Is vulnerability segmentation at the leaf-stem transition a drought resistance mechanism? A theoretical test with a trait-based model for Neotropical canopy tree species

Key message Leaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stem. However, although it has been intensively investigated these past decades, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Based on a trait-based model, this study theoretically supports that vulnerability segmentation enhances shoot desiccation time across 18 Neotropical tree species. Context Leaf-stem vulnerability segmentation predicts lower xylem embolism resistance in leaves than stems thereby preserving expensive organs such as branches or the trunk. Although vulnerability segmentation has been intensively investigated these past decades to test its consistency across species, the extent to which vulnerability segmentation promotes drought resistance is not well understood. Aims We investigated the theoretical impact of the degree of vulnerability segmentation on shoot desiccation time estimated with a simple trait-based model. Methods We combined data from 18 tropical rainforest canopy tree species on embolism resistance of stem xylem (flow-centrifugation technique) and leaves (optical visualisation method). Measured water loss under minimum leaf and bark conductance, leaf and stem capacitance, and leaf-to-bark area ratio allowed us to calculate a theoretical shoot desiccation time (tcrit). Results Large degrees of vulnerability segmentation strongly enhanced the theoretical shoot desiccation time, suggesting vulnerability segmentation to be an efficient drought resistance mechanism for half of the studied species. The difference between leaf and bark area, rather than the minimum leaf and bark conductance, determined the drastic reduction of total transpiration by segmentation during severe drought. Conclusion Our study strongly suggests that vulnerability segmentation is an important drought resistance mechanism that should be better taken into account when investigating plant drought resistance and modelling vegetation. We discuss future directions for improving model assumptions with empirical measures, such as changes in total shoot transpiration after leaf xylem embolism.

Chromosomal-level reference genome of the neotropical tree Jacaranda mimosifolia D. Don

Jacaranda mimosifolia D. Don is a deciduous tree widely cultivated in the tropics and subtropics of the world. It is famous for its beautiful blue flowers and pinnate compound leaves. In addition, this tree has great potential in environmental monitoring, soil quality improvement, and medicinal applications. However, a genome resource for J. mimosifolia has not been reported to date. In this study, we constructed a chromosome-level genome assembly of J. mimosifolia using PacBio sequencing, Illumina sequencing, and Hi-C technology. The final genome assembly was ∼707.32 Mb in size, 688.76 Mb (97.36%) of which could be grouped into 18 pseudochromosomes, with contig and scaffold N50 values of 16.77 and 39.98 Mb, respectively. A total of 30,507 protein-coding genes were predicted, 95.17% of which could be functionally annotated. Phylogenetic analysis among 12 plant species confirmed the close genetic relationship between J. mimosifolia and Handroanthus impetiginosus. Gene family clustering revealed 481 unique, 103 significantly expanded, and 16 significantly contracted gene families in the J. mimosifolia genome. This chromosome-level genome assembly of J. mimosifolia will provide a valuable genomic resource for elucidating the genetic bases of the morphological characteristics, adaption evolution, and active compounds biosynthesis of J. mimosifolia.

Malformation Disease in Tabebuia rosea (Rosy Trumpet) Caused by Fusarium pseudocircinatum in Mexico

Tabebuia rosea (rosy trumpet) is an economically important Neotropical tree in Mexico that is highly valued for the quality of its wood, which is used for furniture, crafts, and packing, and for its use as an ornamental and shade tree in parks and gardens. During surveys conducted in the lower Balsas River Basin region in the states of Guerrero and Michoacán, symptoms of floral malformation were detected in T. rosea trees. The main objectives of this study were to describe this new disease, to determine its causal agent, and to identify it using DNA sequence data. A second set of objectives was to analyze the phylogenetic relationship of the causal agent to Fusarium associated with Swietenia macrophylla trees with malformation surveyed in the same region, and to compare mycotoxin production and the mating type idiomorphs of fusaria recovered from T. rosea and S. macrophylla. Tabebuia rosea showed malformed inflorescences with multiple, tightly curled shoots, and shortened internodes. A total of 31 Fusarium isolates recovered from symptomatic T. rosea (N = 20) and S. macrophylla (N = 11) trees were identified by molecular analysis as F. pseudocircinatum. Pathogenicity tests showed that isolates of F. pseudocircinatum recovered from T. rosea, induced malformation in inoculated T. rosea seedlings. Eighteen F. pseudocircinatum isolates were tested for their ability to produce mycotoxins and other secondary metabolites. Moniliformin, fusaric acid, bikaverin, beauvericin, aurofusarin and 8-O-methylbostrycoidin were produced by at least 1 strain of the 18 isolates tested. A multiplex PCR assay for mating type idiomorph revealed that 22 F. pseudocircinatum isolates were MAT1-1 and 9 were MAT1-2. Here we report a new disease of T. rosea in Mexico caused by F. pseudocircinatum.

Climate Defined but Not Soil-restricted: The Distribution of a Neotropical tree through Space and Time

AimsBrosimum rubescens, a tree species with a Neotropical distribution, can achieve local monodominance in Southern Amazonia forests. Understanding how and why this species varies across space and time is important because the monodominance of some species alters ecosystems complexity. Here we evaluate the fundamental ecological niche of B. rubescens by species distribution models (SDM), combining predictive environmental variables with occurrence points. We specifically aim to 1) determine how the spatial distribution patterns of B. rubescens vary with different environmental predictive variables, and 2) evaluate the temporal and spatial persistence of B. rubescens in the Neotropics. MethodsTo generate the SDMs, the predictive environmental variables were incorporated as main components of climatic, hydric and soil variables. ResultsAll algorithms show higher performance in spatial predictions for hydric variables and for the combination of climatic, hydric and edaphic variables. We identified that the potential niches of B. rubescens seem to be defined by climatic fluctuations, with the edaphic conditions being predictive variables that are not restrictive of their presence on the evaluated spatial scale. From the LMG (Last Glacial Maximum) to the present, the species seems to have increased its spatial amplitude; however, from the present to the future, predictions suggest that B. rubescens will experience a considerable loss of its range. ConclusionsOur findings show the independent and combined effects of different environmental variables, allowing us to identify which limit or facilitate the spatial distribution of B. rubescens. We corroborate the spatial persistence and geographical fidelity of the species' spatial patterns over time.

Photosynthetic plasticity of a tropical tree species, Tabebuia rosea, in response to elevated temperature and [CO2]

Atmospheric and climate change will expose tropical forests to conditions they have not experienced in millions of years. To better understand the consequences of this change we studied photosynthetic acclimation of the neotropical tree species Tabebuia rosea to combined 4°C warming and twice-ambient (800 ppm) CO. We measured temperature responses of the maximum rates of ribulose 1,5-bisphosphate carboxylation (V), photosynthetic electron transport (J), net photosynthesis (P), and stomatal conductance (gs), and fitted the data using a probabilistic Bayesian approach. To evaluate short-term acclimation plants were then switched between treatment and control conditions and re-measured after 1–2 weeks. Consistent with acclimation, the optimum temperatures (T) for V, J and P were 1–5°C higher in treatment than in control plants, while photosynthetic capacity (V, J, and P at T) was 8–25% lower. Likewise, moving control plants to treatment conditions moderately increased temperature optima and decreased photosynthetic capacity. Stomatal density and sensitivity to leaf-to-air vapor pressure deficit were not affected by growth conditions, and treatment plants did not exhibit stronger stomatal limitations. Collectively, these results illustrate the strong photosynthetic plasticity of this tropical tree species as even fully-developed leaves of saplings transferred to extreme conditions partially acclimated.

Cyathea gratissima, a new name for a Neotropical tree fern (Cyatheaceae, Polypodiopsida)

Domin, C. (1930) The species of the genus Cyathea J.Sm. Acta Botanica Bohemica 9: 85–174.Hooker, W.J. & Baker, J.G. (1874) Synopsis Filicum, ed. 2. R. Hardwicke, London. https://doi.org/10.5962/bhl.title.41433Tejedor, A. & Calatayud, G. (2018) Six new scaly tree ferns (Cyathea: Cyatheaceae) from Northern Peru. American Fern Journal 108(4): 117–138. https://doi.org/10.1640/0002-8444-108.4.117Weigend, M. (2002) Observations on the biogeography of the Amotape-Huancabamba Zone in northern Peru. The Botanical Review 68: 38–54. https://doi.org/10.1663/0006-8101(2002)068[0038:OOTBOT]2.0.CO;2