A low watering treatment alters biomass allocation and growth rate but not heteroblastic development in an <Emphasis Type="Italic">Acacia</Emphasis> species

Key message

The Acacia phyllode leaf form is hypothesised to be an adaptation to drought. However, in this experiment, the timing of phyllode development was not related to a low water treatment.

Abstract

Acacia species have markedly different leaf forms known as compound leaves, transitional leaves, and phyllodes, also known as heteroblastic development. The different leaf types are thought to confer an advantage under varying moisture regimes, with phyllodes favoured in drier conditions. The hypothesis that phyllodes develop earlier under low water treatment was tested in this experiment. Three watering level treatments (100, 50, and 25 %) were imposed on seedlings of A. implexa to assess developmental traits (leaf emergence, initial onset of transitional leaves, and phyllodes), biomass allocation patterns (root, stem, compound leaf area/mass, transitional leaf area/mass, and phyllode area/mass), and leaf anatomy traits (epidermis, palisade and spongy mesophyll, and stomatal density). Across watering treatments, there was no difference in the developmental onset of transitional leaves or phyllodes (produced at the 6th and 9th nodes, respectively). Under low watering treatment, there was a decrease in stem height per unit stem diameter, shorter internodes, and greater allocation of biomass to roots. There was no significant difference in leaf anatomy traits. Under the low watering treatment, there was less compound leaf area and mass due to leaf shedding. In this experiment, the low watering treatment did not favour phyllode development at the expense of compound leaf development. Rather, it was found that A. implexa responds to a low water treatment similarly to many other plant species: increased allocation to roots, increased stem area per unit stem height, decrease in leaf area through senescence of older leaves, and lower relative growth rates.

     

Sexes show contrasting patterns of leaf and crown carbon gain in a dioecious rainforest shrub

The sexes of dioecious species may differ in a range of vegetative and reproductive traits as well as in physiological traits. In Siparuna grandiflora, a Neotropical dioecious shrub, we examined differences in leaf-level photosynthesis of different classes of leaf age and, using simulation models, explored whether differences in leaf-level carbon gain led to sex differences in whole-plant daily carbon gain. Male plants had higher photosynthetic capacity at the leaf level. As leaves of both sexes aged their photosynthetic capacity and specific leaf area declined as expected. Simulations of daily carbon gain using the architecturally explicit model Y-Plant and a non-architectural model incorporating a wide range of realistic light environments revealed that the difference in leaf-level photosynthetic capacity did not translate into greater crown-level carbon gain for males. Rather, differences in patterns of allocation to leaf area allow females to achieve higher crown-level carbon gain. The results demonstrate that sex differences at the leaf level do not necessarily predict patterns at the whole-plant level.     

Optimal allocation of resources in response to shading and neighbours in the heteroblastic species, Acacia implexa

BACKGROUND AND AIMS: Heteroblasty is an encompassing term referring to ontogenetic changes in the plant shoot. A shaded environment is known to affect the process of heteroblastic development; however, it is not known whether crowded or high density growing conditions can also alter heteroblasty. Compound leaves of the shade-intolerant Acacia implexa allocate less biomass per unit photosynthetic area than transitional leaves or phyllodes and it is hypothesized that this trait will convey an advantage in a crowded environment. Compound leaves also have larger photosynthetic capture area - a trait known to be advantageous in shade. This studied tested the hypothesis that more compound leaves will be developed under shade and crowded environments. Furthermore, this species should undergo optimal allocation of biomass to shoots and roots given shaded and crowded environments. METHODS: A full factorial design of irradiance (high and low) and density levels (high, medium and low) on three populations sourced from varying rainfall regions (high, medium and low) was established under controlled glasshouse conditions. Traits measured include the number of nodes expressing a compound leaf, biomass allocation to shoots and roots, and growth traits. Key Results A higher number of nodes expressed a compound leaf under low irradiance and in high density treatments; however, there were no significant interactions across treatments. Phenotypes strongly associated with the shade avoidance syndrome were developed under low irradiance; however, this was not observed under high density. There was no significant difference in relative growth rates across light treatments, but growth was significantly slower in a crowded environment. Conclusions Heteroblastic development in Acacia can be altered by shade and crowded environments. In this experiment, light was clearly the most limiting factor to growth in a shaded environment; however, in a crowded environment there were additional limiting resources to growth.     

New insights into leaf and fine‐root trait relationships: implications of resource acquisition among 23 xerophytic woody species

Functional traits of leaves and fine root vary broadly among different species, but little is known about how these interspecific variations are coordinated between the two organs. This study aims to determine the interspecific relationships between corresponding leaf and fine‐root traits to better understand plant strategies of resource acquisition. SLA (Specific leaf area), SRL (specific root length), mass‐based N (nitrogen) and P (phosphorus) concentrations of leaves and fine roots, root system, and plant sizes were measured in 23 woody species grown together in a common garden setting. SLA and SRL exhibited a strong negative relationship. There were no significant relationships between corresponding leaf and fine‐root nutrient concentrations. The interspecific variations in plant height and biomass were tightly correlated with root system size characteristics, including root depth and total root length. These results demonstrate a coordinated plant size‐dependent variation between shoots and roots, but for efficiency, plant resource acquisition appears to be uncoupled between the leaves and fine roots. The different patterns of leaf and fine‐root traits suggest different strategies for resource acquisition between the two organs. This provides insights into the linkage between above‐ and belowground subsystems in carbon and nutrient economy.     

Physiological and morphological correlates of whole-plant light compensation point in temperate deciduous tree seedlings

A range of traits, including metabolic costs, biomass allocation and seed reserves, may contribute to interspecific variation in the shade tolerance of tree seedlings. In addition, shade tolerance may be affected by differential responses of species to soil resource availability at low light. We used a custom-built whole-plant gas-exchange chamber to quantify instantaneous whole-plant light compensation point (WPLCP) and to parameterize whole-plant daily C gain models for seedlings of eight temperate deciduous tree species. We examined the relationship of WPLCP to growth, biomass allocation and gas-exchange under high and low light and nutrient availabilities and compared it to WPCLP of naturally recruited saplings. For species showing a response, both increased light and nutrient availability resulted in increased WPLCP. However, species’ responses to resource availability did not correspond closely with shade tolerance as has generally been predicted. Variation in WPLCP within species was best predicted by whole-plant dark respiration rates, leaf-level light compensation point and leaf mass per area. Among species, seed size was a strong negative correlate of WPLCP, explaining 66% of the variation. Species with the lowest WPLCP maintained lower growth rates across treatments but greater biomass in the low-light treatment compared with more light-demanding species. These data suggest that a number of traits, in particular metabolic costs and seed size, contribute to WPLCP. However, gas-exchange-based WPLCP was 1.5–3.5 times lower than corresponding growth-based field estimates of WPLCP, suggesting that other factors such as biotic interactions or ontogenetic shifts in whole-plant light requirements may substantially increase species’ WPLCP under natural conditions.     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control

We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose-response curves of allocation were constructed by means of a meta-analysis from a wide array of experimental data. They show that the fraction of whole-plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size-induced allocation patterns diminishes the LMF-response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.     

Contribution of root growth responses to leaf traits and relative growth rate of Populus alba under different water-table conditions

Water-table depth variations alter root growth response and may affect whole-plant growth in arid and semi-arid regions. We examined how root biomass allocation and root morphological traits affect the leaf physiological and morphological traits and whole-plant growth of Populus alba growing under different water tables. We exposed 1-year-old P. alba cuttings to contrasting soil–water conditions via water table changes in a greenhouse for 90 days. We examined relationships among net assimilation rate (NAR) and other growth components obtained from our published data for trees harvested every 30 days. Strongly negative correlations were found between RMR and root morphological traits. Root mass ratio had a strong negative relationship with LMR, and proportion of fine-root biomass per total root biomass was positively correlated with SLA and NAR. Both NAR and leaf area ratio were important determinants of variation in relative growth rate (RGR). Leaf mass ratio (LMR) and specific leaf area (SLA) were positively correlated with RGR; the correlation was stronger in the case of LMR. Along a water-table gradient, negative relationships between root growth responses are likely to indirectly influence RGR through changes in NAR, LMR, and SLA.     

Phenotypic plasticity of <Emphasis Type="Italic">Chenopodium murale</Emphasis> across contrasting habitat conditions in peri-urban areas in Indian dry tropics: Is it indicative of its invasiveness?

Phenotypic plasticity is an important plant trait associated with invasiveness of alien plants that reflects its ability to occupy a wide range of environments. We investigated the phenotypic response of Chenopodium murale to resource variability and ontogeny. Its plant-level and leaf-level traits were studied at high-resource (HR) and low-resource (LR) sites in peri-urban areas in Indian dry tropics. Plants at LR had significantly higher root length, root/shoot biomass ratio, stem mass and root mass fractions. Plants at HR had higher shoot length, basal diameter, leaf mass fraction and leaf area ratio. Leaf-level traits like leaf area and chlorophyll a were also higher here. Mean plasticity indices for plant- and leaf-level traits were higher at HR. With increasing total plant biomass, there was significant increase in the biomass of leaf, stem, root, and reproductive parts, and root and shoot lengths, whereas root/shoot length ratio, their biomass ratio, and leaf and root mass fractions declined significantly. Allocation to roots and leaves significantly decreased with increasing plant size at both sites. But, at any size, allocation to roots was greater at LR, indicative of optimization of capture of soil nutrients, whereas leaf allocation was higher at HR. Consistently increasing stem allocation equaled leaf allocation at comparatively higher shoot lengths at HR. Reproductive biomass comprised 10–12% of the plant’s total biomass. In conclusion, the success of alien weed C. murale across environmentally diverse habitat conditions in Indian dry tropics can be attributed to its high phenotypic plasticity, resource utilization capability in low-resource habitats and higher reproductive potential. These characteristics suggest that it will continue to be an aggressive invader.     

Leaf heteroblasty is not an adaptation to shade: seedling anatomical and physiological responses to light

Heteroblastic plants produce markedly different leaf morphologies between juvenile and adult stages, while homoblastic plants exhibit little or gradual changes. We tested the hypothesis that the leaf morphology of the seedling stage of New Zealand heteroblastic species is advantageous in dealing with low light levels found in forest understorey. We used four independent contrasts of heteroblastic and homoblastic seedlings from the genera Aristotelia, Hoheria, Pseudopanax, and Melicope grown in full-sun (100% sunlight) and shade (5% sunlight) light environments in a glasshouse. The four heteroblastic species had consistently smaller leaves and lower specific leaf area than their paired homoblastic species both in sun and shade. In the shade, there were no consistent differences in leaf anatomy (thickness of leaf blade, cuticle, epidermis, and palisade mesophyll, and stomatal density × stomatal aperture length) or physiology (maximum photosynthetic rate, dark respiration, and light compensation point) between homoblastic and heteroblastic species. However, in the sun, heteroblastic A. fruticosa, P. crassifolius, and M. simplex had appreciably thicker leaf blades as well as higher maximum photosynthetic rates than their homoblastic congeners. These traits suggest heteroblastic seedlings possess leaf traits associated with an advantage in high-light environments. We conclude that the heteroblastic seedling leaf morphology is unlikely to be an adaptation to very low light. Alternative explanations for the functional significance of changing leaf morphology in association with life-stage should be sought.     

Diverse Responses of Maple Saplings to Forest Light Regimes

Seedlings of 11 species of forest maples (AcerL.) were grownoutdoors from budburst to senescence under three light regimes:‘gap centre under clear skies’ (approx. 20% opensky irradiance; red:far-red ratio=1.12); ‘gap centre undercloudy skies’ (1.5%, ratio=1.03); and ‘gap edge’(2.5%, ratio=0.6). Seedlings grown under the gap centre (clearsky) regime had significantly greater height growth, greaterspecific leaf mass, higher root:shoot ratio, greater investmentin roots, higher leaf nitrogen concentrations, greater chlorophylla:bratio,lower photosynthetic rates under dim light, higher maximum photosyntheticrate, higher stomatal conductance, and lower leaf internal CO₂concentrationscompared with those grown in either gap edge or gap centre (cloudy)regimes. Responses to the gap edgevs.gap centre (cloudy) treatmentsdiffer little, suggesting that shade acclimation in forest mapleseedlings is mainly a response to light intensity rather thanspectral quality. The ubiquitous and, except for leaf internalCO₂concentration, highly significant interspecific variationin traits was broad-ranging and continuous. These results suggestthat (1) the responses to light quality found in shade intolerantherbaceous and woody species growing in more open habitats maynot have a selective advantage in seedlings of shade tolerantforest trees, and (2) the adaptive plastic response to understoreyvs.gapenvironments in forest maples, which is qualitatively consistentacross species, is founded on co-ordinated, small shifts insets of functionally inter-related traits.Copyright 1998 Annalsof Botany Company Acer,forest gap heterogeneity, plasticity, specific leaf mass, photosynthesis, leaf chlorophyll, nitrogen, stomatal density, root growth, root:shoot ratio, growth form.     

Contrasting modes of light acclimation in two species of the rainforest understory

In tropical rainforests, the increased light associated with the formation of treefall gaps can have a critical impact on the growth and survivorship of understory plants. Here we examine both leaf-level and whole-plant responses to simulated light gap formation by two common shade-tolerant shrubs, Hybanthus prunifolius and Ouratea lucens. The species were chosen because they differed in leaf lifespans, a trait that has been correlated with a number of leaf- and plant-level processes. Ouratea leaves typically live about 5 years, while Hybanthus leaves live less than 1 year. Potted plants were placed in the understory shade for 2 years before transfer to a light gap. After 2 days in high light, leaves of both species showed substantial photoinhibition, including reduced CO₂ fixation, F _v/F _m and light use efficiency, although photoinhibition was most severe in Hybanthus. After 17 days in high light, leaves of both species were no longer photoinhibited. In response to increased light, Ouratea made very few new leaves, but retained most of its old leaves which increased photosynthetic capacity by 50%. Within a few weeks of transfer to high light, Hybanthus had dropped nearly all of its shade leaves and made new leaves that had a 2.5-fold greater light-saturated photosynthetic rate. At 80 days after transfer, the number of new leaves was 4.9-fold the initial leaf number. After 80 days in high light, Hybanthus had approximately tenfold greater productivity than Ouratea when leaf area, photosynthetic capacity, and leaf dark respiration rate were all taken into account. Although both species are considered shade tolerant, we found that their growth responses were quite different following transfer from low to high light. The short-lived Hybanthus leaves were quickly dropped, and a new canopy of sun leaves was produced. In contrast, Ouratea showed little growth response at the whole-plant level, but a greater ability to tolerate light stress and acclimate at the leaf level. These differences are consistent with predictions based on leaf lifespan and are discussed within the context of other traits associated with shade-tolerant syndromes. Received: 25 March 1999 / Accepted: 16 August 1999     

Sun-shade adaptability of the red mangrove,Rhizophora mangle (Rhizophoraceae): Changes through ontogeny at several levels of biological organization

Rhizophora mangle L., the predominant neotropical mangrove species, occupies a gradient from low intertidal swamp margins with high insolation, to shaded sites at highest high water. Across a light gradient, R. mangle shows properties of both “light-demanding” and “shade-tolerant” species, and defies designation according to existing successional paradigms for rain forest trees. The mode and magnitude of its adaptability to light also change through ontogeny as it grows into the canopy. We characterized and compared phenotypic flexibility of R. mangle seedlings, saplings, and tree modules across changing light environments, from the level of leaf anatomy and photosynthesis, through stem and whole-plant architecture. We also examined growth and mortality differences among sun and shade populations of seedlings over 3 yr. Sun and shade seedling populations diverged in terms of four of six leaf anatomy traits (relative thickness of tissue layers and stomatal density), as well as leaf size and shape, specific leaf area (SLA), leaf internode distances, disparity in blade–petiole angles, canopy spread: height ratios, standing leaf numbers, summer (July) photosynthetic light curve shapes, and growth rates. Saplings showed significant sun/shade differences in fewer characters: leaf thickness, SLA, leaf overlap, disparity in bladepetiole angles, standing leaf numbers, stem volume and branching angle (first-order branches only), and summer photosynthesis. In trees, leaf anatomy was insensitive to light environment, but leaf length, width, and SLA, disparities in bladepetiole angles, and summer maximal photosynthetic rates varied among sun and shade leaf populations. Seedling and sapling photosynthetic rates were significantly depressed in winter (December), while photosynthetic rates in tree leaves did not differ in winter and summer. Seasonal and ontogenetic changes in response to light environment are apparent at several levels of biological organization in R. mangle, within constraints of its architectural baiiplan. Such variation has implications for models of stand carbon gain, and suggest that response flexibility may change with plant age.     

Modular growth and functional heterophylly of the phreatophyte Ziziphus lotus: A trait-based study

The variation of plant functional traits, from the cell to the whole-plant level, is a central question in trait-based ecology with regard to understanding ecological strategies and adaptations that result from environmental drivers. Here, we analyzed whole-plant and leaf traits of the phreatophyte Ziziphus lotus (L.) Lam., a long-lived shrub that dominates one of the few terrestrial groundwater-dependent ecosystems (GDEs) in Mediterranean Basin drylands. We (a) assessed architectural traits and growth patterns, (b) analyzed leaf morpho-functional traits (specific leaf area [SLA] and stomata pore index [SPI]) and physiological traits (gas exchange rates), as well as their variations within individuals, and (c) evaluated temporal variations in modular growth (i.e., sequential iteration of structural units) between growing seasons and in leaf traits within seasons. Z. lotus' growth pattern was based on the repetition of modules composed of shoots (short and long) and branches (flowering and plagiotropic) that promoted a functional differentiation between vegetative and reproductive structures, respectively. We identified morpho-functionally distinct leaves (i.e., heterophylly) borne on different types of branches. Leaves on flowering branches had higher SLA and water use efficiency (WUEi), but lower SPI and transpiration rates than leaves on vegetative ones. We also observed trade-offs in the elongation of vegetative and flowering structures between growing seasons: the shorter the long shoots, the larger the flowering branches. The modular differentiation and heterophylly of Z. lotus might contribute to prioritizing the investment of resources of this phreatophyte, either for growth or reproduction, and could improve the efficiency in uptake and conservation of resources in drylands.     

Alpine climate alters the relationships between leaf and root morphological traits but not chemical traits

Leaves and fine roots are among the most important and dynamic components of terrestrial ecosystems. To what extent plants synchronize their resource capture strategies above- and belowground remains uncertain. Existing results of trait relationships between leaf and root showed great inconsistency, which may be partly due to the differences in abiotic environmental conditions such as climate and soil. Moreover, there is currently little evidence on whether and how the stringent environments of high-altitude alpine ecosystems alter the coordination between above- and belowground. Here we measured six sets of analogous traits for both leaves and fine roots of 139 species collected from Tibetan alpine grassland and Mongolian temperate grassland. N, P and N:P ratio of leaves and fine roots were positively correlated, independent of biogeographic regions, phylogenetic affiliation or climate. In contrast, leaves and fine roots seem to regulate morphological traits more independently. The specific leaf area (SLA)–specific root length (SRL) correlation shifted from negative at sites under low temperature to positive at warmer sites. The cold climate of alpine regions may impose different constraints on shoots and roots, selecting simultaneously for high SLA leaves for rapid C assimilation during the short growing season, but low SRL roots with high physical robustness to withstand soil freezing. In addition, there might be more community heterogeneity in cold soils, resulting in multidirectional strategies of root in resource acquisition. Thus our results demonstrated that alpine climate alters the relationships between leaf and root morphological but not chemical traits.     

Drought affects the coordination of belowground and aboveground resource‐related traits in Solidago canadensis in China

Quantifying patterns of variation and coordination of plant functional traits can help to understand the mechanisms underlying both invasiveness and adaptation of plants. Little is known about the coordinated variations of performance and functional traits of different organs in invasive plants, especially in response to their adaptation to environmental stressors. To identify the responses of the invasive species Solidago canadensis to drought, 180 individuals were randomly collected from 15 populations and 212 ramets were replanted in a greenhouse to investigate both the response and coordination between root and leaf functional traits. Drought significantly decreased plant growth and most of the root and leaf functional traits, that is, root length, surface area, volume and leaf size, number, and mass fraction, except for the root length ratio and root mass fraction. Phenotypic plasticity was higher in root traits than in leaf traits in response to drought, and populations did not differ significantly. The plasticity of most root functional traits, that is, root length (RL), root surface area (RSA), root volume (RV), and root mass fraction (RMF), were significantly positively correlated with biomass between control and drought. However, the opposite was found for leaf functional traits, that is, specific leaf area (SLA), leaf area ratio (LAR), and leaf mass fraction (LMF). Drought enhanced the relationship between root and leaf, that is, 26 pairwise root–leaf traits were significantly correlated under drought, while only 15 pairwise root–leaf traits were significantly correlated under control conditions. Significant correlations were found between biomass and all measured functional traits except for leaf size. RV, root length ratio, RMF, total area of leaves, and LMF responded differently to water availability. These responses enable S. canadensis to cope with drought conditions and may help to explain the reason of the vast ecological amplitude of this species.     

Leaf size and leaf display of thirty-eight tropical tree species

Trees forage for light through optimal leaf display. Effective leaf display is determined by metamer traits (i.e., the internode, petiole, and corresponding leaf), and thus these traits strongly co-determine carbon gain and as a result competitive advantage in a light-limited environment. We examined 11 metamer traits of sun and shade trees of 38 coexisting moist forest tree species and determined the relative strengths of intra- and interspecific variation. Species-specific metamer traits were related to two variables that represent important life history variation; the regeneration light requirements and average leaf size of the species. Metamer traits varied strongly across species and, in contrast to our expectation, showed only modest changes in response to light. Intra- and interspecific responses to light were only congruent for a third of the traits evaluated. Four traits, amongst which leaf size, specific leaf area (SLA), and leaf area ratio at the metamer level (LAR) showed even opposite intra- and interspecific responses to light. Strikingly, these are classic traits that are thought to be of paramount importance for plant performance but that have completely different consequences within and across species. Sun trees of a given species had small leaves to reduce the heat load, but light-demanding species had large leaves compared to shade-tolerants, probably to outcompete their neighbors. Shade trees of a given species had a high SLA and LAR to capture more light in a light-limited environment, whereas shade-tolerant species have well-protected leaves with a low SLA compared to light-demanding species, probably to deter herbivores and enhance leaf lifespan. There was a leaf-size-mediated trade-off between biomechanical and hydraulic safety, and the efficiency with which species can space their leaves and forage for light. Unexpectedly, metamer traits were more closely linked to leaf size than to regeneration light requirements, probably because leaf-size-related biomechanical and vascular constraints limit the trait combinations that are physically possible. This suggests that the leaf size spectrum overrules more subtle variation caused by the leaf economics spectrum, and that leaf size represents a more important strategy axis than previously thought. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Zero-sum allocational strategies determine the allometry of specific leaf area

? Premise of the study: Specific leaf area (SLA) is a critical component of the leaf economics spectrum, and many functional leaf traits have been empirically demonstrated to covary with SLA. However, a complete understanding of how change in leaf size influences SLA has not yet emerged. ? Methods: To help develop a more complete understanding of the determinants of variability in SLA, we present a covariation model of leaf allometry that predicts a zero-sum interdependence of leaf thickness, density, and surface area on leaf mass. We test the model's predictions on measurements of 900 leaves from 44 angiosperm species. ? Key results: We observe that "diminishing returns," the negative allometry (slope < 1) of surface area versus mass, does not hold universally across species. Rather, the scaling of SLA is linked to the relative allocation to thickness and density. Specifically, diminishing returns are observed when leaves grow thicker, more than their density decreases, with increasing mass. Finally, we confirm model predictions that the allometric dependence of area, thickness, and density on mass can be well approximated by a zero-sum allocational process. ? Conclusions: Our work adds to the growing body of evidence that allometric covariation is a hallmark of the scaling behavior of complex plant and leaf traits. Moreover, because our model makes predictions based on allocational constraints, it provides a foundation to understand how deviations from zero-sum tradeoffs in allocation to leaf thickness, density, or area determine the allometry of SLA and, ultimately, underlie adaptive strategies within and across plant species.     

Heteroblasty in Arabidopsis thaliana (L.) Heynh

 Heteroblasty in Arabidopsis thaliana was analyzed in a variety of plants with mutations in leaf morphology using a tissue-specific β-glucuronidase gene marker. Some mutants exhibited their mutant phenotypes specifically in foliage leaves. The phenotypes associated with the foliage-leaf-specific mutations were also found to be induced ectopically in cotyledons in the presence of the lec1 mutation. Moreover, the features of an emf1lec1 double mutant showed that cotyledons can be partially converted into carpelloids. When heteroblastic traits were examined in foliage leaves in the presence of certain mutations or natural deviations by histochemical analysis of the expression of the tissue-specific marker gene, it was found that ectopic expression of the developmental program for the first foliage leaves in lec1 cotyledons seemed to affect the heteroblastic features of the first set of foliage leaves, while foliage leaves beyond the third position appeared normal. Similarly, in wild-type plants, discrepancies in heteroblastic features, relative to standard features, of foliage leaves at early positions seemed to be eliminated in foliage leaves at later positions. These results suggest that heteroblasty in foliage leaves might be affected in part by the heteroblastic stage of the preceding foliage leaves but is finally controlled autonomously at each leaf position. Received: 9 July 1999 / Accepted: 17 August 1999     

Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.