A system, including a specialized treatment vessel for pesticide uptake studies, was developed and experiments were carried out to determine the ability of Glomus intraradices (Schenck & Smith), G. vesiculiferium (Thaxter), and indigenous vesicular-arbuscular mycorrhizae (VAM) fungi to influence 14C-atrazine uptake and transfer to corn. Uptake by root systems with and without VAM infection was compared to uptake by VAM hyphal systems by controlling access to 14C-atrazine-treated soil. Hyphal systems of Glomus species were able to remove 14C-residue from soil and transfer these to corn. Amount of 14C-residue transferred to corn through hyphal systems was highly correlated to the level of VAM root infection which varied among VAM species. In root systems, variations in 14C-residue uptake resulting from mycorrhization were largely explained in terms of the negative correlation between level of infection and root mass located in 14C-atrazine-treated soil. Allocation of 14C-residue to shoot tissues of corn was greater when 14C-residues were taken up through root systems rather than through hyphal systems. There were significant effects of VAM species on 14C-residue compartmentalization between methanol extractable and nonextractable portions of corn tissues. Data from these experiments in a confined soil system were related to VAM cost-benefit relationships and indicate a significant role for VAM in determining atrazine fate in agricultural systems.

The development of functional-structural plant models has opened interesting perspectivesfor a better understanding of plant growth as well as for potential applications inbreeding or decision aid in farm management. Parameterization of such models is however adifficult issue due to the complexity of the involved biological processes and theinteractions between these processes. The estimation of parameters from experimental databy inverse methods is thus a crucial step. This paper presents some results anddiscussions as first steps towards the construction of a general framework for theparametric estimation of functional-structural plant models. A general family of models ofCarbon allocation formalized as dynamic systems serves as the basis for our study. Anadaptation of the 2-stage Aitken estimator to this family of model is introduced as wellas its numerical implementation, and applied in two different situations: first amorphogenetic model of sugar beet growth with simple plant structure, multi-stage anddetailed observations, and second a tree growth model characterized by sparse observationsand strong interactions between functioning and organogenesis. The proposed estimationmethod appears robust, easy to adapt to a wide variety of models, and generally provides asatisfactory goodness-of-fit. However, it does not allow a proper evaluation of estimationuncertainty. Finally some perspectives opened by the theory of hidden models arediscussed.

Perennial grain crops would address many agricultural problems, including soil erosion, nutrient loss and pesticide contamination. Doubts about the possibility of perennial grain crops rest upon two assumptions: (1) that the relationship between yield and longevity is a fixed function that cannot be influenced by selection, mutation or environmental changes; and (2) that yield and longevity trade off in a bivariate manner to the exclusion of all other traits. These assumptions are consistent with the phenotypic trade-off model, but recent research suggests that a quantitative genetic model is a more appropriate approach to trade-offs. In the quantitative genetic model, environmental and genetic changes can result in increases in two traits simultaneously even when a trade-off, or negative correlation, exists between the two traits. Empirical evidence that the trade-off between perenniality and reproductive allocation is not fixed comes from wild, herbaceous perennials that can produce more than 2000 kg seed ha−1 in the temperate zone, and herbaceous perennial crops that produce on average 8900 kg fruit ha−1 in the tropics. Ecological literature suggests that most perennials produce small amounts of seed relative to their vegetative growth not as a physiological absolute, but rather as a result of natural selection in a stable, competitive environment favoring longevity. By selecting strongly for seed yield in a population of perennial plants, the plant breeder can likely achieve that which is rare in nature—a high seed-yielding perennial plant. The same general methodologies that have allowed annual grain breeders to increase grain yield and push many combinations of negatively correlated traits to levels of expression not seen in nature are available to the perennial grain breeder. Perennial grain breeders are integrating ecological principles and traditional plant breeding methods in their efforts to develop perennial grain wheat (Triticum spp.), sorghum (Sorghum spp.), sunflower (Helianthus spp.), Illinois bundleflower (Desmanthus illinoensis) and rice (Oryza spp.).

How ecosystems adapt to climate changes depends in part on how individual trees allocate resources to their components. A review of research using tree seedlings provides some support for the hypothesis that some tree species respond to exposure to drought with increases in root[ratio ]shoot ratios but little change in total root biomass. Limited research on mature trees over moderately long time periods (2–10 yr), has given mixed results with some studies also providing evidence for increases in root: shoot ratios. The Throughfall Displacement Experiment (TDE) was designed to simulate both an increase and a decrease of 33% in water inputs to a mature deciduous forest over a number of years. Belowground research on TDE was designed to examine four hypothesized responses to long-term decreases in water availability; (1) increases in fine-root biomass, (2) increases in fine root[ratio ]foliage ratio, (3) altered rates of fine-root turnover (FRT), and (4) depth of rooting. Minirhizotron root elongation data from 1994 to 1998 were examined to evaluate the first three hypotheses. Differences across treatments in net fine-root production (using minirhizotron root elongation observations as indices of biomass production) were small and not significant. Periods of lower root production in the dry treatment were compensated for by higher growth during favorable periods. Although not statistically significant, both the highest production (20 to 60% higher) and mortality (18 to 34% higher) rates were found in the wet treatment, resulting in the highest index of FRT. After 5 yr, a clear picture of stand fine-root-system response to drought exposure has yet to emerge in this forest ecosystem. Our results provide little support for either an increase in net fine-root production or a shift towards an increasing root[ratio ]shoot ratio with long-term drought exposure. One possible explanation for higher FRT rates in the wet treatment could be a positive relationship between FRT and nitrogen and other nutrient availability, as treatments have apparently resulted in increased immobilization of nutrients in the forest floor litter under drier conditions. Such hypotheses point to the continued need to study the interactions of water stress, nutrient availability and carbon-fixation efficiency in future long-term studies.

Growth of crops in CO2-enriched atmospheres typically results in significant changes in root growth and development. Increased root carbohydrates stimulate root growth either directly (functioning as substrates) or indirectly (functioning as signal molecules) by enhancing cell division or cell expansion, or both. Although highly variable, the literature suggests that, generally, initiation and stimulation of lateral roots is favored over the elongation of primary roots, leading to more highly branched, shallower root systems. Such architectural shifts can render root systems less efficient, perhaps contributing to the lower specific root activities often reported. Allocation of carbon (C) to roots fluctuates through the life of the plant; root functional and growth responses should therefore not be viewed as static. In annual crops, C allocation to belowground processes changes as vegetative growth switches to reproduction and maturation. Reductions in C allocation to roots over time might cause temporal shifts in root deployment, perhaps affecting root demography. However, significant changes in root turnover (defined here as root flux or mortality relative to total root pool size) as a result of decreased root longevities in crop plants are unlikely. Consideration of changing C allocation to roots, a more thorough understanding of the mechanistic controls on root longevity, and a better characterization of the rooting habits (life histories) of different crop species will further our understanding of how increasing atmospheric [CO2] will affect root demography. This knowledge will lead the way toward a more thorough understanding of the linkage of atmosphere with belowground plant function and also that of plant function with soil biology and structure. Ultimately, successful modeling of global C and nitrogen (N) cycles will require empirical data concerning spatial and temporal deployment of roots for a range of crop species grown under different agricultural management systems.

Ectomycorrhizal seedlings of Scots pine (Pinus sylvestris) inoculated with the nitrotolerant Laccaria bicolor and the nitrophobic Suillus bovinus were exposed to ambient (350 μl l−1) and elevated (700 μl l−1) [CO2]. After 79 d the seedlings were labelled for 28 d with 14CO2, after which they were harvested. 14C was determined in shoots, roots plus mycorrhizas, soil, and below-ground respiration; nitrogen was determined in shoots and roots. Total net 14C uptake increased under elevated [CO2]. The extra carbon did not increase the shoot mass but was translocated to the roots and resulted in a decreased shoot-to-root ratio in the Suillus-inoculated seedlings. Laccaria-inoculated seedlings did not incorporate the additional carbon in root or fungal tissue but only increased below-ground respiration. S. bovinus acquired or transferred nitrogen better than L. bicolor and enabled the seedlings to perform better with regard to net carbon uptake under elevated [CO2]. This resulted in nitrogen concentrations in shoots of Suillus-inoculated seedlings that were twice as high as in Laccaria-inoculated seedlings, irrespective of [CO2]. The higher nitrogen concentration in the shoots resulted in a doubling of the 14C uptake per unit shoot mass. Our results suggest that the ability of ectomycorrhizal Scots pine seedlings to respond positively to elevated atmospheric [CO2] is strongly fungal-species specific.

The effects of partial defoliation on photosynthesis, whole-seedling carbon allocation, partitioning and growth were studied for two species with contrasting foliar traits. Field-grown seedlings of deciduous Japanese larch (Larix leptolepis) and evergreen red pine (Pinus resinosa) were defoliated by hand in early summer for 2 consecutive years. In the first year (1990), seedlings were defoliated by removing the distal 0, 25, 50 or 75% of each needle. In the second year (1991), seedlings were defoliated either 0 or 50%, regardless of previous defoliation treatments. Defoliation had little effect on photosynthesis and starch concentration in whole seedlings of either species in the first year. In the second year, photosynthesis increased in both species in response to the 1991 defoliation treatment, and in red pine also increased in response to the 1990 defoliation treatment. Further, in 1991 both larch and pine had decreased whole-seedling total non-structural carbohydrate concentrations in all seedlings that were defoliated at least once over the 2-yr period. This decrease was noted mostly in the starch component of the non-structural carbohydrates, and was similar in both species. In 1991, biomass was similarly decreased in both species in response to 1991 defoliation. Both species showed overcompensation in total and component biomass in seedlings defoliated by 25% in 1990. Overall, the results do not support the widely held belief that evergreen trees are substantially more affected than deciduous trees by defoliation.

Influence of habitat on physiological and structural characteristics was investigated for broad-leaved tropical monocotyledons in the genus Heliconia (Heliconiaceae). Seven species were selected from three different light regimes, enabling an analysis of the extent to which this genus has adapted its photosynthetic strategies and morphological characteristics to different daily photon flux densities (PFD). Predictably, light response curves showed a clear gradient with respect to light saturation and rates of maximum net assimilation (Amax). Heliconia latispatha, an open site species, showed saturation at higher PFD (1400 μmol m−2 s−1) and higher Amax (14–16 μmol m−2 s−1) than H. mathiasiae, a forest edge species (PFD 1000 μmol m−2 s−1; Amax 7.5–8.5 μmol m−2 s−1) and H. irrasa of deep-shade forest understorey (PFD 250 μmol m−2 s−1; Amax 3.5 mol m−2 s−1). Leaf blade areas were largest in open sites, and leaf specific mass was also significantly higher, but leaf support efficiency was highest in understorey species. Species in open sites had thicker leaves with more chlorenchyma, whereas deep-shade species had very thin leaves and low stomatal densities. These rapidly growing herbaceous perennials appear to allocate much of their above-ground biomass to leaf tissues and have a relatively low investment in support tissues. This contrasts with understorey palms, in which leaf form and structural investment has been interpreted as a trade-off between economy and protection against tissue loss from falling branches. Presence of below-ground rhizomes in Heliconia may provide the key to this difference.

We grew cuttings of two early (mid Oct.) and two late (early Nov.) leaf-fall Populus tremuloides Michx. genotypes (referred to as genotype pairs) for c. 150 d in open-top chambers to understand how twice-ambient (elevated) CO2 and soil N availability would affect growth and C allocation. For the study, we selected genotypes differing in leaf area duration to find out if late-season photosynthesis influenced C allocation to roots. Both elevated CO2 and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO2 stimulated leaf-level photosynthesis rates, whereas high soil N availability resulted in greater total plant leaf area. The early leaf-fall genotype pair had significantly higher photosynthesis rates per unit leaf area than the late leaf-fall genotype pair and elevated CO2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair, and their rate of leaf area development decreased earlier in the season. Across both genotype pairs, high soil N availability significantly increased fine root length production and mortality by increasing both the amount of root length present, and by decreasing the life span of individual roots. Elevated CO2 resulted in significantly increased fine root production and mortality in high N but not low N soil and did not affect fine root life span. The early leaf-fall genotype pair had significantly greater fine root length production than the late leaf-fall genotype pair across all CO2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availability and will increase under elevated atmospheric CO2. In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO2, within and among species.

Eucalyptus coccifera Hook., a plant capable of forming both arbuscular mycorrhizas and ectomycorrhizas, was used to compare the effects of the two mycorrhizal types on phosphorus uptake and C allocation. Seedlings were grown in a P-deficient soil/sand mixture inoculated with peat/vermiculite spawn of Laccaria bicolor (Maire) Orton or Thelephora terrestris (Ehrh.) Fr.; or with 250-μm sievings from leek colonized by Glomus caledonium (Nicol. & Gerd.) Trappe & Gerde., Glomus sp. type E3 or Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or with autoclaved spawn (non-mycorrhizal control). Before the 89-d harvest, a subset of the harvested plants was labelled with 14C (45–60-min pulse, 202-h chase). Growth promotion and the increase in seedling P content was largest in the two ectomycorrhizal treatments. Production of fluorescein diacetate-stained external hyphae was three to seven times higher by ectomycorrhizal (ECM) fungi compared with arbuscular mycorrhizal (AM) fungi and was highly correlated with P uptake and shoot weight. Phosphorus inflow rates of ECM and AM seedlings were 3·8 times, and 2·0–2·7 times those of non-mycorrhizal seedlings. Phosphorus acquisition efficiencies were similar (11·2 and 10·0 μmol P mmol−1C for T. terrestris and Glomus E3 plants, respectively) for the two mycorrhizal types, and appeared to be greater than in uninoculated plants (7.2 μmol P mmol−1C) grown at the same P level.

Spring ephemerals have evolved specific growth strategies that take advantage of the high photon fluence rates of early spring. These strategies involve a sequential growth of different organs. Erythronium americanum (Ker-Gawl), a common spring ephemeral of northern maple forests, produces its roots in the autumn, although stems and leaves develop in the spring. Mycorrhizal infection of the root system occurs very rapidly and intensively. The fungi thus depend on carbohydrate reserves accumulated in the corm (underground reserve organ) for their growth and development through the winter. We found that the presence of mycorrhizas drastically decreases root growth during the cold period and is more costly of carbohydrate reserves than root growth alone. However, during the spring, the presence of mycorrhizas fully benefits the plant: we measured an annual growth rate of mycorrhizal plants twice that of fungicide-treated plants which were non-mycorrhizal. We suspect that E. americanum is highly susceptible to water stress during the growing season and might rely on its mycorrhizas for water supply to a greater extent than for nutrient supply. Nutrient uptake might occur mainly in the autumn when arbuscules are at their most abundant.