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We estimated daily use of stored water by Scots pine (Pinus sylvestris L.) trees growing in a temperate climate with the ANAFORE model (ANAlysis of FORest Ecosystems) and compared the simulation results with sap flow measurements. The original model was expanded with a dynamic water flow and storage model that simulates sap flow dynamics in an individual tree. ANAFORE was able to accurately simulate diurnal patterns of measured sap flow under microclimatic conditions that differ from those of the calibration period. Strong relationships were found between stored water use and several tree characteristics (diameter at breast height, sapwood area, leaf area), but not with tree height. Relative to transpiration, stored water use varied over time (between less than 1% and 44% of daily transpiration). On days when transpiration was high, trees were more dependent on stored water, indicating that the contribution of internal water to transpiration is not a constant in the water budget of trees. [ABSTRACT FROM AUTHOR]

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO2 efflux rate (FCO2 ) and xylem CO2 concentration ([CO2* ]). Before and after the drought treatment, diurnal fluctuations in FCO2 and [CO2*] corresponded well with variations in stem temperature (Tst). Daytime depressions in FCO2 did not occur. During the drought treatment, FCO2 still responded to stepwise changes in temperature, but diurnal fluctuations in FCO2 were no longer correlated with diurnal fluctuations in Tst. From the moment daily growth rate of the stem became zero, diurnal fluctuations in FCO2 became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in FCO2 were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO2*] showed clear daytime depressions in response to drought. When the tree was re-watered, FCO2 and [CO2*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in FCO2 and [CO2*] disappeared and diurnal fluctuations in FCO2 and [CO2*] corresponded again with variations in Tst. [ABSTRACT FROM AUTHOR]

Calibration of a recently developed water flow and storage model based on experimental data for a young diffuse-porous beech tree (Fagus sylvatica L.) and a young ringporous oak tree (Quercus robur L.) revealed that differences in stem wood anatomy between species strongly affect the calibrated values of the hydraulic model parameters. The hydraulic capacitance (C) of the stem storage tissue was higher in oak than in beech (939.8 versus 212.3 mg MPa-1). Model simulation of the elastic modulus (ε) revealed that this difference was linked to the higher elasticity of the stem storage tissue of oak compared with beech. Furthermore, the hydraulic resistance (Rx) of beech was about twice that of oak (0.1829 versus 0.1072 MPa s mg-1). To determine the physiological meaning of the Rx parameter identified by model calibration, we analyzed the stem wood anatomy of the beech and oak trees. Calculation of stem specific hydraulic conductivity (ks) of beech and oak with the Hagen-Poiseuille equation confirmed the differences in Rx predicted by the model. The contributions of different vessel diameter classes to the total hydraulic conductivity of the xylem were calculated. As expected, the few big vessels contributed much more to total conductivity than the many small vessels. Compared with beech, the larger vessels of oak resulted in a higher ks (10.66 versus 4.90 kg m-1 s-1 MPa-1). The calculated ratio of ks of oak to beech was 2, confirming the Rx ratio obtained by model calibration. Thus, validation of the Rx parameter of the model led to identification of its physiological meaning. [ABSTRACT FROM AUTHOR]

• Background and Aims Stomatal oscillations have been reported in many plant species, but they are usually induced by sudden step changes in the environment when plants are grown under constant conditions. This study shows that in navel orange trees (Citrus sinensis) pronounced stomatal oscillations occur and persist under natural climatic conditions.• Methods Oscillations in stomatal conductance were measured, and related to simultaneous measurements of leaf water potential, and flow rate of sap in the stems of young, potted plants. Cycling was also observed in soil-grown, mature orchard trees, as indicated by sap flow in stem and branches.• Key Results Oscillations in stomatal conductance were caused by the rapid propagation and synchronization of changes in xylem water potential throughout the tree, without rapid changes in atmospheric conditions.• Conclusions The results show marked stomatal oscillations persisting under natural climatic conditions and underscore the need to discover why this phenomenon is so pronounced in orange trees. [ABSTRACT FROM AUTHOR]

Abstract: The use of plant water status indicators such as midday stem water potential (Ψ stem) and maximum daily trunk shrinkage (MDS) in irrigation scheduling requires the definition of a reference or threshold value, beyond which irrigation is necessary. These reference values are generally obtained by comparing the seasonal variation of plant water status with the environmental conditions under non-limiting soil water availability. In the present study an alternative approach is presented based on the plant’s response to water deficit. A drought experiment was carried out on two apple cultivars (Malus domestica Borkh. ‘Mutsu’ and ‘Cox Orange’) in which both indicators (Ψ stem and MDS) were related to several plant physiological responses. Sap flow rates, maximum net photosynthesis rates and daily radial stem growth (DRSG) (derived from continuous stem diameter variation measurements) were considered in the assessment of the approach. Depending on the chosen plant response in relationship with Ψ stem or MDS, the obtained reference values varied between −1.04 and −1.46MPa for Ψ stem and between 0.17 and 0.28mm for MDS. In both cultivars, the approach based on maximum photosynthesis rates resulted in less negative Ψ stem values and smaller MDS values, compared to the approaches with sap flow and daily radial stem growth. In the well-irrigated apple trees, day-to-day variations in midday Ψ stem and MDS were related to the evaporative demand. These variations were more substantial for MDS than for midday Ψ stem. [Copyright &y& Elsevier]