Needle length matters. A commentary on 'Needle length in pines as a key trait regulating hydraulic resistance'.

Background and AimsIn conifers, leaf length exhibits remarkable variation across and within species, even within the same individual. Leaves are often shorter in drier sites and at the tops of taller trees. Several hypotheses have been proposed to explain this shortening, but a clear causal framework is lacking. We hypothesize that conifer needles should exhibit a low rate of tip-to-base conduit widening leading to higher hydraulic resistance in long needles, explaining adaptive leaf shortening.MethodsWe sampled needles from 22 Pinus species and one Sequoia sempervirens across a range of environmental conditions. We conducted a detailed intraspecific analysis on four Pinus species by measuring tracheid diameter along the needle, and an interspecific comparison by measuring tracheid diameter at the needle base across all species. In both analyses, we fitted tracheid diameter against distance from the needle tip and calculated the slope (b) of tip-to-base tracheid widening.Key ResultsA low mean intraspecific widening slope (b = 0.12) was found, indicating that tracheid diameter increases only slightly from tip to base. This low widening rate cannot fully compensate for the increase in hydraulic resistance, which therefore increases with needle length. The interspecific slope of mean tracheid diameter at the needle base vs. needle length (0.25) was higher than the intraspecific mean, suggesting that longer-needled species may have wider conduits at the needle apex, offsetting needle length-imposed resistance.ConclusionsOur findings suggest that shorter needles should reduce hydraulic resistance under dry conditions or with height growth, maintaining leaf-specific conductance. We offer a novel explanation for the commonly observed pattern of needle shortening, interpreting it as an adaptive response rather than a physiological limitation.

Microneedles have been an expanding medical technology in recent years due to their ability to penetrate tissue and deliver therapy with minimal invasiveness and patient discomfort. Variations in design have allowed for enhanced fluid delivery, biopsy collection, and the measurement of electric potentials. Our novel microneedle design attempts to combine many of these functions into a single length of silica tubing capable of both light and fluid delivery terminating in a sharp tip of less than 100 microns in diameter. This manuscript focuses on the fluid flow aspects of the design, characterizing the contributions to hydraulic resistance from the geometric parameters of the microneedles. Experiments consisted of measuring the volumetric flow rate of de-ionized water at set pressures (ranging from 69-621 kPa) through a relevant range of tubing lengths, needle lengths, and needle tip diameters. Data analysis showed that the silica tubing (~150 micron bore diameter) adhered to within ±5% of the theoretical prediction by Poiseuille's Law describing laminar internal pipe flow at Reynolds numbers less than 700. High hydraulic resistance within the microneedles correlated with decreasing tip diameter. The hydraulic resistance offered by the silica tubing preceding the microneedle taper was approximately 1-2 orders of magnitude less per unit length, but remained the dominating resistance in most experiments as the tubing length was >30 mm. These findings will be incorporated into future design permutations to produce a microneedle capable of both efficient fluid transfer and light delivery.

Predicting the physiological responses of tree species under future hydroclimate scenarios is essential for understanding and mitigating the impacts of anthropogenic climate change. Here, we present our work on reconstructing the intraseasonal physiological responses of one of the longest living tree species on Earth – Great Basin bristlecone pine (Pinus longaeva, Pinaceae). This species is infamous for its tree ring chronologies that can extend beyond 5,000 years, yet the key physiological traits that will determine its ability to tolerate warmer, drier conditions in the future remain to be characterized. Moreover, the extent to which localized changes in topoclimate and seasonal water availability will impact overall growth performance and survival is also uncertain. To address this, we collected needle samples from trees along an elevation gradient near Great Basin National Park, NV, USA. Using the unique phyllotaxy of this species, we isolated annual needle samples corresponding to five distinct growth years (2018 – 2022). We then developed a novel method for quantifying intraseasonal variation in carbon isotope discrimination and intrinsic water use efficiency by using fine-scale, sequential measurements of needle δ13C in-situ via laser ablation isotope ratio mass spectrometry. We obtained an average of 25 individual δ13C measurements along the lengths of each needle sample, which were all consistent with whole-needle δ13C values measured via traditional elemental analyzer isotope ratio mass spectrometry. However, these sequential δ13C values varied in excess of 1 ‰ (VPDB) along the lengths of each needle sample, likely reflecting intraseasonal changes in water availability through the time in which individual needles were being constructed. Paired with our previous measurements of annual ring width, stomatal density, and needle length from trees at this site, we discuss how this new method provides a more comprehensive understanding of the role of intraseasonal variation in water availability on the overall physiological performance of this species in the past as well as under future hydroclimate scenarios.

Leaf morphology in the upper canopy of trees tends to be different from that lower down. The effect of long-term water stress on leaf growth and morphology was studied in seedlings of Metasequoia glyptostroboides to understand how tree height might affect leaf morphology in larger trees. Tree height increases water stress on growing leaves through increased hydraulic resistance to water flow and increased gravitational potential, hence we assume that water stress imposed by soil dehydration will have an effect equivalent to stress induced by height. Seedlings were subjected to well-watered and two constant levels of long-term water stress treatments. Drought treatment significantly reduced final needle count, area and mass per area (leaf mass area, LMA) and increased needle density. Needles from water-stressed plants had lower maximum volumetric elastic modulus (ε(max)), osmotic potential at full turgor (Ψ¹⁰⁰(π)) (and at zero turgor (Ψ⁰(π)) (than those from well-watered plants. Palisade and spongy mesophyll cell size and upper epidermal cell size decreased significantly in drought treatments. Needle relative growth rate, needle length and cell sizes were linear functions of the daily average water potential at the time of leaf growth (r² 0.88-0.999). We conclude that water stress alone does mimic the direction and magnitude of changes in leaf morphology observed in tall trees. The results are discussed in terms of various models for leaf growth rate.

The response of mature forest stands to a reduction in water availability has received little attention. In particular, the extent to which a short-term reduction in gas exchange can be alleviated in the long-term by acclimation processes is not well understood. We studied the impact of a severe reduction in water availability on the water relations and growth of 35-year-old Pinus laricio Poiret. trees in a replicated experiment. Sapwood and needle increments, soil and tree water status, stand transpiration, xylem embolism and plant hydraulic architecture were monitored over a 3-year period in control and drought-treated plots. Needle length was reduced in drought-treated trees by 30, 19 and 29%, and sapwood increments by 50, 27 and 24% over the 3 years. Drought did not result in tree mortality or in extensive xylem embolism or foliage dieback. On the contrary, a conservative water-use strategy was observed, because minimum leaf water potentials did not differ between treatments or over the season. Plant hydraulic resistance was also unaffected by restricted water availability. Stand transpiration was strongly reduced by drought treatment over the summer, but not during the winter, despite significant differences in leaf area between control and drought-treated trees, implying higher transpiration rates per unit leaf area in the droughted plants. This suggests that water transport capacity, rather than the amount of leaf area, controlled stand transpiration, which is at variance with expectations based on experiments with seedlings and short-term experiments with mature trees.

Conifers have the highest rates of mortality during their first year, often attributed to water stress; yet, this tree life stage is the least studied in terms of hydraulic properties. Previous work has revealed correlations between xylem anatomy to both hydraulic transport capacity and resistance to hydraulic dysfunction. In this study, we compared xylem anatomical and plant functional traits of Pseudotsuga menziesii, Larix occidentalis, and Pinus ponderosa seedlings over the first 10 wk of growth to evaluate potential maximum hydraulic capabilities and resistance to drought-induced embolism. We hypothesized that, based on key functional traits of the xylem, predicted xylem embolism resistance of the species will reflect their previously determined drought tolerances with L. occidentalis, P. menziesii, and P. ponderosa in order of least to most embolism-resistant xylem. Xylem and pit anatomical characteristics and additional hydraulic-related functional traits were compared at five times during the first 10 wk of growth using confocal laser scanning microscopy (CLSM). Based on thickness to span ratio, torus to pit aperture overlap, and torus thickness, primary xylem appeared to be not only more hydraulically conductive but also less embolism-resistant than secondary xylem. By week 10, P. menziesii was predicted to have the most embolism-resistant xylem followed by P. ponderosa and L. occidentalis. Theoretical measurements suggest that hydraulic transport capacities and vulnerability to embolism varied for each species over the first 10 wk of growth; thus, the timing of germination and onset of limited soil moisture is critical for growth and survival of seedlings.

PremiseTrees in wet forests often have features that prevent water films from covering stomata and inhibiting gas exchange, while many trees in drier environments use foliar water uptake to reduce water stress. In forests with both wet and dry seasons, evergreen trees would benefit from producing leaves capable of balancing rainy‐season photosynthesis with summertime water absorption.MethodsUsing samples collected from across the vertical gradient in tall redwood (Sequoia sempervirens) crowns, we estimated tree‐level foliar water uptake and employed physics‐based causative modeling to identify key functional traits that determine uptake potential by setting hydraulic resistance.ResultsWe showed that Sequoia has two functionally distinct shoot morphotypes. While most shoots specialize in photosynthesis, the axial shoot type is capable of much greater foliar water uptake, and its within‐crown distribution varies with latitude. A suite of leaf surface traits cause hydraulic resistance, leading to variation in uptake capacity among samples.ConclusionsShoot dimorphism gives tall Sequoia trees the capacity to absorb up to 48 kg H2O h−1 during the first hour of leaf wetting, ameliorating water stress while presumably maintaining high photosynthetic capacity year round. Geographic variation in shoot dimorphism suggests that plasticity in shoot‐type distribution and leaf surface traits helps Sequoia maintain a dominate presence in both wet and dry forests.

Mild water and salt stress improve water use efficiency by decreasing stomatal conductance via osmotic adjustment in field maize

Soil salinity results in reduced productivity in chickpea. However, breeding for salinity tolerance is challenging because of limited knowledge of the key traits affecting performance under elevated salt and the difficulty of high-throughput phenotyping for large, diverse germplasm collections. This study utilised image-based phenotyping to study genetic variation in chickpea for salinity tolerance in 245 diverse accessions. On average salinity reduced plant growth rate (obtained from tracking leaf expansion through time) by 20%, plant height by 15% and shoot biomass by 28%. Additionally, salinity induced pod abortion and inhibited pod filling, which consequently reduced seed number and seed yield by 16% and 32%, respectively. Importantly, moderate to strong correlation was observed for different traits measured between glasshouse and two field sites indicating that the glasshouse assays are relevant to field performance. Using image-based phenotyping, we measured plant growth rate under salinity and subsequently elucidated the role of shoot ion independent stress (resulting from hydraulic resistance and osmotic stress) in chickpea. Broad genetic variation for salinity tolerance was observed in the diversity panel with seed number being the major determinant for salinity tolerance measured as yield. This study proposes seed number as a selection trait in breeding salt tolerant chickpea cultivars.

AimUnderstanding tree mortality under climate change‐induced droughts requires knowledge of hydraulic trait distribution across environmental gradients. The spatial distribution of embolism resistance (i.e., P50), a key trait linked to hydraulic failure, is currently unknown across the Amazon. Here, we tested how precipitation, water table depth (WTD) and soil fertility interact as filters modulating the geographic hydraulic trait distribution.LocationFour sites in three key regions across the Amazon basin: Central, Western peripheral area and Brazilian Shield.Time PeriodPresent.Major Taxa StudiedTree species.MethodWe measured hydraulic vulnerability curves of 64 tree species (165 individuals) using an orthogonal design contrasting seasonal precipitation gradients, soil fertility and WTD. To estimate the geographical distribution of hydraulic traits for the basin scale, we use the coefficients of the best generalized linear model, projected on a 1 km grid in each environmental layer.ResultsWe show that WTD and soil fertility are the main drivers of embolism resistance, while precipitation has a secondary effect. Trees in shallow WTD and fertile soils had riskier hydraulic strategies. Our spatial projection identified the Amazonian southern band and valleys across the basin as the most vulnerable areas. High soil fertility in these regions increases the variability of hydraulic traits and thus uncertainty in predictions of drought responses.Main ConclusionAt the scale of the Amazon basin, we show that dominant tree species traits converge toward hydraulic resistance when resource (water and nutrients) levels are low and towards hydraulic vulnerability (but with higher trait variability) when resources increase. These results highlight the importance of WTD and soil fertility in structuring hydraulic traits and their effects on tree mortality under climate change‐induced droughts. Further empirical tests covering a wider spatial range and more tree species will help validate our biogeographical distribution model.

Strategic optimisation of Root System Architecture (RSA) represents a critical frontier for stabilising crop productivity amid increasingly unpredictable moisture-deficit regimes. Understanding key root traits underlying effective drought response is necessary to harness the genetic diversity associated with root growth patterns and environmental adaptations. Many functionally significant root architectural traits have been reported, and the mechanistic importance of some of the anatomical ideotypes, such as the increased metaxylem vessel diameter to reduce axial hydraulic resistance to maintain leaf water potential and change in root growth angle to promote geotropic deep-soil moisture foraging, are discussed in this review. Despite the identification of these characteristics, the knowledge gap in their integration into predictive breeding frameworks remains. This review addresses this fragmentation by critically evaluating how the bottleneck of the ‘phenotyping’ process is being broken down through non-invasive high-throughput phenotyping modalities. Dynamic root-soil interfaces can be spatio-temporally quantified in situ using non-destructive technologies such as X-ray computed tomography and MRI, which can detect developmental plasticity masked by destructive sampling. Artificial Intelligence (AI), especially Convolutional Neural Networks, enables automated extraction of high-dimensional topological parameters from complex digital rhizograms. Present review integrates recent advances in phenotyping with molecular regulatory mechanisms, bridging two traditionally disparate fields. By focusing on the DRO1/qSOR1 loci and ABA-auxin crosstalk, we establish critical connections between molecular regulation and field-scale architectural performance. The resulting multi-scale roadmap may help in targeted selection of climate-resilient cultivars to maximize resource use efficiency.