Plant assimilation and nitrogen cycling

Nitrogen, an abundant and yet limiting nutrient for crop and food production, enters the plant as nitrate or ammonium, or as dinitrogen through biological fixation by procaryotes associated with the plant. Nitrogen incorporation into the soil-plant-animal system is ultimately restricted by rates of biological and industrial fixation. Biological fixation conserves fossil fuel, but fertilization is preferred in most present agriculture. Nitrogen-metabolism research has the practical objectives of allowing more efficient N-fertilizer utilization by plants, including those that fix N₂ but benefit from fertilizer_N supplements. Nitrogen accumulation by harvested crops results in changes in soil acidity, with the direction of change depending on the N-source. There is little information on long-term effects of crop N-nutrition on acidity, and acidity is a critical factor that affects agricultural productivity in many tropical soils. Thus, plant control of pH and the acid/base balance in the soil as a consequence of nitrogen uptake and assimilation are important areas of future research.     

The sensitivity of shoot growth of corn to the least limiting water range of soils

The least limiting water range (LLWR), the range in soil water content within which limitations to plant growth associated with water potential, aeration and mechanical resistance to root penetration are minimal, has been proposed as an index of the structural quality of soils for crop growth. An hypothesis that is implicit in the proposed use of LLWR as an index of soil structural quality is that crop growth is negatively related to the proportion of the total number of measurements in which the water content falls outside the LLWR (p_out) and therefore given a certain climate positively related to the magnitude of the LLWR. The objective of this investigation was to test the hypothesis that plant response, specifically shoot growth rate of corn (Zea mays L.), can be functionally related to p_out and the LLWR of soils. The study was carried out on a farm with a side-by-side comparison of no-till and conventional-till. Thirty two paired sampling sites were located along the two transects. The LLWR and p_out were calculated for the 0–20 cm depth in each sampling site. Shoot growth rate (SGR) was measured during a 17 and 16 day period in 1992 and 1993, respectively that corresponded to the 10–11 leaf stage. Although the variation in p_out accounted for a larger percentage of the variation in SGR than did LLWR, the correlation between SGR and LLWR was high and justifies further studies to determine if crop yield can be related to LLWR under different soil and climatic conditions.     

Using the expolinear growth equation for modelling crop growth in year-round cut chrysanthemum

The aim of this study was to predict crop growth of year-round cut chrysanthemum (Chrysanthemum morifolium Ramat.) based on an empirical model of potential crop growth rate as a function of daily incident photosynthetically active radiation (PAR, MJ m-2 d-1), using generalized estimated parameters of the expolinear growth equation. For development of the model, chrysanthemum crops were grown in four experiments at different plant densities (32, 48, 64 and 80 plants m-2), during different seasons (planting in January, May-June and September) and under different light regimes [natural light, shading to 66 and 43 % of natural light, and supplementary assimilation light (ASS, 40-48 micro mol m-2 s-1)]. The expolinear growth equation as a function of time (EXPOT) or as a function of incident PAR integral (EXPOPAR) effectively described periodically measured total dry mass of shoot (R2 > 0.98). However, growth parameter estimates for the fitted EXPOPAR were more suitable as they were not correlated to each other. Coefficients of EXPOPAR characterized the relative growth rate per incident PAR integral [rm,i (MJ m-2)-1] and light use efficiency (LUE, g MJ-1) at closed canopy. In all four experiments, no interaction effects between treatments on crop growth parameters were found. rm,i and LUE were not different between ASS and natural light treatments, but were increased significantly when light levels were reduced by shading in the summer experiments. There was no consistent effect of plant density on growth parameters. rm,i and LUE showed hyperbolic relationships to average daily incident PAR averaged over 10-d periods after planting (rm,i) or before final harvest (LUE). Based on those relationships, maximum relative growth rate (rm, g g-1 d-1) and maximum crop growth rate (cm, g m-2 d-1) were described successfully by rectangular hyperbolic relationships to daily incident PAR. In model validation, total dry mass of shoot (Wshoot, g m-2) simulated over time was in good agreement with measured ones in three independent experiments, using daily incident PAR and leaf area index as inputs. Based on these results, it is concluded that the expolinear growth equation is a useful tool for quantifying cut chrysanthemum growth parameters and comparing growth parameter values between different treatments, especially when light is the growth-limiting factor. Under controlled environmental conditions the regression model worked satisfactorily, hence the model may be applied as a simple tool for understanding crop growth behaviour under seasonal variation in daily light integral, and for planning cropping systems of year-round cut chrysanthemum. However, further research on leaf area development in cut chrysanthemum is required to advance chrysanthemum crop growth prediction.     

Agronomic and environmental aspects of diazotrophic bacteria in rice fields

This article provides an overview of the free-living and plant-associated nitrogen (N)-fixing bacterial communities in wet rice fields, with a focus on describing the elements affecting community assemblages in this waterlogged soil–plant system. Nitrogen is a crucial nutrient for rice yield and growth. Characteristics of the rice paddy ecosystem promote N-fertilizer losses, resulting in negative impacts on the environment. Public concerns on sustainable rice crop production and food security have accentuated interest in exploring biological supplementary nitrogen sources. Biological N-fixation is a significant source of the nitrogen in agroecosystems. The nitrogen requirement of rice crops can be partly remedied by managing and promoting the activities of N-fixing microorganisms. These changes are leading towards a cleaner approach that maintains sustainability while simultaneously improving crop production targets. The use of N-fixing microorganisms as biofertilizers and the factors driving the success of this technology in wet rice paddies are also discussed.     

Computing competition for light in the GREENLAB model of plant growth: a contribution to the study of the effects of density on resource acquisition and architectural development

BACKGROUND AND AIMS: The dynamical system of plant growth GREENLAB was originally developed for individual plants, without explicitly taking into account interplant competition for light. Inspired by the competition models developed in the context of forest science for mono-specific stands, we propose to adapt the method of crown projection onto the x-y plane to GREENLAB, in order to study the effects of density on resource acquisition and on architectural development. METHODS: The empirical production equation of GREENLAB is extrapolated to stands by computing the exposed photosynthetic foliage area of each plant. The computation is based on the combination of Poisson models of leaf distribution for all the neighbouring plants whose crown projection surfaces overlap. To study the effects of density on architectural development, we link the proposed competition model to the model of interaction between functional growth and structural development introduced by Mathieu (2006, PhD Thesis, Ecole Centrale de Paris, France). KEY RESULTS AND CONCLUSIONS: The model is applied to mono-specific field crops and forest stands. For high-density crops at full cover, the model is shown to be equivalent to the classical equation of field crop production (Howell and Musick, 1985, in Les besoins en eau des cultures; Paris: INRA Editions). However, our method is more accurate at the early stages of growth (before cover) or in the case of intermediate densities. It may potentially account for local effects, such as uneven spacing, variation in the time of plant emergence or variation in seed biomass. The application of the model to trees illustrates the expression of plant plasticity in response to competition for light. Density strongly impacts on tree architectural development through interactions with the source-sink balances during growth. The effects of density on tree height and radial growth that are commonly observed in real stands appear as emerging properties of the model.     

Analysis and modelling of effects of leaf rust and Septoria tritici blotch on wheat growth

A model to predict Septoria tritici blotch (STB) and leaf rust effects on wheat growth was constructed and evaluated in two steps. At the leaf scale, Bastiaans' approach that predicts the relative photosynthesis of a wheat leaf infected with a single disease, was extended to the case of two diseases, one biotrophic and one necrotrophic by considering the leaf rust-STB complex. A glasshouse experiment with flag leaves inoculated either singly with one disease or with two diseases combined was performed to check the leaf damage model. No interaction of the two diseases on photosynthesis loss was observed when they occurred simultaneously on the same leaf. In a second step, the single-leaf model was extended to the canopy scale to model the effects of the leaf rust-STB complex on the growth of a wheat crop. The model predicts the effects of disease on the growth of an affected crop relative to the growth of a healthy crop. The canopy model accounted for different contributions to photosynthetic activity of leaf layers, derived from their position in the canopy and their natural leaf senescence. Treatments differing in nitrogen fertilization, microclimatic conditions, and wheat cultivars were implemented in a field experiment to evaluate the model. The model accurately estimated the effect of disease on crop growth for each cultivar, with differences from experimental values lower than 10%, which suggests that this model is well suited to aid an understanding of disease effects on plant growth. A reduction in green leaf area was the main effect of disease in these field experiments and STB accounted for more than 70% of the reduction in plant growth. Simulations suggested that the production of rust spores may result in a loss of biomass from diseased crops and that stem photosynthesis may need to be considered in modelling diseased crop growth.     

Biomass Accumulation and Main Stem Elongation of Durum Wheat Grown under Mediterranean Conditions

Detecting and exploiting genetic variation in biomass accumulationis of great importance for increasing wheat yield when the harvestindex is close to its upper limit. This study was undertakento analyse the pattern of biomass accumulation and main stemelongation in 25 durum wheat (Triticum turgidum L. ‘Durum’)genotypes. Field experiments were conducted over 2 years intwo environments contrasting in the amount of available water,in northeastern Spain. Plants were sampled at the main stagesof Zadoks' scale, and dry weight per plant, crop dry weight(CDW) and main stem length were measured at each stage. Measurementsfor growth traits and thermal time from sowing fitted betterto an asymmetric logistic peak curve than to the Richards logisticmodel. Four biological variables were computed from the curve.Differences among curves describing changes in biomass werefound to be greater between irrigated and rainfed sites thanbetween years. Drought stress had less effect on main stem elongationthan on biomass accumulation. Average dry weight per plant andCDW were reduced by drought by 42 and 38%, respectively, duemainly to similar reductions in the mean rate of growth of thetwo variables. In contrast, cycle length from sowing to themaximum values of dry weight per plant and CDW was only slightlymodified by drought. Copyright 2001 Annals of Botany Company Triticum turgidum L. ‘Durum’, durum wheat, biomass, crop dry weight, stem length, rate of growth, modelling, growth analysis, logistic peak curve     

Drainage and nitrate leaching in a crop rotation under different N-fertilizer strategies: application of capacitance probes

Quantification of drainage and nitrate leaching from cropping systems is necessary to optimize N-fertilizer application and determine the impact on groundwater quality. The objectives of this work were to (i) assess the use of capacitance probes for the continuous determination of the volume of drainage water and the amount of nitrate leached in a crop production system, and (ii) compare the effect of different N-fertilizer strategies to control nitrate leaching in a crop rotation in humid Mediterranean climate. A factorial (control and three fertilizer strategies) experiment was conducted during three cropping seasons in Navarra (Spain). Wheat (Triticum aestivum L.) was planted in 2002, barley (Hordeum vulgare L.) in 2003, and rapeseed (Brassica napus L.) in 2004. Daily soil water content measurements based on capacitance probes were used to calculate drainage at 1 m depth, by applying the water balance equation. Nitrate leaching was calculated as the drainage volume multiplied by the nitrate concentration of the soil solution extracted in ceramic cups. The results revealed distinct behaviour in three crop phases, viz.: (i) from planting to GS-25, with high risk of drainage and nitrate leaching, (ii) from GS-25 to the end of the drainage period, with little drainage and leaching, and (iii) from then to harvest, when no drainage or nitrate leaching took place. Drainage and soil mineral N content before planting were the main factors determining the amount of N leached. Splitting N-fertilizer application and the use of nitrification inhibitors are not likely to have a significant impact on subsequent N-leaching losses, provided that the N-fertilizer application is adjusted to crop N needs corrected by soil N supply.     

Variation in and correlation between intrinsic rate of increase and carrying capacity

Intrinsic population growth rate and density dependence are fundamental components of population dynamics. Theory suggests that variation in and correlations between these parameters among patches within a population can influence overall population size, but data on the degree of variation and correlation are rare. Replicate populations of a specialist aphid (Chaetosiphon fragaefolii) were followed on 11 genotypes of host plant (Fragaria chiloensis) in the greenhouse. Population models fit to these census data provide estimates of intrinsic growth rate and carrying capacity for aphid populations on each plant genotype. Growth rate and carrying capacity varied substantially among plant genotypes, and these two parameters were not significantly correlated. These results support the existence of spatial variation in population dynamic parameters; data on frequency distributions and correlations of these parameters in natural populations are needed for evaluation of the importance of variation in growth rate and density dependence for population dynamics in the field.     

Inherent trait differences explain wheat cultivar responses to climate factor interactions: New insights for more robust crop modelling

Climate change predictions foresee a combination of rising CO₂, temperature and altered precipitation. Effects of single climatic variables are well defined, but the importance of combined variables and genotypic effects is less known, although pivotal for assessing climate change impacts, for example, with crop growth models. This study provides developmental and physiological data from combined climatic factors for two distinct wheat cultivars (Paragon and Gladius), as a basis to improve predictions for climate change scenarios. The two cultivars were grown in controlled climate chambers in a fully factorial setup of atmospheric CO₂ concentration, growth temperature and watering regime. The cultivars differed considerably in their developmental rate, response pattern and the parameters responsible for most of their variation. The growth of Paragon was linked to climatic effects on photosynthesis and mainly affected by temperature. Paragon was overall more negatively affected by all treatment combinations compared to Gladius. Gladius was mostly affected by watering regime. The cultivars' acclimation strategies to climate factors varied significantly. Thus, considering a single factor is an oversimplification very likely impacting the accuracy of crop growth models. Intraspecific crop variation could help understanding genotype by environment variation. Cultivars with high phenotypic plasticity may have greater resilience against climatic variability.     

Effect of nitrogen supply and Azospirillum brasilense Sp-248 on the response of wheat to seawater irrigation

Response of wheat to Azospirillum brasilense Sp-248 inoculation with different N-fertilizer levels using seawater irrigation was investigated. All inoculated treatments increased plant height, shoot and root dry weight, and tiller number in compared with uninoculated treatments. Yield parameters measured were also increased due to the inoculation. In terms of the effect of saline irrigation, there were no significant differences in growth and yield parameters in plants treated with tap water and others irrigated with 8.0% seawater concentration. This would indicate a relatively high tolerance of A. brasilense to saline irrigation and its ability to reduce the deleterious effects of saline on growth by increasing the plant’s adaptation. However, increasing the seawater concentration in the irrigation water to 16.0% significantly decreased all tested parameters. Inoculation treatments generally increased NPKCa contents and decreased sodium ratio of the grains in compared with the uninoculated treatments. Overall results clearly revealed that the Azospirillum inoculation saved about 20 units of N-fertilizer and that saving was made economically feasible by decreasing the chemical fertilizers needed, improving the nitrogen content and counteracting the effects of salinity.     

Growth Rate and % N of Field Grown Crops: Theory and Experiments

Mechanistic equations were developed to link the % N in plantdry matter to plant mass per unit area and to various expressionsof growth rate. They were tested against the results of multi-harvest,multi-N-level fertilizer field experiments on French beans,potatoes, tall fescue and winter wheat. Sub-critical values of % N measured at different times duringthe growing period were for each crop almost linearly relatedto relative growth rate, but the values of the regression coefficientsvaried from experiment to experiment. A single equation was developed that denned the % N in the drymatter at any time with any sub-optimal level of fertilizer-N,in terms of the plant d.wt per unit area of N-sufficient plantsmeasured at that time, and the ratio of the growth rate coefficient(see below) of the N-deficient crop to that of the N-sufficientcrop. Even though there were no other variables, the equationremoved 86.5% of the variance of all the measured sub-optimaland critical values of % N for the different crops. The growthrate coefficient K_x(F) was defined by the eqn:     

Quantitative genetic analysis of plant growth: biases arising from vegetative propagation

Propagation through vegetative cuttings is a widely used technique that may bias estimates of genetic and environmental effects on plant growth. Leafy stem cuttings from 210 genotypes from eight populations of Salix pulchra were rooted and raised at three levels of nitrogen availability. Cuttings showed a complex suite of responses to vegetative propagation. Population and/or genotypic variation in response to vegetative propagation was observed in (1) retention of leaves during rooting, (2) date that cuttings resumed shoot growth after rooting, and (3) the frequency of cuttings that remained shoot dormant throughout the experiment. Nitrogen treatments also caused different responses to vegetative propagation, influencing date that cuttings resumed shoot growth and frequency of shoot dormancy. Because each of these responses had a direct effect on final plant size, I concluded that final size was a product of both differences among genotypes and treatments in plant growth rate, as well as genotype- and treatment-specific responses to vegetative propagation. This study shows that plant growth experiments can be designed to quantify responses to vegetative propagation and statistically remove these artifacts of propagation from estimates of genetic and environmental effects on plant growth.     

Spore production and mycorrhizal development in various tropical crop hosts infected with Glomus clarum

Plant growth, mycorrhizal development and vesicular arbuscular spore production were examined in five tropical crop host species inoculated with Glomus clarum and grown in a glasshouse. In one of the two experiments, sequential harvests of maize, sorghum and chickpea were made in order to study spore production in relation to plant growth and mycorrhizal development. Spore numbers in each of these hosts increased at a fairly constant rate until maximum plant dry weight, when spore production ceased. Sorghum and maize produced considerably more spores than chickpea, with spore numbers being closely correlated with mycorrhizal root length. In the second experiment, Glomus clarum was cultured on each of maize, millet, sorghum, groundnut and chickpea for three consecutive generations before cross-inoculation of the spores from each host onto all five hosts. Sporulation with respect to host size was generally greatest when the inoculum used to infect a host had been produced on that host. The growth-promoting effects of the fungus were not influenced by the source of the inoculum. More spores were produced on the cereals than the legumes. Differences in spore numbers amongst hosts and plant generations were apparently influenced mainly by infected root length and by the growth period.     

Growth and competition in clonal plants-persistence of shoot populations and species diversity

This paper reviews studies on growth and size-structure dynamics of shoots and clones in clonal plants in comparison with those in non-clonal plants, and discusses the characteristics of clonal plants. The mode of competition between individuals (symmetric versus asymmetric, degree of competitive asymmetry), growth dynamics of individuals, allocation pattern between organs and spatial pattern of individuals are closely correlated with each other in non-clonal plant populations. Theoretical and field studies based on the diffusion model revealed that plants of “height-growth” type (mostly early-successional tree species) and plants of “diameter-growth” type (mostly late-successional tree species) tend to exhibit asymmetric competition and symmetric competition respectively. Moreover, asymmetrically competing plants show smaller effects of variation in individual growth rate and spatial pattern on the size-structure dynamics of the population than symmetrically competing plants. Thefefore, the spatial pattern of inviduals should be considered especially for plants undergoing symmetric competition. These results for non-clonal plants should have a significant implication also for the growth dynamics and competition in clonal plants. The mean growth rate of shoots [G(t,x) function] and hence the mode of competition between shoots differs among clonal plant species as in non-clonal plants. However, a large magnitude and size-independence (or slightly negative size-dependence) of the variation in growth rate of shoots [D(t,x) function], especially at the early stage in a growing season is a common characteristic of many clonal plant species, in contrast to the positively size-dependent variation in individual growth rate in non-clonal plants. This type of variation in shoot growth rate leads to the persistence of stable shoot populations even when the mean growth rate function is changed, and also in cases where the shoot population structure would be unstable in the absence of variation in growth rate. It is suggested that competition between clones is symmetric in most clonal plant species, which brings about small-scale spatio-temporal changes in species abundance and hence species diversity.     

Vermicompost treatment differentially affects seed germination,seedling growth and physiological status of vegetable crop species

Vermicompost preparations are increasingly used in agricultural practice. There is a possibility, that crop plants are sensitive to negative effect of vermicompost at early stages of development. The aim of the present study was to test the effects of vermicompost on seed germination and seedling growth of different vegetable crop species. Vermicompost substitution inhibited seed germination and seedling growth with almost linear decrease of growth with increasing concentration of vermicopost in the substrate. However, both leaf chlorophyll content and photochemical activity of photosynthesis increased in all crop species with the exception of pea seedlings. Vermicompost extract as a watering solution showed positive effect on growth of bean and pea seedlings. Germination response of vermicompost extract-imbibed seeds was clearly crop species-dependent. Hypocotyl growth was stimulated by low and moderate vermicompost extract concentrations. Radicle growth was more sensitive to negative effect of vermicompost extract. It is reported that both solid vermicompost and vermicompost extract contain number of active substances of both phenolic and humic nature, each with own dose- and genotype-dependent effect of seed germination and early stages of seedling development. Findings of this study suggests that vermicompost must be used cautiously for practical purposes of plant propagation.     

Effects of contemporary agricultural land cover on Colorado potato beetle genetic differentiation in the Columbia Basin and Central Sands

Landscape structure, which can be manipulated in agricultural landscapes through crop rotation and modification of field edge habitats, can have important effects on connectivity among local populations of insects. Though crop rotation is known to influence the abundance of Colorado potato beetle (CPB; Leptinotarsa decemlineata Say) in potato (Solanum tuberosum L.) fields each year, whether crop rotation and intervening edge habitat also affect genetic variation among populations is unknown. We investigated the role of landscape configuration and composition in shaping patterns of genetic variation in CPB populations in the Columbia Basin of Oregon and Washington, and the Central Sands of Wisconsin, USA. We compared landscape structure and its potential suitability for dispersal, tested for effects of specific land cover types on genetic differentiation among CPB populations, and examined the relationship between crop rotation distances and genetic diversity. We found higher genetic differentiation between populations separated by low potato land cover, and lower genetic diversity in populations occupying areas with greater crop rotation distances. Importantly, these relationships were only observed in the Columbia Basin, and no other land cover types influenced CPB genetic variation. The lack of signal in Wisconsin may arise as a consequence of greater effective population size and less pronounced genetic drift. Our results suggest that the degree to which host plant land cover connectivity affects CPB genetic variation depends on population size and that power to detect landscape effects on genetic differentiation might be reduced in agricultural insect pest systems.     

Role of Foliage Habit in the Competition Between Differently Sized Plants in Carrot Crops

The length of the tallest leaf on a plant varied considerablywithin each of three carrot crops, and in each crop there wasa high positive correlation between shoot height and root weight.Lowering the petiole angle of carrot leaves greatly reducedthe ability of plants to compete against untreated neighbours,but did not affect growth when all plants were treated similarly.Cutting treatments which produced a more uniform shoot heightfor all plants, allowed small plants temporarily to overcomethe dominance of their larger neighbours. Similar cutting treatmentsapplied in a field experiment reduced crop growth and variationin root weight. These findings support the view that shoot heightdifference is an important component of inter-plant competitionin carrot crops and hence is a source of root-weight variation. Daucus carota L., carrot, foliage habit, competition     

Woody plant assemblages in isolated forest patches in a semiarid agricultural matrix

Spatial and temporal isolation and environmental variability are important factors explaining variation in plant species composition. The effect of fragmentation and disturbance on woody plant species composition was studied using data from 32 remnant church forest patches in northern Ethiopia. The church forests are remnants of dry Afromontane forest, embedded in a matrix of intensively used crop and grazing lands. We used canonical correspondence analysis and partial canonical correspondence analysis to analyze the effects of fragmented and isolated forest-patch identity, environmental and spatial variables on woody plant species composition in different growth stages. The dominance of late successional species was higher at the adult growth stage than seedlings and saplings growth stages. In the adult stages, late successional species like Olea europaea subsp. cuspidate had high frequency of occurrence. Forest patch identity was more important in explaining woody plant assemblages than environmental and spatial variables. For all growth stages combined, environmental variables explained more of the explained total fraction of variation in species composition than spatial variables. Topographic variables best explained variations in species composition for saplings, adults and all growth stages combined, whereas the management regime was most important for seedlings species composition. Our results show that in a matrix of cultivated and grazing land, fragmented and isolated forest patches differ in woody plant species assemblages. Some species are widely distributed and occurred in many patches while other occurred only in one or a few forest patches. Thus, our results indicate that remnant forest patches are important for preserving rare plant species and therefore management practices should focus on minimizing disturbance to the church forests and if possible increase church forest patch size.