Acropetal leaflet initiation of Eschscholzia californica is achieved by constant spacing of leaflets and differential growth of leaf

In compound leaves, leaflet primordia are initiated directionally along the lateral sides. Our understanding of the molecular basis of leaflet initiation has improved, but the regulatory mechanisms underlying spatio-temporal patterns remain unclear. In this study, we investigated the mechanisms of acropetal (from the base to the tip) progression of leaflet initiation in Eschscholzia californica. We established an ultraviolet-laser ablation system to manipulate compound-leaf development. Local ablation at the leaflet incipient site generated leaves with asymmetric morphology. In the majority of cases, leaflets that were initiated on the ablated sides shifted apically. Finite time-course observation revealed that the timing of leaflet initiation was delayed, but the distance from the leaf tip did not decrease. These results were suggestive of the local spacing mechanism in leaflet initiation, whereby the distance from the leaf tip and adjacent pre-existing leaflet determines the position of leaflet initiation. To understand how such a local patterning mechanism generates a global pattern of successive leaflet initiation, we assessed the growth rate gradient along the apical–basal axis. Our time-course analysis revealed differential growth rates along the apical–basal axis of the leaf, which can explain the acropetal progression of leaflet initiation. We propose that a leaflet is initiated at a site where the distances from pre-existing leaflets and the leaf tip are sufficient. Furthermore, the differential growth rate may be a developmental factor underlying the directionality of leaflet initiation.     

DOES SHADE PROLONG JUVENILE DEVELOPMENT? A MORPHOLOGICAL ANALYSIS OF LEAF SHAPE CHANGES IN CUCURBIT A ARGYROSPERMA SUBSP. SORORIA (CUCURBITACEAE)

Several studies have concluded that shade extends the juvenile phase of plant development based on the prolonged production of juvenile-looking leaves along the shoot. Until now, the alternative hypothesis that leaves produced in shade converge in shape with more juvenile leaves through plastic responses of individual leaves has not been investigated. The literature has shown that differences in shape among leaves in a heteroblastic series are manifest very early in development, often at or near inception, whereas divergence in development between sun and shade leaves does not become apparent until considerably later. This study is the first to distinguish between these alternatives by comparing the developmental morphology of young leaves of the heteroblastic plant Cucurbita argyrosperma subsp. sororia. Differences in shapes of mature leaves along the shoot in sun and shade were quantified in terms of leaf area, perimeter, and shape using truss analysis. Developmental morphology from initiation through expansion was examined for representative transition and for later (adult) leaves using scanning electron microscopy and allometry. Determinants of shape established very early in development were the same for leaves at the same position grown in sun and shade. Differences in morphology between sun and shade leaves at the same position did not arise until these leaves reached lamina lengths greater than 1,000 μm. Thus, the less-lobed, more juvenile looking leaf produced at later positions in the shade arose through later developmental responses of individual leaves to shade, rather than through a prolonged phase of juvenile development.     

Roles of the af and tl genes in pea leaf morphogenesis: characterization of the double mutant (afaftltl)

The pleiofila phenotype (afaftltl double mutant) of Pisum sativum arises from two single-gene, recessive mutations known to affect the identity of leaf pinnae, afila (af), and acacia (tl). The wild-type leaf consists of proximal leaflets and distal tendrils, whereas the pleiofila leaf consists of branched pinnae terminating in small leaflets. Using morphological measurements, histology, and SEM, we characterized the variation in leaf form along the plant axis, in leaflet anatomy, and in leaf development in embryonic, early postembryonic, and late postembryonic leaves of aftl and wild-type plants. Leaves on aftl plants increase in complexity more rapidly during shoot ontogeny than those on wild-type plants. Leaflets of aftl plants have identical histology to wild-type leaflets although they have smaller and fewer cells. Pinna initiation is acropetal in early postembryonic leaves of aftl plants and in all leaves of wild-type plants, whereas in late postembryonic leaves of aftl plants pinna initiation is bidirectional. Most phenotypic differences between these genotypes can be attributed to differential timing (heterochrony) of major developmental events.     

HETEROCHRONY AND HETEROBLASTIC LEAF DEVELOPMENT IN TWO SUBSPECIES OF CUCURBITA ARGYROSPERMA (CUCURBITACEAE)

To date, reports of paedomorphosis at the whole plant or shoot level have been loosely based on whole plant form or on the sequence of leaf shapes produced along the shoot (heteroblasty). However, interpreting the significance of heterochrony in the evolutionary loss or gain of heteroblasty based on mature leaf forms assumes that all leaves with the same shape arose through very similar modes of organogenesis. This study examines this assumption in two subspecies of Cucurbita argyrosperma, one that is wild and heteroblastic and a second that is cultivated and not markedly heteroblastic. All leaves of the cultivar are visually similar to early leaves of the wild subspecies. The cultivar is considered to be the progenitor of the wild subspecies. Scanning electron microscopy and allometry of developing leaves showed that at early nodal positions along the primary shoot, leaf development in both subspecies was similar. At later nodal positions, very young leaves of both subspecies were more similar to each other than to leaves at earlier nodal positions within the same plant at the same stage of development. This early similarity was masked in the mature shapes of later leaves due to subsequent differences in allometric growth. Thus a simple hypothesis of paedomorphosis in which the early leaf form in the progenitor is simply reiterated at later nodal positions in the cultivar is not supported by patterns of leaf development.     

Heteroblasty and preformation in mayapple, Podophyllum Peltatum (Berberidaceae): developmental flexibility and morphological constraint

Developmental preformation can constrain growth responses of shoots to current conditions, but there is potential for flexibility in development preceding formation of the preformed organs. Mayapple (Podophyllum peltatum) is strongly heteroblastic, producing rhizome scales, bud scales, and either a single vegetative foliage leaf or two foliage leaves on a sexual shoot. To understand how and when preformation constrains growth responses, we compare (1) how leaf homologs of the renewal shoot differ in development, (2) whether there are differences in shoot development that occur in advance of morphological determination of shoot type, and (3) whether there are points of developmental flexibility in renewal shoot growth prior to preformation of the foliage and floral organs. We use scanning electron microscopy and histology to show that the three vegetative leaves (both types of scale leaves and the vegetative foliage leaf) are similar in the initial establishment of an encircling and overarching leaf base. Differences among them are found in the timing of differentiation of the leaf base and in the relative timing and degree of growth of the lamina and petiole. In contrast, foliage leaves on sexual shoots show less expression of the leaf base and precocious growth of the lamina and petiole. Prior to shoot type determination, there are no morphological differences in the sequence or position of leaf homologs that predict final shoot type. In this colony, leaves at positions 12 and 13, on average, appear to be identical in development until they are between 700 and 800 μm in length, when it becomes possible to distinguish leaves that will become vegetative foliage leaves from additional bud scale leaves on vegetative or sexual shoots. We suggest that late developmental determination of leaves at positions 12 and 13 reflects ontogenetic sensitivity to a transition to flowering. Thus, in mayapple, heteroblasty appears to facilitate developmental flexibility prior to the point where shoot growth becomes constrained by preformation of determined aerial structures.     

Developmental analysis of a Medicago truncatula smooth leaf margin1 mutant reveals context-dependent effects on compound leaf development

Compound leaf development requires highly regulated cell proliferation, differentiation, and expansion patterns. We identified loss-of-function alleles at the SMOOTH LEAF MARGIN1 (SLM1) locus in Medicago truncatula, a model legume species with trifoliate adult leaves. SLM1 encodes an auxin efflux carrier protein and is the ortholog of Arabidopsis thaliana PIN-FORMED1 (PIN1). Auxin distribution is impaired in the slm1 mutant, resulting in pleiotropic phenotypes in different organs. The most striking change in slm1 is the increase in the number of terminal leaflets and a simultaneous reduction in the number of lateral leaflets, accompanied by reduced expression of SINGLE LEAFLET1 (SGL1), an ortholog of LEAFY. Characterization of the mutant indicates that distinct developmental domains exist in the formation of terminal and lateral leaflets. In contrast with the pinnate compound leaves in the wild type, the slm1 sgl1 double mutant shows nonpeltately palmate leaves, suggesting that the terminal leaflet primordium in M. truncatula has a unique developmental mechanism. Further investigations on the development of leaf serrations reveal different ontogenies between distal serration and marginal serration formation as well as between serration and leaflet formation. These data suggest that regulation of the elaboration of compound leaves and serrations is context dependent and tightly correlated with the auxin/SLM1 module in M. truncatula.     

Shoot and compound leaf comparisons in eudicots: dynamic morphology as an alternative approach

Recent developmental studies suggest that the compound leaf is a more or less incompletely developed shoot. Instead of treating compound leaves and shoots as non-homologous, this interpretation draws a continuum between them. The present work considers the plant as a hierarchical series of units on which similar developmental processes are at work, and where each level (shoot, compound leaf, leaflet) is 'repeated' by the next higher level. Measurements related to the expression of developmental processes operating on leaves at the shoot level and on leaflets at the compound leaf level were used to determine if similar processes are at work at these different levels during early stages of organogenesis. Plants with compound leaves showing acropetal leaflet inception, representing a total of 16 species from ten eudicot families, were studied. Based on several types of quantitative analyses, there appears to be a continuum between so-called shoots, compound leaves and leaflets in the species studied. This perspective, qualified as dynamic morphology, both parallels and complements the classical interpretation.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 143, 219−230.     

Reduced leaf complexity in tomato wiry mutants suggests a role for PHAN and KNOX genes in generating compound leaves

Recent work on species with simple leaves suggests that the juxtaposition of abaxial (lower) and adaxial (upper) cell fates (dorsiventrality) in leaf primordia is necessary for lamina outgrowth. However, how leaf dorsiventral symmetry affects leaflet formation in species with compound leaves is largely unknown. In four non-allelic dorsiventrality-defective mutants in tomato, wiry, wiry3, wiry4 and wiry6, partial or complete loss of ab-adaxiality was observed in leaves as well as in lateral organs in the flower, and the number of leaflets in leaves was reduced significantly. Morphological analyses and expression patterns of molecular markers for ab-adaxiality [LePHANTASTICA (LePHAN) and LeYABBY B (LeYAB B)] indicated that ab-adaxial cell fates were altered in mutant leaves. Reduction in expression of both LeT6 (a tomato KNOX gene) and LePHAN during post-primordial leaf development was correlated with a reduction in leaflet formation in the wiry mutants. LePHAN expression in LeT6 overexpression mutants suggests that LeT6 is a negative regulator of LePHAN. KNOX expression is known to be correlated with leaflet formation and we show that LeT6 requires LePHAN activity to form leaflets. These phenotypes and gene expression patterns suggest that the abaxial and adaxial domains of leaf primordia are important for leaflet primordia formation, and thus also important for compound leaf development. Furthermore, the regulatory relationship between LePHAN and KNOX genes is different from that proposed for simple-leafed species. We propose that this change in the regulatory relationship between KNOX genes and LePHAN plays a role in compound leaf development and is an important feature that distinguishes simple leaves from compound leaves.     

Juvenile Rhus glabra leaves have higher temperatures and lower gas exchange rates than mature leaves when compared in the field during periods of high irradiance

We sought to test the hypothesis that stomatal development determines the timing of gas exchange competency, which then influences leaf temperature through transpirationally driven leaf cooling. To test this idea, daily patterns of gas exchange and leaflet temperature were obtained from leaves of two distinctively different developmental stages of smooth sumac (Rhus glabra) grown in its native habitat. Juvenile and mature leaves were also sampled for ultrastructural studies of stomatal development. When plants were sampled in May-June, the hypothesis was supported: juvenile leaflets were (for part of the day) from 1.4 to 6.0 degrees C warmer than mature leaflets and as much as 2.0 degrees C above ambient air temperature with lower stomatal conductance and photosynthetic rates than mature leaflets. When measurements were taken from July to October, no significant differences were observed, although mature leaflet gas exchange rates declined to the levels of the juvenile leaves. The gas exchange data were supported by the observations that juvenile leaves had approximately half the number of functional stomata on a leaf surface area basis as did mature leaves. It was concluded that leaf temperature and stage of leaf development in sumac are strongly linked with the higher surface temperatures observed in juvenile leaflets in the early spring possibly being involved in promoting photosynthesis and leaf expansion when air temperatures are cooler.     

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     

A comparative study of the genetic bases of natural variation in tomato leaf,sepal, and petal morphology

In an effort to better understand the dramatic differences in vegetative and floral morphology that differentiate species within the genus Lycopersicon, quantitative trait loci (QTL) for leaflet and perianth size and shape characters were mapped in an interspecific F₂ population of tomato (Lycopersicon esculentum × L. pennellii). Thirty-six highly significant (P0.001) QTL were associated with 18 separate traits. QTL for correlated traits were generally not colocalized in the genome unless there was a clear codependence between the traits (e.g., organ length and area). Little or no overlap in QTL positioning between different organs was observed, suggesting that the genes determining the size and shape of leaflets, sepals, and petals are organ specific. Thus, while leaves are considered the developmental and evolutionary precursors to floral organs, genes acting late in development to determine certain aspects of morphology (namely shape and size) must have specialized to exert control over individual organs. Five of the leaflet-trait QTL map to analogous regions in the genome of eggplant, and therefore it appears there has been some conservation in the genes controlling leaf morphology within the Solanaceae.     

Elevated CO2 and leaf shape: Are dandelions getting toothier?

Heteroblastic leaf development in Taraxacum officinale is compared between plants grown under ambient (350 ppm) vs. elevated (700 ppm) CO₂ levels. Leaves of elevated CO₂ plants exhibited more deeply incised leaf margins and relatively more slender leaf laminae than leaves of ambient CO₂ plants. These differences were found to be significant in allometric analyses that controlled for differences in leaf size, as well as analyses that controlled for leaf developmental order. The effects of elevated CO₂ on leaf shape were most pronounced when plants were grown individually, but detectable differences were also found in plants grown at high density. Although less dramatic than in Taraxacum, significant effects of elevated CO₂ on leaf shape were also found in two other weedy rosette species, Plantago major and Rumex crispus. These observations support the long-standing hypothesis that leaf carbohydrate level plays an important role in regulating heteroblastic leaf development, though elevated C0₂ may also affect leaf development through direct hormonal interactions or increased leaf water potential. In Taraxacum, pronounced modifications of leaf shape were found at CO₂ levels predicted to occur within the next century.     

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.     

Light-Induced Indeterminacy Alters Shade-Avoiding Tomato Leaf Morphology

Plants sense the foliar shade of competitors and alter their developmental programs through the shade-avoidance response. Internode and petiole elongation, and changes in overall leaf area and leaf mass per area, are the stereotypical architectural responses to foliar shade in the shoot. However, changes in leaf shape and complexity in response to shade remain incompletely, and qualitatively, described. Using a meta-analysis of more than 18,000 previously published leaflet outlines, we demonstrate that shade avoidance alters leaf shape in domesticated tomato (Solanum lycopersicum) and wild relatives. The effects of shade avoidance on leaf shape are subtle with respect to individual traits but are combinatorially strong. We then seek to describe the developmental origins of shade-induced changes in leaf shape by swapping plants between light treatments. Leaf size is light responsive late into development, but patterning events, such as stomatal index, are irrevocably specified earlier. Observing that shade induces increases in shoot apical meristem size, we then describe gene expression changes in early leaf primordia and the meristem using laser microdissection. We find that in leaf primordia, shade avoidance is not mediated through canonical pathways described in mature organs but rather through the expression of KNOTTED1-LIKE HOMEOBOX and other indeterminacy genes, altering known developmental pathways responsible for patterning leaf shape. We also demonstrate that shade-induced changes in leaf primordium gene expression largely do not overlap with those found in successively initiated leaf primordia, providing evidence against classic hypotheses that shaded leaf morphology results from the prolonged production of juvenile leaf types.Not only is the shape of a single leaf highly multivariate, but the shape of leaves within and between plants is influenced by evolutionary, genetic, developmental, and environmental factors (Chitwood et al., 2012a, 2012b, 2013, 2014; Chitwood and Topp, 2015). Over a lifetime, a plant will produce numerous leaf shapes, influenced by the development of individual leaves as their blades unequally expand (allometric expansion; Hales, 1727; Remmler and Rolland-Lagan, 2012; Rolland-Lagan et al., 2014) and the different types of leaf shapes a plant produces at successive nodes, a result of the temporal development of the shoot apical meristem (SAM; heteroblasty; Goebel, 1900; Ashby, 1948; Poethig, 1990, 2010; Kerstetter and Poethig, 1998). Therefore, leaf shape in a single plant cannot be reduced to a single shape, as shapes are ephemeral, changing from one moment to the next in individual leaves, and the shapes of leaves emerging from successive nodes are not necessarily constant.When environmental conditions induce changes in leaf shape (plasticity), it is within the above-mentioned developmental context that morphology must be considered (Diggle, 2002). For example, a once prevailing hypothesis was that changes in leaf shape across successive nodes were dependent on nutrition. The rationale for this premise rested on the unique (often irregular) shapes of first emerging leaves, thought to result from abortive development because of reduced photosynthetic support from any previous leaves. Similarly, many plants produce juvenile-looking leaves when shaded, interpreted again as resulting from reduced photosynthate (Goebel, 1908; Allsopp, 1954). This hypothesis has recently been revisited, as sugar has been found to be a signal mediating vegetative phase change (Yang et al., 2013; Yu et al., 2013). Careful morphological studies of leaf development can separate the different effects of shade and heteroblasty, refuting these ideas, at least at a morphological level in some species. In Cucurbita spp., the changes in successively emerging leaves are morphologically observable in early leaf primordia. Despite light intensity-induced changes in the heteroblastic progression of mature leaf morphology, leaf primordia initiated under low light resemble those initiated in sun. This suggests that, in Cucurbita spp., light intensity-induced morphology results from plastic responses later in leaf development after initiation rather than through changes in heteroblasty and timing (Jones, 1995).In addition to the responses to decreased light intensity discussed above, plants can also sense changes in light quality. Phytochrome proteins, which sense decreases in the ratio of red light to far-red light (R:FR), initiate the shade-avoidance response upon detecting deflected light from competitors (Smith and Whitelam, 1997). Shade-avoiding plants typically exhibit increases in internode and petiole length, reduced leaf mass per area, alterations in stomatal patterning, and shoot/root resource reallocation as an adaptive response to overgrow competitors and better intercept light (Casal, 2012). The changes in leaf shape in response to shade are more ambiguous and can be radically different based on morphological context (such as simple versus complex leaves) and species. For example, in Arabidopsis (Arabidopsis thaliana), shade avoidance is typified by greater increases in petiole length relative to the blade region and inhibited blade outgrowth (Tsukaya et al., 2002; Kim et al., 2005; Kozuka et al., 2005), but in wild relatives of domesticated tomato (Solanum lycopersicum), both the petiole and rachis region expand equally and blade outgrowth is increased. These shade-avoiding responses inversely correlate with the amount of vegetation present in the native locale of an accession, implying adaptive significance in tomato (Chitwood et al., 2012a).Here, we begin by characterizing the effects of simulated foliar shade on leaf morphology in domesticated tomato and its wild relatives through a meta-analysis of more than 18,000 previously published leaflets (Chitwood et al., 2012a, 2012b, 2014). We find that the effects of decreased R:FR on leaf shape are strong, but only when multiple morphometric parameters are considered across leaflets, both within leaves and across the leaf series, of individual plants. Circularity (a measure of leaflet serration in tomato) and leaf complexity are the most strongly affected individual traits during the shade-avoidance response. We then seek to determine when different shade-avoiding traits manifest during leaf development by swapping plants between light treatments during early leaf development. Leaves will plastically increase their blade area late into development if moved into simulated foliar shade from an initial sunlight treatment, but other traits, such as stomatal patterning, are irrevocably specified earlier during development. Observing increases in SAM size under low R:FR conditions, we then perform laser-capture microdissection to analyze the effects of simulated foliar shade on gene expression in the first emerging leaf primordium (P1) and the meristem. The most conspicuous change in gene expression is increased expression of LeT6 (the tomato ortholog of SHOOTMERISTEMLESS) and other indeterminacy-related genes in the P1, consistent with the increases in leaflet serration and leaf complexity observed in tomato during the shade-avoidance response. Finally, to determine the developmental context of our observations, we compare gene expression changes during shade avoidance with heteroblastic gene expression (i.e. successively initiating leaves at the same developmental stage) in the SAM and young leaf primordia. Gene expression induced by decreased R:FR in light and that which changes with progression through the heteroblastic series in leaf primordia are largely distinct, suggesting not only that the shade-avoidance response is not mediated through heteroblastic changes but also that increases in leaf complexity in these two contexts employ distinct suites of genes.     

叶发育的遗传调控机理研究进展

叶是植物进行光合作用的主要器官。高等植物叶原基起始于顶端分生组织的周边区,在一系列基因精确调控下,叶原基建立近一远轴、基一顶轴和中．侧轴极性,引导原基细胞朝着特定的方向分裂和分化,最终发育戍一定形态和大小的叶片。近年来分子遗传学研究结果表明,数个转录因子家族基因、小分子RNA和细胞增殖相关因子组成一个复杂的遗传控制网络,调节叶片极性建成过程。此外,复叶的形态建成还受到另外一些转录因子的调控。本文对近年来叶发育遗传调控机理研究的新进展做简要介绍。     

New perspectives on the Mesozoic seed fern order Corystospermales based on attached organs from the Triassic of Antarctica

A new Triassic corystosperm is described from the Shackleton Glacier region of Antarctica. The compression fossils include cupulate organs (Umkomasia uniramia) and leaves (Dicroidium odontopteroides) attached to short shoot-bearing branches. The cupulate organs occur in groups near the apices of the short shoots, and each consists of a single axis with a pair of bracts and a subapical whorl of five to eight ovoid cupules. This unique architecture indicates that the cupules are individual megasporophylls rather than leaflets of a compound megasporophyll. A branch bearing an attached D. odontopteroides leaf provides the first unequivocal evidence that Umkomasia cupulate organs and Dicroidium leaves were produced by the same plants. Although this had previously been assumed based on organ associations, the new specimens are important in demonstrating that a single species of corystosperm produced the unique cupulate organs described here and the geographically and stratigraphically widespread and common D. odontopteroides leaf. Therefore, biostratigraphic, paleoecological, and phylogenetic studies that treat Dicroidium leaf morphospecies as proxies for biological species of entire plants should be reconsidered. Phylogenetic analysis suggests that the corystosperm cupule is an unlikely homologue for the angiosperm carpel or outer integument.     

Heteroblastic development and the optimal partitioning of traits among contrasting environments in Acacia implexa

Background and Aims: Optimal partitioning theory (OPT) predicts plants will allocatebiomass to organs where resources are limiting. Studies of OPTfocus on root, stem and leaf mass ratios where roots and stemsare often further sub-divided into organs such as fine roots/taproots or branches/main stem. Leaves, however, are rarely sub-dividedinto different organs. Heteroblastic species develop juvenileand adult foliage and provide an opportunity of sub-dividingleaf mass ratio into distinct organs. Acacia implexa (Mimosaceae)is a heteroblastic species that develops compound (juvenile),transitional and phyllode (adult) leaves that differ dramaticallyin form and function. The aims of the present study were togrow A. implexa to examine patterns of plastic development ofwhole-plant and leaf traits under the OPT framework. Methods: Plants were grown in a glasshouse under contrasting nutrient,light and water environments in a full factorial design. Allocationto whole-plant and leaf-level traits was measured and analysedwith multivariate statistics. Key Results: Whole-plant traits strongly followed patterns predicted by OPT.Leaf-level traits showed a more complex pattern in responseto experimental treatments. Compound leaves on low nutrientplants had significantly lower specific leaf area (SLA) andwere retained for longer as quantified by a significantly greatercompound leaf mass ratio after 120 d. There was no significantdifference in SLA of compound leaves in the light treatment,yet transitional SLA was significantly higher under the lowlight treatment. The timing of heteroblastic shift from compoundto transitional leaves was significantly delayed only in thelow light treatment. Therefore, plants in the light treatmentresponded at the whole-plant level by adjusting allocation toproductive compound leaves and at the leaf-level by adjustingSLA. There were no significant SLA differences in the watertreatment despite strong trends at the whole-plant level. Conclusion: Explicitly sub-dividing leaves into different types providedgreater insights into OPT.     

Heteroblasty on Chatham Island: A comparison with New Zealand and New Caledonia

We used a comparative approach to investigate heteroblasty in the Chatham Islands. Heteroblasty refers to abrupt changes in the morphology of leaves and shoots with plant height. Common on isolated islands such as New Caledonia and New Zealand, which once had flightless, browsing birds, heteroblasty is hypothesised to be an adaptation to deter bird browsing. The Chatham Islands are a small archipelago located 800 km off the east coast of New Zealand, which has clear floristic links to New Zealand. However, unlike New Caledonia and New Zealand, the Chathams never had flightless, browsing birds. We investigated heteroblasty on the Chatham Islands by: (1) comparing height-related changes in leaf morphology and branching architecture in several plant taxa with heteroblastic relatives on the New Zealand mainland; (2) characterising changes in leaf morphology in heteroblastic tree species endemic to the Chathams; and (3) comparing overall trends in leaf heteroblasty on the Chathams with New Caledonia and New Zealand. Reversions to homoblasty were observed in the three Chatham Island taxa with heteroblastic relatives on the New Zealand mainland. However, two endemic tree species were clearly heteroblastic; both produced dramatically larger leaves as juveniles than as adults. Inter-archipelago comparisons showed that this trend in leaf morphology is rare among heteroblastic species in New Caledonia and New Zealand. Therefore, while some of our results were consistent with the hypothesis that heteroblasty is an adaptation to avoid bird browsing, other processes also appear to have shaped the expression of heteroblasty on Chatham Island.     

Reconstruction of the Leaves of Two New Species of Pseudosmodingium (Anacardiaceae) from Oligocene Strata of Puebla, Mexico

Leaves of two new plants are reconstructed from their isolated leaflets collected from the Oligocene Los Ahuehuetes locality near Tepexi de Rodríguez in Puebla, Mexico. The leaves of Pseudosmodingium mirandae Ramírez-Gardu?o et al. are compound imparipinnate with leaflets of variable morphology. The leaflets of five leaf morphotypes vary from narrow elliptic to lanceolate or lorate; they are symmetrical to slightly asymmetrical, with acute to attenuate apex, acute to cuneate base, and entire to serrate margin. Venation is simple pinnate craspedodromous, with secondary veins slightly curved near their base; secondary veins may dichotomize near the margin to become tertiary veins, and intersecondary veins are small and oblique to the secondary veins. A small number of leaflets assigned to Pseudosmodingium terrazasiae Ramírez-Gardu?o et al. are distinguished from P. mirandae by the leaflet shape, length&rcolon;width ratio, base shape, and apex angle. Morphological comparison of the fossil leaves with leaves of extant species of Anacardiaceae based on numerical analyses indicates a close similarity between P. mirandae and Pseudosmodingium multifolium Rose, while P. terrazasiae is more similar to Pseudosmodingium perniciosum (HBK) Engl. The presence of fossil species with extant relatives that are endemic to Mexico, along with previous reports, indicates that by the Oligocene, some lineages were already in place, although today they form part of the more xeric communities in southern North America.