Water ascends in woody plants : so what?

Woody plants are often considered as static individuals, taking up water via the root system and losing it again via leaf transpiration. Quite boring one might conclude when considering that more than 95% of the water taken up by the roots is transpired by the leaves. But the story suddenly becomes much more intriguing when the dynamics happening during the ascent of water in woody plants are included. This keynote talk will elaborate on water transport in woody plants, highlighting where the dynamics come from, what the relevance is and which tissues are involved. The power to decipher water transport dynamics with plant-based measurements, where sap flow and stem diameter variations are at the forefront, will be illustrated. The concept of plant-based measurements will be complemented with a viewpoint on how sophisticated mechanistic water transport models can assist in plant-based irrigation scheduling or early warning and stress detection systems

Putting two water transport models to the test under wet and dry conditions

In order to improve fruit quality and quantity, accurate monitoring of the water status is necessary. The water status can be continuously predicted by using a mechanistic water transport and storage model (e.g. Steppe et al., 2006; 2008). This model typically links measurements of sap flow rate (SF) and stem diameter variations (D) to simulate stem water potential (Ψstem), which is recognised as one of the best indicators for evaluating plant water status. Despite good model performance under sufficient water availability, the model fails under dry conditions. However, a proper simulation of water transport under drought is essential for many applications. For example, grapevines are often subjected to some level of drought stress during the growing season in order to improve the quality of the grapes. Therefore, we aim at adjusting the existing model to improve its performance in simulating water transport during drought conditions. First, a dynamic function describing changes in hydraulic xylem resistance is used to replace the former constant parameter, and represents the resistances encountered in the soil, root and stem (RX). Second, also the former constant radial flow resistance between xylem and storage tissues has been replaced by an equation (RS). For the first time, equations for RX and RS instead of parameters were used in the model, and simulations were compared to the original ones. Both models functioned well under wet conditions, but where the original model failed under dry events, the adapted model could still accurately simulate D and Ψstem under these conditions. The adapted model is thus capable of describing the grapevine’s hydraulic response to both wet and (severe) drought conditions and seems very promising within the context of an automatic plant-based system for water status monitoring

The potential of the tree water potential

Non-invasive quantification of tree water potential is one of the grand challenges for assessing the fate of trees and forests in the coming decades. Tree water potential is a robust and direct indicator of tree water status and is preferably used to track how trees, forests and vegetation in general respond to changes in climate and drought. In this issue of Tree Physiology, Dietrich et al. (2018) predict the daily canopy water potential of mature temperate trees from tree water deficit derived from stem diameter variation measurements

A Mariotte-based verification system for heat-based sap flow sensors

Determination of the accuracy of commonly used techniques for measuring sap flux density in trees often presents a challenge. We therefore designed and built a verification system for heat-based sap flow sensors typically used at stem level. In the laboratory, a Mariotte's bottle device was used to maintain a constant flow rate of water through freshly cut stem segments of American beech (Fagus grandifolia Ehrh.). This verification system was used to determine the accuracy of three heat-based sap flux density techniques: heat pulse velocity, thermal dissipation and heat field deformation. All three techniques substantially underestimated sap flux density when compared against gravimetric measurements. On average the actual sap flux density was underestimated by 35% using heat pulse velocity, 46% using heat field deformation and 60% using thermal dissipation. These differences were consistent across sap flux densities ranging from 5 to 80 cm(3) cm(-2) h(-1). Field measurements supported the relative sensor performance observed in the laboratory. Applying a sensor-specific correction factor based on the laboratory test to the field data produced similar estimates of sap flux density from all three techniques. We concluded that a species-specific calibration is therefore necessary when using any of these techniques to insure that accurate estimates of sap flux density are obtained, at least until a physical basis for error correction can be proposed

Beneficial effects of zeolites on plant photosynthesis

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Understanding plant responses to drought: how important is woody tissue photosynthesis?

Within trees, it is known that a part of the respired CO2 is assimilated in chlorophyll-containing stem and branch tissues. However, the role of this woody tissue photosynthesis in tree functioning remains unclear, in particular under drought stress conditions. In this study, stem diameter and leaf photosynthesis were measured for one-year-old cutting-derived plants of Populus nigra 'Monviso' under both well-watered and drought stress conditions. Half of the plants were subjected to a stem and branch light-exclusion treatment to prevent woody tissue photosynthesis to occur, while the other trees served as controls. Drought stress was induced in both treatments by limiting the water supply. We found that under well-watered conditions, light-exclusion resulted in reduced stem radial daily growth rate (DG) relative to DG observed for control trees. In response to drought, stem shrinkage of the light-excluded trees was more pronounced as compared to the control trees. Maximum leaf net photosynthesis (A(max)) decreased more rapidly in light-excluded trees compared to the controls during drought stress. Our results are the first to report on the potentially significant role of woody tissue photosynthesis in tree drought stress tolerance. Moreover, our study implies that the impact of assimilation of respired CO2 on tree functioning extends beyond local stem processes and indicates that woody tissue photosynthesis is potentially a key factor in understanding plant responses to drought stress