Characterization of the Transpiration of a Vineyard under Different Irrigation Strategies Using Sap Flow Sensors

Lysimeters are the reference method for determining ETc, but they are expensive and complex, which limits their use. The first objective of this work was to adjust and evaluate the robustness of sap flow sensors in order to determine the transpiration of a vineyard and, together with an evaporation model, to calculate the ETc of the vineyard. For this purpose, we compared water consumption data obtained from a vineyard weighing lysimeter (ETcLys) with the sum of transpiration obtained from sap flow sensors (TSF) and evaporation estimated empirically over four years (2012, 2013, 2014 and 2015). The second objective was to obtain the relationship between the vegetative growth and transpiration of the vines with different water availability (irrigation and rainfed treatments), as an alternative method for estimating vine water needs adjusted to their real development. The third and last objective was to evaluate the transpiration response of the vines when subjected to water stress. We carried out the work in an experimental vineyard which has a well-established weighing lysimeter. As a result, a good match was obtained between vine sap flow and transpiration (R2 = 0.85) as well as a good relationship between vegetative growth and vine transpiration (FiPAR: R2Irrigation = 0.34. R2Rainfed = 0.54; LAI: R2Irrigation = 0.68. R2Rainfed = 0.53).

Assessing reliability of HRM sap-flow sensors under large range of vapor pressure deficit

<p>Evapotranspiration (ET) is a major water flux of ecosystems and represents globally 60-80% of the incoming precipitation lost by terrestrial environments. In forested lands, tree transpiration (TR) is the dominant component of ET, yet remains challenging to measure. Over the years, sap-flow sensors have become the standard tool for quantifying tree TR and different methods based on thermal approaches have been developed. Heat ratio methods (HRM) are considered as the most reliable and accurate method to quantify absolute flows. Leading commercial brands ensure an accurate measurement of positive flows up to 100 cm hr<sup>-1</sup> but different studies have highlighted a saturation effect at high flows with threshold for accuracy remaining elusive[RS1] . Due to climate change, the occurrence, the severity and the duration of extreme events like heat waves and dry periods are expected to increase in future, so the potential for high TR rate periods will also increase. Therefore, it is crucial to determine the species-specific environmental conditions allowing a reliable measurement of TR in order to improve or understanding of eco-hydrological and physiological processes during high potential TR periods that can be crucial for vegetation survival. In this study, we tested the accuracy of HRM sap-flow sensors for beech (Fagus sylvatica) and oak (Quercus robur) tree species under extreme vapor pressure deficit (VPD) conditions in order to determine threshold for reliable measurements. In greenhouse conditions, we collected a complete and dense series of TR response to VPD between 0.7 to 8.3 kPa for potted beech and oak trees using three different methods: infrared gas analyser, gravimetric method, and HRM sap-flow sensors. Responses shown a linear trend at the low-canopy leaf level (41.5 and 45.1 mg H<sub>2</sub>O m<sup>-2</sup> s<sup>-1</sup> kPa<sup>-1</sup> respectively for beech and oak) but a bi-linear conformation at the whole plant level (1<sup>st</sup> slope = 12.04 ± 0.7 mg H<sub>2</sub>O m<sup>-2</sup> s<sup>-1</sup> kPa<sup>-1</sup> and break-point at 3.9 ± 0.07 kPa for beech trees). Sap-flow sensors using the HRM method displayed a clear inability to reliably measure flows under high VPD conditions. Thresholds of 2.25 ± 0.04 and 2.87 ± 0.14 kPa were identified as the maximum limit of method reliability for beech and oak respectively. In highly demanding environments, we suggest a bi-linear extrapolation beyond VPD threshold for better quantifying tree TR. Further experiments aiming at characterizing TR responses to VPD for a broad range of species and in different water deficit conditions are certainly needed for better understanding tree transpiration at the whole stand level.</p>

Effect of soil straw cover on evaporation, transpiration, and evapotranspiration in sugarcane cultivation

Residual straw affects the physical, chemical, and biological attributes of soil and can influence plant transpiration and the evaporation of water from the soil. Therefore, in this study, we evaluated the effect of straw on evaporation and transpiration of sugarcane. The experiment was conducted in a 2.5 ha area irrigated via a central pivot. The experiment consisted of two treatments, namely, with and without straw removal of soil. Evaporation was determined by means of equations and transpiration with sap flow sensors using the heat balance method. Evapotranspiration of the crop was measured using the Bowen ratio method and compared with the sum of the collected soil evaporation and transpiration data. On the basis of relationship analysis between the evapotranspiration of the crop measured using the Bowen ratio method and the sum of the sap flow combined with estimates of the evaporation of soil water, we obtained coefficient of determination values of 0.65 and 0.69, and angular coefficients of 1.01 and 0.96 for treatments with and without straw, respectively. We accordingly found that the use of straw on the soil reduces the evaporation of soil water and increases the transpiration and evapotranspiration of the crop.

Water-use efficiency, photosynthetic characteristics, and high through-put phenotyping methods for soybean (glycine max) genotypes contrasting in carbon isotope discrimination

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Drought is a huge concern for soybean growers across the world, and in the Midwestern US is the main limitation to grain yield. A way to protect against drought stress is for plants to use water more efficiently. Carbon isotope discrimination (CID) is a measured trait that is related to water-use efficiency (WUE), and can be used to screen genotypes for higher WUE. Several genotypes were studied in multiple greenhouse and field experiments with varying drought stress treatments. Genotypes exhibiting less CID were shown to have a higher WUE, and CID was related to WUE. The higher WUE genotypes also exhibited differences in photosynthetic traits, especially in their stomatal behavior to restrict water loss. In terms of grain yield, very few differences were observed between the genotypes. Thermal images to estimate canopy temperature and sap flow sensors to estimate field water use provided excellent insight into differences among watering treatments and genotypes for transpiration rates. This research demonstrates, that in soybean, CID can be used as a screening tool to select for higher WUE, and higher WUE is likely a result of increased stomatal restrictions to prevent water loss during periods of drought stress. However, these genotypes exhibiting less transpiration showed minimal, if any grain yield reduction. Further, whole field imaging can also be utilized to identify higher WUE genotypes, and sap flow sensors can be expected to estimate water use in the field. Both resulting in reduced labor and more efficient time use.