Jianming Deng

Insights into plant size-density relationships from models and agricultural crops

There is general agreement that competition for resources results in a tradeoff between plant mass, M, and density, but the mathematical form of the resulting thinning relationship and the mechanisms that generate it are debated. Here, we evaluate two complementary models, one based on the space-filling properties of canopy geometry and the other on the metabolic basis of resource use. For densely packed stands, both models predict that density scales as M−3/4, energy use as M0, and total biomass as M1/4. Compilation and analysis of data from 183 populations of herbaceous crop species, 473 stands of managed tree plantations, and 13 populations of bamboo gave four major results: (i) At low initial planting densities, crops grew at similar rates, did not come into contact, and attained similar mature sizes; (ii) at higher initial densities, crops grew until neighboring plants came into contact, growth ceased as a result of competition for limited resources, and a tradeoff between density and size resulted in critical density scaling as M−0.78, total resource use as M−0.02, and total biomass as M0.22; (iii) these scaling exponents are very close to the predicted values of M−3/4, M0, and M1/4, respectively, and significantly different from the exponents suggested by some earlier studies; and (iv) our data extend previously documented scaling relationships for trees in natural forests to small herbaceous annual crops. These results provide a quantitative, predictive framework with important implications for the basic and applied plant sciences.

Trade-Offs between the Metabolic Rate and Population Density of Plants

The energetic equivalence rule, which is based on a combination of metabolic theory and the self-thinning rule, is one of the fundamental laws of nature. However, there is a progressively increasing body of evidence that scaling relationships of metabolic rate vs. body mass and population density vs. body mass are variable and deviate from their respective theoretical values of 3/4 and −3/4 or −2/3. These findings questioned the previous hypotheses of energetic equivalence rule in plants. Here we examined the allometric relationships between photosynthetic mass (M p) or leaf mass (M L) vs. body mass (β); population density vs. body mass (δ); and leaf mass vs. population density, for desert shrubs, trees, and herbaceous plants, respectively. As expected, the allometric relationships for both photosynthetic mass (i.e. metabolic rate) and population density varied with the environmental conditions. However, the ratio between the two exponents was −1 (i.e. β/δ = −1) and followed the trade-off principle when local resources were limited. Our results demonstrate for the first time that the energetic equivalence rule of plants is based on trade-offs between the variable metabolic rate and population density rather than their constant allometric exponents.

Models and tests of optimal density and maximal yield for crop plants

We introduce a theoretical framework that predicts the optimum planting density and maximal yield for an annual crop plant. Two critical parameters determine the trajectory of plant growth and the optimal density, , where canopies of growing plants just come into contact, and competition: (i) maximal size at maturity, , which differs among varieties due to artificial selection for different usable products; and (ii) intrinsic growth rate, g, which may vary with variety and environmental conditions. The model predicts (i) when planting density is less than , all plants of a crop mature at the same maximal size, , and biomass yield per area increases linearly with density; and (ii) when planting density is greater than , size at maturity and yield decrease with −4/3 and −1/3 powers of density, respectively. Field data from China show that most annual crops, regardless of variety and life form, exhibit similar scaling relations, with maximal size at maturity, , accounting for most of the variation in optimal density, maximal yield, and energy use per area. Crops provide elegantly simple empirical model systems to study basic processes that determine the performance of plants in agricultural and less managed ecosystems.

A theoretical framework for whole-plant carbon assimilation efficiency based on metabolic scaling theory: a test case using Picea seedlings

Simultaneous and accurate measurements of whole-plant instantaneous carbon-use efficiency (ICUE) and annual total carbon-use efficiency (TCUE) are difficult to make, especially for trees. One usually estimates ICUE based on the net photosynthetic rate or the assumed proportional relationship between growth efficiency and ICUE. However, thus far, protocols for easily estimating annual TCUE remain problematic. Here, we present a theoretical framework (based on the metabolic scaling theory) to predict whole-plant annual TCUE by directly measuring instantaneous net photosynthetic and respiratory rates. This framework makes four predictions, which were evaluated empirically using seedlings of nine Picea taxa: (i) the flux rates of CO(2) and energy will scale isometrically as a function of plant size, (ii) whole-plant net and gross photosynthetic rates and the net primary productivity will scale isometrically with respect to total leaf mass, (iii) these scaling relationships will be independent of ambient temperature and humidity fluctuations (as measured within an experimental chamber) regardless of the instantaneous net photosynthetic rate or dark respiratory rate, or overall growth rate and (iv) TCUE will scale isometrically with respect to instantaneous efficiency of carbon use (i.e., the latter can be used to predict the former) across diverse species. These predictions were experimentally verified. We also found that the ranking of the nine taxa based on net photosynthetic rates differed from ranking based on either ICUE or TCUE. In addition, the absolute values of ICUE and TCUE significantly differed among the nine taxa, with both ICUE and temperature-corrected ICUE being highest for Picea abies and lowest for Picea schrenkiana. Nevertheless, the data are consistent with the predictions of our general theoretical framework, which can be used to access annual carbon-use efficiency of different species at the level of an individual plant based on simple, direct measurements. Moreover, we believe that our approach provides a way to cope with the complexities of different ecosystems, provided that sufficient measurements are taken to calibrate our approach to that of the system being studied.

Scaling the respiratory metabolism to phosphorus relationship in plant seedlings

There are empirical indications of an isometric scaling relationship between plants’ respiratory metabolism rates and nitrogen contents. To test the hypothesis that there may be a similar relationship between plants’ respiratory metabolism and phosphorus contents we used data obtained from 150 laboratory and field-grown seedlings representing 30 herbaceous species and 20 woody deciduous species. Our results show that whole-plant respiration rates strongly scaled to the 0.81-power of the whole-plant phosphorus content, across wide ranges of growth conditions and functional classifications. Moreover, we also found a similar scaling exponent between whole-plant respiration rates and total nitrogen contents for the same set of samples. The similarities of the metabolic scaling relationships suggest that similar mechanisms may be involved in the transport and storage of phosphorus and nitrogen in plants.

Variations between Diploids and Tetraploids of Allium przewalskianum, an Important Vegetable and/or Condiment in the Himalayas

Allium przewalskianum, a wild onion species growing at altitudes ranging from 1800 to 4500 m, has long been commonly used as an important vegetable and/or condiment by Tibetans, Indians, and Nepalese in the highlands of the Himalayas and adjacent regions. This species comprises both diploids and tetraploids. In this study, we examined the nutritional content and biomass accumulation profiles of two cytotypes, collected from 29 sites, with different altitudinal origins but cultivated in a common garden. On an average, this species has superior qualities in the minerals and amino acids compared to other edible congeners. When compared with the diploids, the tetraploids grew faster and accumulated more biomass; in addition, the tetraploids had higher values of moisture and energy, higher contents of cystine and phosphorus, but lower fiber levels. Moreover, the tetraploids from the higher altitudes had greater biomasses than the other tetraploids, in addition to having increased levels of proteins, fats, and the minerals Mg, Fe, Mn, and Cu. These results illustrate the large variations in nutritional efficacy and growth within this single morphological species, and provide critical information for its effective consumption in the future.

Response of three dominant shrubs to soil water and groundwater along the oasis-desert ecotone in Northwest China

The ecotone from oasis to desert is an important area for combating sandy desertification. Three dominant desert shrubs (Nitraria tangutorum, Calligonuum mongolicum, Haloxylon ammodendrori) were selected in Minqin Oasis, Northwest China, to determine the groundwater level; soil water potential; and change of the three shrubs in density, coverage, and biomass along the natural and seminatural oasis-desert ecotone (ODE), respectively. The results indicated that traits of desert plant interaction with the topsoil water and groundwater depth along the ODE play an important role in generating complex desert vegetation spatial dynamics. Some natural desert plant species with shallow root systems will distribute themselves according to distribution of topsoil water. Thus, the distribution of Nitraria tangutorum had a decreasing trend in distribution along ODE. Calligonuum mongolicum occurs in different trends in natural and seminatural ODE due to utilizing groundwater as well as topsoil water. Some plant species with deep roots, such as Haloxylon ammodendron, will show more degradation near oases and will exhibit an ascending trend along ODE. Therefore, it is of primary importance to protect the integrity of groundwater depth in order to protect the stability of the oasis-desert ecotones.

Spatial distribution of leaf form and the self-thinning exponent are affected by the sensitivity of the response to abscisic acid in anArabidopsis thaliana population

The spatial distribution of leaves is related to the exponent of the self-thinning relationship in plant populations. In this study, we evaluated the fractal dimension of rosette leaves of wild-type (WT)Arabidopsis thaliana and of an abscisic acid (ABA) -insensitive mutant (abi2-1) to test a model of the spatial distribution of leaf form in anArabidopsis population based on subdivision of a cube surrounding the leaf into uniform boxes and to investigate ABA’s affect on this model of the leaf. The values of the self-thinning exponent were -1.31 and -1.45 for WT andabi2-1. The mean dimensions of the box used to model the spatial distribution of leaf form, estimated using our model, were 2.08 and 2.03, respectively. By assuming that the box dimension equals the fractal dimension within the populations, the predicted self-thinning exponent equaled -1.40 for WT and -1.49 forabi2-1. When exogenous ABA was applied to both genotypes, the self-thinning exponent became -1.26 and -1.43 for WT andabi2-1, and the exponents predicted using the dimensions of the box were -1.37 and -1.46, respectively. The empirically predicted exponent equaled that predicted using the dimensions of the box (95% confidence interval). Empirical prediction of the spatial pattern using the two genotypes with and without ABA showed that ABA influenced the spatial form of the rosette leaves. Therefore, sensitivity to ABA can affect self-thinning through genetically determined changes in leaf form and its spatial distribution.

Ecogeographical variation of 12 morphological traits within Pinus tabulaeformis: the effects of environmental factors and demographic histories

Aims More data are needed about how genetic variation (GV) and environmental factors influence phenotypic variation within the natural populations of long-lived species with broad geographic distributions. To fill this gap, we examined the correlations among environmental factors and phenotypic variation within and among 13 natural populations of Pinus tabulaeformis consisting of four demographically distinct groups within the entire distributional range. Methods Using the Akaikes Information Criterion (AIC) model, we measured 12 morphological traits and constructed alternative candidate models for the relationships between each morphological trait and key climatic variables and genetic groups. We then compared the AIC weight for each candidate model to identify the best approximating model for ecogeographical variation of P. tabulaeformis. The partitioning of variance was assessed subsequently by evaluating the independent variables of the selected best models using partial redundancy analysis. Important Findings Significant phenotypic variation of the morphological traits was observed both within individual populations and among populations. Variation partition analyses showed that most of the phenotypic variation was co-determined by both GV and climatic factors. GV accounted for the largest proportion of reproductive trait variation, whereas local key climatic factors (i.e. actual evapotranspiration, AET) accounted for the largest proportion of phenotypic variation in the remaining investigated traits. Our results indicate that both genetic divergence and key environmental factors affect the phenotypic variation observed among populations of this species, and that reproductive and vegetative traits adaptively respond differently with respect to local environmental conditions. This partitioning of factors can inform those making predictions about phenotypic variation in response to future changes in climatic conditions (particularly those affecting AET).