Nadezhda Nadezhdina

Many tree species have developed extensive root systems that allow them to survive in arid environments by obtaining water from a large soil volume. These root systems can transport and redistribute soil water during drought by hydraulic redistribution (HR). A recent study revealed the phenomenon of evaporation-driven hydraulic redistribution (EDHR), which is driven by evaporative demand (transpiration). In this study, we confirmed the occurrence of EDHR in Chinese white poplar (Populus tomentosa) through root sap flow measurements. We utilized micro-computed tomography technology to reconstruct the xylem network of woody lateral roots and proposed conceptual models to verify EDHR from a physical perspective. Our results indicated that EDHR is driven by the internal water potential gradient within the plant xylem network, which requires three conditions: high evaporative demand, soil water potential gradient, and special xylem structure of the root junction. The simulations demonstrated that during periods of extreme drought, EDHR could replenish water to dry roots and improve root water potential up to 38.9%–41.6%. This highlights the crucial eco-physiological importance of EDHR in drought tolerance. Our proposed models provide insights into the complex structure of root junctions and their impact on water movement, thus enhancing our understanding of the relationship between xylem structure and plant hydraulics.

Trees interconnected through functional root grafts can exchange resources, but the effect of exchange on trees remains under debate. A mechanistic understanding of resource exchange via functional root grafts will help understand their ecological implications for tree water exchange for individual trees, groups of trees and forest stands. To identify the main patterns qualitatively describing the movement of sap between grafted trees, we reviewed the available literature on root grafting in woody plants that focus on tree allometry and resource translocation via root grafts. We then extended the BETTINA model, which simulates mangrove (Avicennia germinans) tree growth on the individual tree scale, to synthesize the available empirical information. Using allometric data from a field study in mangrove stands, we simulated potential water exchange and analysed movement patterns between grafted trees. In the simulations, relative water exchange ranged between −9.17 and 20.3 %, and was driven by gradients of water potential, i.e. differences in tree size and water availability. Moreover, the exchange of water through root grafts alters the water balance of trees and their feedback with the soil: grafted trees that receive water from their neighbours reduce their water uptake. Our individual-tree modelling study is a first theoretical attempt to quantify root graft-mediated water exchange between trees. Our findings indicate that functional root grafts represent a vector of hydraulic redistribution, helping to maintain the water balance of grafted trees. This non-invasive approach can serve as a basis for designing empirical studies to better understand the role of grafted root interaction networks on a broader scale.

One of the giants in Czechoslovakia – now the Czech Republic – in plant water relations, Professor Jan Čermák, passed away on December 23, 2021, in Brno, Czech Republic. Following in the footsteps of earlier noted Czech physiological ecologists – Jiří Čatský, Pavol Eliáš, Josef Huzulák, Zdeněk Šesták and Bohdan Slavík – Jan Čermák carved a reputation in plant water relations. Unlike the others, Čermák collaborated globally in his research endeavors, and these collaborations spanned almost four decades.

Maintaining the activity and function of the shallow root system of plants is essential for withstanding drought stress, but the associated mechanism is poorly understood. By investigating sap flow in 14 lateral roots (LRs) randomly selected from trees of a Chinese white poplar (Populus tomentosa) plantation receiving three levels of irrigation, an unknown root water transport mode of simultaneous daytime bi-directional water flow was discovered. This mode existed in five LRs confined to the surface soil without attached sinker roots. In the longer term, the bi-directional water flow was correlated with the soil water content. However, within the day, it was associated with transpiration. Our data demonstrated that bi-directional root sap flow occurred during the day, and was driven by evaporative demand, further suggesting the existence of circumferential water movement in the LR xylem. We named this phenomenon evaporation-driven hydraulic redistribution (EDHR). A soil-root water transport model was proposed to encapsulate this water movement mode. EDHR may be a crucial drought-tolerance mechanism that allows plants to maintain shallow root survival and activity by promoting root water recharge under extremely dry conditions.

The Heat Field Deformation (HFD) technique is a thermodynamic method for measuring sap flow. Based on continuous heating the HFD method allows for high time resolution measurements which are highly important when studying plant responses to abrupt environmental changes. This work provides a succinct review of previously described features of the HFD methodology. Analyzing symmetrical and asymmetrical temperature differences around a measured linear heater (dTsym and dTas) relative to their ratio dTsym/dTas (so called a K-diagram) is at the heart of this methodology. This key concept, however, has to date only been generally described in previous works on the HFD technique. My objective here is to provide a comprehensive overview describing different types of K-diagrams, their interpretation and application for determining K-values or dTas for a zero flow condition. The K-value is a measured parameter which is particularly important for objectively characterizing heat conducting properties at the sensor insertion point under specific local measurement conditions. Correctly determining the K-value is critical for accurately calculating sap flow based on recorded temperature measurements. I have included in this review several examples demonstrating how the K-value is dependent upon changes to the environment and its important role in sap flow estimation.

Water relations for two remote populations of Dracaena tree species from the dragon tree group, Dracaena cinnabari Balfour f. and Dracaena draco (L.) L., were studied to test our hypothesis that morphological and anatomical differences in leaf structure may lead to varied functional responses to changing environmental conditions. Sap flow measurements were performed using the heat field deformation method for four Dracaena seedlings grown in one glasshouse and two greenhouses, and leaf traits related to plant–water relationships were characterised. All traits studied confirmed that D. cinnabari leaves are more xeric in their morpho-anatomical structure compared with D. draco leaves. No radial sap flow variability was detected in D. draco plant stems, whereas sap flow was found to be higher in the inner part of D. cinnabari stems. The regular occurrence of reverse sap flow at night in both Dracaena species was consistent with a staining experiment. Vapour pressure deficit (VPD) was found to be the main driver for transpiration for both Dracaena species. However, the relationship between VPD and sap flow appeared to be different for each species, with a clockwise or no hysteresis loop for D. draco and a counter-clockwise hysteresis loop for D. cinnabari. This resulted in a shorter transpiration cycle in D. cinnabari. The observed superior water-saving strategy of D. cinnabari corresponds to its more xeric morpho-anatomical leaf structure compared with D. draco.

Hydraulic redistribution plays a relevant role in the water relations of trees in climates with alternation between warm/dry and cold/wet periods. We aim to illustrate the ability of Mediterranean deep‐rooted rainfed olive trees to maintain transpiration during the hot dry season and to redistribute soil water through roots, tending to temporarily homogenize soil moisture vertically. Sap flow was monitored by the heat field deformation method for 2·5 years in the stem, lignotuber, medium and shallow roots of an olive tree tracing the long‐term variations in the patterns of transpiration, water uptake and hydraulic redistribution. During the same period, soil water content and meteorological data were measured and related to sap flow. Results show that hydraulic redistribution within the rhizosphere buffers the seasonal and long‐term water deficits. Under high evaporative demand during the dry summer, the deeper roots connected to stem through the lignotuber uptake water supporting transpiration needs and reducing the intensive drying of the upper soil layers. Copyright © 2014 John Wiley & Sons, Ltd.