Constant hydraulic supply enables optical monitoring of transpiration in a grass, a herb, and a conifer

Using long-term field data to quantify water potential regulation in response to VPD and soil moisture in a conifer tree

The regulation of vascular water potential (Ψstem) by stomata is one of the most dynamic and important behaviours in vascular plants, playing a central role in determining gas exchange and vulnerability to drought. Yet, the species-specific characterization of Ψstem regulatory behaviour in response to soil or atmospheric dryness remains elusive. We hypothesize that Ψstem regulatory behaviour can only be defined when the combination of both vapour pressure deficit (VPD) and soil water potential (Ψsoil) effects is considered. To test this hypothesis, we collected a high-resolution time series of Ψstem using optical dendrometers from trees of a hardy conifer, Callitris rhomboidea, monitored across multiple highly variable growing seasons. The regulatory behaviour of Ψstem collected over a total of 571 d could be predicted on the basis of diurnal Ψsoil and VPD (R2 = 0.74) using five mechanism-aligned parameters that describe specific stomatal regulation. Our novel approach to predict species-specific water potential variation in response to seasonal change using data from a continuous Ψstem monitoring technique creates a new opportunity to quantitatively compare water use and climatic sensitivity between diverse species or genotypes in the field or laboratory.

Increasing photosynthetic benefit with decreasing irrigation frequency in an Australian temperate pasture exposed to elevated carbon dioxide

Elevated atmospheric CO2 (e[CO2]) often enhances plant photosynthesis and improves water status. However, the effects of e[CO2] vary significantly and are believed to be influenced by water availability. With a future warmer climate expected to increase the frequency and severity of extreme rainfall, the response of plants to e[CO2] under changing precipitation patterns remains uncertain. We examined the effects of e[CO2] and different irrigation regimes on perennial ryegrass in a free-air CO2 enrichment (FACE) experiment. Immediately after irrigation, the mean net photosynthetic rate was 21.2% higher under e[CO2] compared with ambient conditions. This benefit increased over time, reaching a 31.3% higher rate as days since watering increased, indicating a substantial increase in photosynthetic benefit with longer intervals between watering. Mean stomatal conductance was 21% lower in ryegrass under e[CO2] immediately after irrigation compared with ambient plots. However, the reduction in stomatal conductance under e[CO2] decreased as the interval between irrigation events increased, showing no difference 7-10 d after an irrigation event. These results imply that plants benefit most from carbon fertilization, assimilating relatively more carbon and losing less water, during periods with less frequent rainfall. These findings have significant implications for understanding leaf-level responses to climate change.

Polyploidy drives changes in tissue allocation modifying whole-plant water relations

Polyploid plants often display functional trait values distinct from those of diploids, influencing their stress tolerance and adaptive capacity. These differences shape how polyploids interact with their environment, a factor that is crucial to their evolutionary success. Here, we investigated the species complex Dianthus broteri, where ploidy level is known to correlate with water availability, as a model system to understand the possible link between ploidy and whole-plant water relations. We quantified allocation between leaves, xylem, and roots in 4 different ploidies of D. broteri (2×, 4×, 6×, 12×), and examined its relationship with hydraulic efficiency (Kr-s), water potential regulation, and stomatal conductance (gc) in response to varying leaf-to-air vapor pressure deficits (VPDL). A gradient in tissue allocation with increasing ploidy led to contrasting water-use strategies within D. broteri. Higher ploidy was associated with greater allocation to roots and xylem, resulting in higher Kr-s and gc and lower water potential gradients. Despite these differences, gc responses to VPDL were largely consistent across ploidies. In D. broteri 12×, the significant investment in water uptake and transport without a proportional increase in leaf area appeared suboptimal, incurring high xylem costs per unit water transport. However, this trade-off also led to increased water uptake and transport efficiency, potentially advantageous under water-limited conditions. Overall, our results indicate that multiple rounds of genome duplication cause substantial changes in whole-plant water relations, likely impacting water stress exposure in the field.

Long-Term in vivo Observation of Maize Leaf Xylem Embolism, Transpiration and Photosynthesis During Drought and Recovery

Plant water transport is essential to maintain turgor, photosynthesis and growth. Water is transported in a metastable state under large negative pressures, which can result in embolism, that is, the loss of function by the replacement of liquid xylem sap with gas, as a consequence of water stress. To avoid experimental artefacts, we used an optical vulnerability system to quantify embolism occurrence across six fully expanded maize leaves to characterize the sequence of physiological responses (photosynthesis, chlorophyll fluorescence, whole-plant transpiration and leaf inter-vein distance) in relation to declining water availability and leaf embolism during severe water stress. Additionally, we characterize the recovery of leaf function in the presence of sustained embolism during a 6-day recovery period. Embolism formation occurred after other physiological processes were substantially depressed and were irreversible upon rewatering. Recovery of transpiration, net CO2 assimilation and photosystem II efficiency were aligned with the severity of embolism, whereas these traits returned to near pre-stress levels in the absence of embolism. A better understanding of the relationships between embolism occurrence and downstream physiological processes during stress and recovery is critical for the improvement of crop productivity and resilience.

Stomatal response to VPD is not triggered by changes in soil-leaf hydraulic conductance in Arabidopsis or Callitris

Stomatal closure under high VPDL (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψleaf) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψleaf appear to drive stomatal VPDL response, but many argue that dynamic changes in soil-to-leaf hydraulic conductance (Ks-l) make an important contribution to this response pathway, even in well-hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance (gc) in response to typical changes in daytime VPDL is influenced by dynamic changes in Ks-l. We use well-watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia-0) and the dry forest conifer Callitris rhomboidea. The dynamics of Ks-l and gc were continuously monitored by combining concurrent in situ measurements of Ψleaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPDL were imposed to induce stomatal closure and observe the impact on Ks-l. In both species, gc was observed to decline substantially as VPDL increased, while Ks-l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in Ks-l. Static Ks-l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.

Development and application of an inexpensive open-source dendrometer for detecting xylem water potential and radial stem growth at high spatial and temporal resolution

There is currently a need for inexpensive, continuous, non-destructive water potential measurements at high temporal resolution (<1 min). We describe here the development and testing of an entirely open-source dendrometer that, when combined with periodic Scholander pressure chamber measurements, provides sub-minute resolution estimates of water potential when placed on tissues exhibiting little or no secondary growth (petioles, monocotyledon stems). The dendrometer can also be used to measure radial growth of stems and branches when placed on dicotyledon and gymnosperm species. The dendrometer can be interfaced directly with a computer in real time in the lab or greenhouse, or connected to a datalogger for long periods of use in the field on batteries. We tested this device on a herbaceous dicotyledon (Helianthus annuus) (petioles and stems) and a monocotyledon (Zea mays) species (stems) for 1 week during dehydration and re-watering treatments under laboratory conditions. We also demonstrated the ability of the device to record branch and trunk diameter variation of a woody dicotyledon (Rhus typhina) in the field. Under laboratory conditions, we compared our device (hereafter 'contact' dendrometer) with modified versions of another open-source dendrometer (the 'optical' dendrometer). Overall, contact and optical dendrometers were well aligned with one another, with Pearson correlation coefficients ranging from 0.77 to 0.97. Both dendrometer devices were well aligned with direct measurements of xylem water potential, with calibration curves exhibiting significant non-linearity, especially at water potentials near the point of incipient plasmolysis, with pseudo R2 values (Efron) ranging from 0.89 to 0.99. Overall, both dendrometers were comparable and provided sufficient resolution to detect subtle differences in stem water potential (ca. 50 kPa) resulting from light-induced changes in transpiration, vapour pressure deficit and drying/wetting soils. All hardware designs, alternative configurations, software and build instructions for the contact dendrometers are provided.

In situ characterisation of whole-plant stomatal responses to VPD using leaf optical dendrometry

Vapour pressure deficit (VPD) plays a crucial role in regulating plant carbon and water fluxes due to its influence on stomatal behaviour and transpiration. Yet, characterising stomatal responses of the whole plant to VPD remains challenging due to methodological limitations. Here, we develop a novel method for in situ assessment of whole-plant stomatal responses (gc ) to VPD in the herbaceous plant Tanacetum cinerariifolium. To do this, we examine the relationship between daytime VPD and the corresponding soil-stem water potential gradient (ΔΨ) monitored using the optical dendrometry in well-hydrated plants under nonlimiting light in both glasshouse and field conditions. In glasshouse plants, ΔΨ increased proportionally with the VPD up to a threshold of 1.53 kPa, beyond which the slope decreased, suggesting a two-phase response in gc . This pattern aligned with corresponding gravimetrically measured gc behaviour, which also showed a decline when VPD exceeded a similar threshold. This response was then compared with that of field plants monitored using the optical dendrometry technique over a growing season under naturally variable VPD conditions and nonlimiting light and water supply. Field plants exhibited a similar threshold-type response to VPD but were more sensitive than glasshouse individuals with a VPD threshold of 0.74 kPa. The results showed that whole-plant gc responses to VPD can be characterised optically in T. cinerariifolium, introducing a new tool for the monitoring and characterisation of stomatal behaviour in situ.

Functional responses to multiple sequential abiotic stress (waterlogging-drought) in three woody taxa with different root systems and stress tolerance

There is generally a trade-off in the resistance to drought and to waterlogging. However, several species are sequentially subjected to both stressors in many environments. We evaluated the ecophysiological strategies to cope with multiple sequential stress of waterlogging and drought (W + D) of three taxa differing in stress resistance and root morphology: the phreatophic Eucalyptus camaldulensis (Ec) and two shallow-rooted willow clones: Salix matsudana x Salix alba (SmxSa) and Salix nigra (Sn4). Individuals of the three taxa were grown in pots and assigned to either of four treatments: Control (well-watered plants), well-watered followed by drought (C + D); waterlogged for 15 days followed by drought (W15d + D) and waterlogged for 30 days followed by drought (W30d + D). Biomass allocation, growth (diameter, height, length of leaves, and roots), specific leaf area, stomatal conductance, water potential, hydraulic conductivity of roots and branches, leaf C13 and root cortical aerenchyma formation were determined at different stages of the experiment. Ec growth was not affected by W + D, developing tolerance strategies at leaf and whole plant levels. Differential effects of W + D were observed in both Salix clones depending on the time of waterlogging. In Sn4 and SmxSa, the root biomass was affected in W15d + D treatment, but a root tolerance response (aerenchyma and adventitious root formation) was observed in W30d + D. In the three taxa, and contrary to expectations, the previous exposure to a waterlogging period did not increase the susceptibility of the plants to a subsequent drought event. On the contrary, we found tolerance, which depended on the time of waterlogging exposure.

Constant hydraulic supply and ABA dynamics facilitate the trade-offs in water and carbon

Carbon-water trade-offs in plants are adjusted through stomatal regulation. Stomatal opening enables carbon uptake and plant growth, whereas plants circumvent drought by closing stomata. The specific effects of leaf position and age on stomatal behavior remain largely unknown, especially under edaphic and atmospheric drought. Here, we compared stomatal conductance (g_s ) across the canopy of tomato during soil drying. We measured gas exchange, foliage ABA level and soil-plant hydraulics under increasing vapor pressure deficit (VPD). Our results indicate a strong effect of canopy position on stomatal behavior, especially under hydrated soil conditions and relatively low VPD. In wet soil (soil water potential > -50 kPa), upper canopy leaves had the highest g_s (0.727 ± 0.154 mol m^-2 s^-1) and assimilation rate (A; 23.4 ± 3.9 µmol m^-2 s^-1) compared to the leaves at a medium height of the canopy (g_s : 0.159 ± 0.060 mol m² s^-1; A: 15.9 ± 3.8 µmol m^-2 s^-1). Under increasing VPD (from 1.8 to 2.6 kPa), g_s , A and transpiration were initially impacted by leaf position rather than leaf age. However, under high VPD (2.6 kPa), age effect outweighed position effect. The soil-leaf hydraulic conductance was similar in all leaves. Foliage ABA levels increased with rising VPD in mature leaves at medium height (217.56 ± 85 ng g^-1 FW) compared to upper canopy leaves (85.36 ± 34 ng g^-1 FW). Under soil drought (< -50 kPa), stomata closed in all leaves resulting in no differences in g_s across the canopy. We conclude that constant hydraulic supply and ABA dynamics facilitate preferential stomatal behavior and carbon-water trade-offs across the canopy. These findings are fundamental in understanding variations within the canopy, which helps in engineering future crops, especially in the face of climate change.