environments. The concentrations were lower in eral-2mutant than in abil-1mutants, and the concentrations in eral-2mutant didn’t increased as high level as those in abil-1mutants. The results indicated that ABA sensitivity might influence the adoption and use strategy of N and P with increasing drought stress. However, there were no conserved scaling relationships of nitrogen with phosphorus in mutants and WTs under both benign and drought environment, which was in contrary to the generality of universal nitrogen/phosphorus scaling relationships.The main results confirmed our hypothesis, and ascertained the effects of ABA sensitivity on the variation of population mass-density relationships with increasing drought stress. This research may therefore provide a physiological mechanism to explain variable mass-density relationships in diverse environments.