Ping Lu

Regulation of canopy conductance and transpiration and their modelling in irrigated grapevines

Whole-vine transpiration was estimated for well-watered nine-year-old Sultana grapevines (Vitis vinifera L. cv. Sultana) from xylem sap flow measured with Graniers heat-dissipation probes. Canopy conductance of the grapevine was calculated by inverting the Penman-Monteith equation. Transpiration from grapevine canopies was strongly controlled by the canopy conductance. Canopy conductance decreased exponentially with increasing vapour pressure deficit (VPD) except in the morning when solar radiation was less than 200 W m-2 and the canopy conductance was predominantly limited by the solar radiation. A non-linear model of canopy conductance as a function of the solar radiation and VPD explained > 90% of the variation observed in canopy conductance. Under contrasting VPD conditions (daytime maximum of 3 kPa vs 8 kPa), grapevines were able to regulate their canopy conductance from 0.006 to 0.001 m s-1 to maintain a near constant transpiration. Whole-canopy transpiration calculated from modelled canopy conductance using the Penman-Monteith equation was highly correlated with the measured transpiration (sap flow) values over the range of 0-0.20 mm h-1 (R2 > 0.85). Cross-validation shows that these mechanistic models based on solar radiation and VPD provide good predictions of canopy conductance and transpiration under the conditions of the study.

A simple field validation of daily transpiration derived from sapflow using a porometer and minimal meteorological data

Heat-pulse techniques are routinely used to estimate transpiration from canopies of woody plants typically without any local calibration, mainly because of the difficulty of doing so in the field and, frequently, lack of detailed weather data. This is despite concerns that the techniques may produce erroneous values under certain conditions, such as when evaporative demand is high. In this study, we used a micrometeorological approach to validate transpiration from irrigated olives deduced from heat-pulse technique by ascertaining precise values for the parameters that are critical for converting heat-pulse velocity to sapflow. The micrometeorological approach involved limited data on stomatal conductance (gs), obtained hourly with a porometer on four contrasting days, and was used to calibrate a simple model for predicting conductance. Predicted stomatal conductance (gsm) agreed well with that measured, and when both were used to calculate hourly transpiration, they produced values that were within 10% of each other. This was despite brief underestimations of transpiration based on gsm (Tm) in the early hours of the day that arose from poor determination of incident radiation at this time. We then used Tm to iteratively set the values for the various parameters, including the time-out value that accounts for zero-flow conditions, needed to convert heat-pulse velocity to sapflow, for the four days. The best fit between Tm and transpiration from sapflow (Ts) was obtained with time-out value set to 120xa0s. All heat-pulse velocity data were therefore analysed with this time-out value to obtain sapflow and, hence, transpiration (Ts). Comparison of Tm and Ts for the whole season showed that the former tended to produce higher values on certain days when vapour pressure deficit (D) was high in summer (December–February). While Ts occasionally produced larger values than Tm under the mild conditions of autumn (March–April). Totals of the daily transpiration during the 190-day period were within 10% of each other.

Characterization of Thermal Stress Phenomena Induced Into a Packaged Planar Lightwave Circuit Using Fiber Bragg Grating Sensors

We present a technique for the characterization and analysis of the thermal stress in the optical substrate of packaged photonic devices. This method allows optimization of the package geometry in order to improve the passive compensation of the thermal sensitivity of photonic devices. To the best of our knowledge, we report for the first time the use of strongly chirped, weakly apodized fiber Bragg grating (FBG) sensors to evaluate the stress distribution induced by the package in the planar lightwave circuit (PLC) substrate. We also evaluated the substrate thermal stress using finite element analysis (FEA). We investigated some of the package design parameters that can be used to control and tune the amount of stress that can be applied to the photonic device optical substrate. Our goal is to optimize the design of a package that applies tensile stress to the optical device to compensate unwanted effects due to ambient temperature variation.Copyright