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PREMISE OF THE STUDY: The flip‐leaved podocarp Retrophyllum has a disjunct extant distribution in South American and Australasian tropical rainforests and a Gondwanic fossil record since the Eocene. Evolutionary, biogeographic, and paleoecological insights from previously described fossils are limited because they preserve little foliar variation and no reproductive structures. METHODS: We investigated new Retrophyllum material from the terminal Cretaceous Lefipán, the early Eocene Laguna del Hunco, and the early/middle Eocene Río Pichileufú floras of Patagonian Argentina. We also reviewed type material of historical Eocene fossils from southern Chile. KEY RESULTS: Cretaceous Retrophyllum superstes sp. nov. is described from a leafy twig, while Eocene R. spiralifolium sp. nov. includes several foliage forms and a peduncle with 13 pollen cones. Both species preserve extensive damage from sap‐feeding insects associated with foliar transfusion tissue. The Eocene species exhibits a suite of characters linking it to both Neotropical and West Pacific Retrophyllum, along with several novel features. Retrophyllum araucoensis (Berry) comb. nov. stabilizes the nomenclature for the Chilean fossils. CONCLUSIONS: Retrophyllum is considerably older than previously thought and is a survivor of the end‐Cretaceous extinction. Much of the characteristic foliar variation and pollen‐cone morphology of the genus evolved by the early Eocene. The mixed biogeographic signal of R. spiralifolium supports vicariance and represents a rare Neotropical connection for terminal‐Gondwanan Patagonia, which is predominantly linked to extant Australasian floras due to South American extinctions. The leaf morphology of the fossils suggests significant drought vulnerability as in living Retrophyllum, indicating humid paleoenvironments.

Premise of the study: Leaves at the tops of most trees are smaller, thicker, and in many other ways different from leaves on the lowermost branches. This height-related variation in leaf structure has been explained as acclimation to differing light environments and, alternatively, as a consequence of hydrostatic, gravitational constraints on turgor pressure that reduce leaf expansion. METHODS: To separate hydrostatic effects from those of light availability, we used anatomical analysis of height-paired samples from the inner and outer tree crowns of tall redwoods (Sequoia sempervirens). Key results: Height above the ground correlates much more strongly with leaf anatomy than does light availability. Leaf length, width, and mesophyll porosity all decrease linearly with height and help explain increases in leaf-mass-to-area ratio and decreases in both photosynthetic capacity and internal gas-phase conductance with increasing height. Two functional traits--leaf thickness and transfusion tissue--also increase with height and may improve water-stress tolerance. Transfusion tissue area increases enough that whole-leaf vascular volume does not change significantly with height in most trees. Transfusion tracheids become deformed with height, suggesting they may collapse under water stress and act as a hydraulic buffer that improves leaf water status and reduces the likelihood of xylem dysfunction. CONCLUSIONS: That such variation in leaf structure may be caused more by gravity than by light calls into question use of the terms \"sun\" and \"shade\" to describe leaves at the tops and bottoms of tall tree crowns.

Key messageIn Sakhalin fir trees from nine different source elevation provenances, we found genetic differentiation of traits related to mechanical reinforcement, hydraulic efficiency, and photosynthetic capacity.Climatic conditions change with elevation and trees must cope with the resulting variation in stresses. Thus, trees may differentiate into elevational ecotypes with genetic-based variations in morphological and physiological traits. To explore genetically differentiated traits related to elevational adaptation, needles and stems were analyzed in 43-year-old Sakhalin fir [Abies sachalinensis (Fr. Schm.) Masters] trees which derived from nine source elevations (230–1250 m above sea level) and grown in a nursery plantation at 230 m above sea level. Trees from a high-elevation provenance showed greater mechanical reinforcement in needles and stems. Needles from high-elevation provenances were shorter and thicker, and developed more sclerenchyma in transfusion tissue. Shorter and thicker stems and larger reaction wood portions were also found. Moreover, needles and stems from high-elevation provenance trees also exhibited xylem traits associated with higher hydraulic efficiency and lower hydraulic safety. In the midrib xylem, the theoretical conductivity was greater due to higher number of tracheids. Pit architecture of stem-xylem tracheid indicated a higher hydraulic efficiency, but lower hydraulic safety due to larger pit apertures. Furthermore, high-elevation provenance trees exhibited a thicker bark, which may reduce water losses and act as a water reservoir in winter. Leaf nitrogen content and stomata number per needle were higher in high-elevation provenance trees, both of which were related to high photosynthetic capacity. Overall, the data suggested genetic differentiation of traits related to various trade-offs and optimization for mechanical resistance, hydraulic efficiency, and photosynthetic capacity at high elevation in Sakhalin fir.

Key messageElement profile signatures of needle tissues differentiated four tissues: epidermis (main contributor: calcium), endodermis (main contributors: magnesium, sulphur and manganese), mesophyll (main contributor: potassium), and transfusion parenchyma (main contributor: zinc).Distribution of elements in cross-sections of Scots pine (Pinus sylvestris L.) needles was investigated using micro-proton-induced X-ray emission. Tissue-specific distributions of magnesium (Mg), sulphur (S), calcium (Ca), phosphorus (P), potassium (K), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), aluminium (Al) and silicon (Si) were resolved in a quantitative manner. Distribution maps and tissue-specific concentrations revealed the largest concentration of Ca in epidermis, of Mg, S and Mn in endodermis, of K in mesophyll and phloem and of Zn in transfusion parenchyma. Phosphorus, Cl, Fe, Al and Si did not exhibit apparent tissue-specific distribution. Inverse allocation of P and Ca was observed, a likely mechanism to prevent their precipitation. Taking the area of tissues into account, relative element distribution calculations indicated that mesophyll contained the majority of the elements studied, except Ca, which predominated in the epidermis (79% of total Ca concentration) and Mn, which predominated in the endodermis (40% of total Mn concentration). When considering a complete element profile of a particular tissue, four clusters were differentiated, which generally supported single-element observations. The first cluster differentiated mesophyll, xylem, phloem, transfusion tracheids and Strasburger cells with predominance of K, the second cluster differentiated epidermis on the basis of Ca, the third cluster differentiated endodermis with contributions from Mg, S and Mn, and the fourth cluster differentiated transfusion parenchyma with contribution from Zn. Information on tissue-specific-element allocations will complement structural and functional knowledge of needle tissues and advance our understanding of element/nutrient transfers in Scots pine.

KEY MESSAGE : In Cryptomeria japonica , transfusion tissue in leaves may have functions of water storage and supply, which could compensate for hydraulic constraints with increasing height. The tallest trees of Cryptomeria japonica occur in climatic regions similar to the world’s tallest trees. We hypothesized that tall C. japonica trees would have evolved adaptive mechanisms to overcome height growth limitation. Here, we focused on foliar water storage, a mechanism recently discovered in Sequoia sempervirens. In C. japonica, leaf water potential at turgor loss did not change with height or light availability, while leaf hydraulic capacitance and succulence (water content per leaf surface area) increased, suggesting hydraulic compensation. Plasticity of leaf morphology could contribute to avoiding negative effects of height on photosynthesis. We also focused on the structure and function of transfusion tissue in leaves and its role in water storage and supply. Cross-sectional area of transfusion tissue increased with height, whereas that of xylem was constant. We confirmed that water flowed from vascular bundle to mesophyll via the transfusion tissue. Cryo-scanning electron microscopy images of leaf cross sections showed that transfusion cells were flattened, but not fully dehydrated when leaf water potential decreased in situ and by experimental dehydration, and cell deformation was more marked for treetop leaves than for lower-crown leaves. The shape of transfusion cells recovered at predawn as well as after experimental rehydration. As in S. sempervirens, transfusion tissue of C. japonica may function as a hydraulic buffer, absorbing and releasing water according to leaf water status. Anatomical and hydraulic properties contributing to foliar water storage may be an adaptive mechanism acquired by tall Cupressaceae trees to overcome the hydraulic constraints on physiological function with increasing height.

PREMISE OF THE STUDY: Leaves respond to environmental signals and acclimate to local conditions until their ecological limits are reached. Understanding foliar response to climate change, as it expands our knowledge of tree physiology. METHODS: We examined foliar anatomy and morphology of the largest plant species, Sequoiadendron giganteum, from leafy shoot samples collected throughout crowns of trees up to 95 m tall and assessed the functionality of within-crown variation with a novel drought/recovery experiment. KEY RESULTS: We found phenotypic variation in response to water availability in 13 anatomical traits of Sequoiadendron leaves. Shoot expansion was constrained by the hydrostatic gradient of maximum water potential, while functional traits supporting succulence and toughness were associated with sites of peak hydraulic limitation. Water-stress tolerance in experimental shoots increased dramatically with height. CONCLUSION: We propose a heat-sink function for transfusion tissue and uncover a suite of traits suggesting rapid hydraulic throughput and flexibility in water-stress tolerance investments as strategies that help this montane species reach such enormous size. Responses to water stress alter the amount of carbon stored in foliage and the rate of the eventual release of carbon.

De acuerdo con una hipótesis común, algunos rasgos estructurales en las hojas de plantas portadoras de traqueidas \"compensan\" la baja conductividad específica del leño sin vasos. La información sobre este tema es contradictoria, lo que puede explicarse por el hecho de que las relaciones hídricas en las hojas no dependen solo de los rasgos estructurales, sino también de la estructura de otros tejidos foliares. En este estudio nuestro objetivo fue evaluar la diversidad de los sistemas de transporte de agua en las hojas de plantas leñosas portadoras de traqueidas en especies del bosque templado lluvioso del centro sur de Chile. Para esto recolectamos hojas de cuatro especies de Podocarpaceae y dos Winteraceae en hábitats naturales, estudiamos su anatomía foliar mediante microscopía de luz y electrónica de transmisión, determinamos caracteres anatómicos cuantitativos y analizamos los datos usando análisis de componentes principales. Las hojas de las especies analizadas se diferencian en la anatomía del mesófilo y xilema. Cuatro especies tienen rasgos que aceleran el transporte de agua a través de los tejidos foliares mediante el apoplasto (Prumnopitys andina), el tejido de transfusión accesorio (Podocarpus saligna) y la red de venas (especies de Drimys). Por el contrario, las hojas de Saxegothaea conspicua y Podocarpus nubigena acumulan agua en el tejido de almacenamiento de esta (hidrénquima), pero su ecología sugiere que el hidrénquima no es una adaptación a las condiciones ambientales. Los datos obtenidos indican la existencia de diferentes formas de suministro de agua al tejido fotosintético en las hojas de plantas sin vasos. En el caso de que el suministro de agua a través de traqueidas sea insuficiente, es posible que el hidrénquima mantenga la hidratación de las hojas.

The aim of the study was to determine the adaptive characteristics of pine needles associated with age and different growing conditions. The length of the needles decreases and its variability reduces with increasing dryness and poverty of the soil. In oppressed trees, the coefficient of variability of the length of the needles on the tree is 8%. The coefficient of variation in the length of needles approaching 20% will indicate the best conditions for the growth of a particular tree. Trends of the dependence of width and thickness of needles on growing conditions were not identified. The area of needles in pine forests with optimal water regime of soils (blueberry, cowberry type) varies in the range of 112–124 mm2. In extreme growing conditions pine needles area is reduced by 27–33% and equals 76–86 mm2. These ranges of values of the areas of needles are typical for plantings of the third and fourth classes of age. Changing the width and thickness of the needles is aimed at compensating for changes in the length of the needles in the direction of maintaining the optimal area for these conditions needles. In extreme conditions, the area of the assimilating tissue increases, and the area of the conducting tissue (stele) decreases. Correlation dependences of the area of the stele of needles with the cross-sectional area, with the area of conducting beams, with the number of resin canals and with the cover fabric are revealed.

Despite more than 130 years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms.

Pines are generally absent from tropical rainforests. An important exception, Pinus krempfii, is a unique tree that bears flattened needles and competes with evergreen angiosperm trees in southern Vietnam. Here, the photosynthetic and hydraulic physiology of P. krempfii leaves were examined to determine whether this species departs from the widespread pattern of high-light-demanding photosynthetic physiology displayed in needle-leaved Pinus species. Maximum photosynthesis and light saturation of photosynthesis, as well as stem and leaf hydraulic efficiencies, were all very low in P. krempfii compared with other Pinus species. These characteristics were consistent with our observations of P. krempfii seedling regeneration under the forest canopy. By possessing shade tolerance coupled with the production of flattened leaves, P. krempfii has converged morphologically and physiologically with many genera of the southern hemisphere conifer family Podocarpaceae. This convergence extends to a key feature of leaf anatomy, the production of tubular sclereids in the leaf for radial transport of water from the vein to the margin. These observations suggest that few adaptive possibilities are open to conifers when moving into tropical rainforest, meaning that Pinus is forced into direct competition with southern hemisphere conifers for a narrow niche in the equatorial zone.