1. Jacobs et al. (1996) Stomatal behavior and photosynthetic rate of unstressed grapevines in semi-arid conditions. Agricultural and Forest Meteorology, 2, 111-134. Shimoda and Oikawa (2006). Temporal and spatial variations of canopy temperature over a C3-C4 mixture grassland. Hydrol. Process., 20, 3503-3516. Tanner (1963). Plant temperature. Agron. J., 55, 210-211. Van der Ploeg et al. (2008). Matric potential measurements by polymer tensiometers in cropped lysimeters under water-stressed conditions. Vadose Z. J., 7, 1048-1054. Continuous observations (every 30 minutes): Pressure head h (Polymer Tensiometers measuring till -1.6 MPa): 2 observation points each plot at 0.05, 0.15 and 0.30 m depth. T air and RH: 1 observation point each plot. T canopy : 1 observation point each plot (infrared thermometry). Theoretical Aspects of Crop Water Stress Root water uptake rate is determined by root water uptake dynamic and soil water content → Root water extraction model (De Jong van Lier et al., 2008) Transpiration rate is determined by stomatal conductance and microclimatic elements, as VPD (Vapor Pressure Deficit) → CO 2 Assimilation model (Jacobs et al., 1996) How to identify plant water stress? Relationship ∆T canopy-air x VPD: linear when there is enough water supply to plants. Difference of ∆T canopy-air between irrigated and non irrigated plot. Comparison between T canopy x wet-bulb temperature T wb : if resistances in healthy plants are low, T canopy must be constantly higher than T wb ; as soon as plant resistance to transpiration increases, T canopy become even higher than T wb. Acknowledgements CAPES-WUR Agreement (proj. n.° 019/06) WUR/The Netherlands (proj. n.° 5100184-01) FAPESP (proj. n.° 2009/02117-7) University of São Paulo - Post-Graduation Section Infrared temperature to assess plant transpiration reduction Angelica Durigon 1*, Quirijn de Jong van Lier 1 and Klaas Metselaar 1 Department of Biosystems Engineering, ESALQ-University of São Paulo, Brazil. *adurigon@esalq.usp.br 2 Department of Environmental Sciences, Wageningen University and Research Centre, The Netherlands. University of São Paulo Main Bibliographic References Bakker et al. (2007). New polymer tensiometers: measuring matric pressures down to wilting point. Vadose Z.J., 6, 196-202. De Jong van Lier et al. (2008) Macroscopic root water uptake distribution using a matric flux potential approach. Vadose Z. J., p. 1065-1078. Ehrler (1973). Cotton leaf temperatures as related to soil water depletion and meteorological factors. Agron. J., 65, 404-409. Fucks (1990). Infrared measurement of canopy temperature and detection of plant water stress. Theor. Appl. Climatol., 42, 253-261. Idso et al. (1981). Normalizing the stress-degree-day parameter for environmental variability. Agricultural Meteorology, 24, 45–55. Campaign observations: Root density: 3 times each plot. Stomatal resistance and transpiration rate: 11 days at midday. Leaf Area Index (LAI): 5 times with a ceptometer. Introduction Resistances in the soil-plant-atmosphere pathway determine transpiration rate. Stomatal conductance can be changed by the plant in a reaction to environmental conditions, e.g. a dry soil or a dry atmosphere. A direct effect of stomata closure is increased stomatal resistance leading to reduced transpiration and CO 2 -uptake rate. Indirect consequences are a reduction in energy dissipation and photosynthesis and an increase in leaf temperature. Leaf temperature can be used to evaluate plant water status and transpiration reduction. Objective Study the physical mechanisms and the interaction between factors related to soil and atmosphere that lead to crop water stress. Identify plant water stress occurrence using canopy temperature T canopy. Determine how pressure head h, ∆T canopy-air and vapor pressure deficit VPD in field conditions are related with plant water stress. Use mechanistic models of root water extraction and CO 2 assimilation by leaves to determine which part (soil and/or atmosphere) is responsible for plant water stress occurrence. EGU General Assembly, April 3-8, 2011, Vienna, Austria HS 8.3 Subsurface Hydrology - Unsaturated Zone HS 8.3.1: Soil-plant interactions from the rhizosphere to field scale BEANS (Phaseolus vulgaris L.) Area: 990 m 2 (22 m x 45 m) Two plots: 22 m x 22,5 m (one irrigated) Dry period: 02-Aug to 02-Sep Identifying Plant Water Stress Next steps Simulate the dry period with the root water extraction model of De Jong van Lier et al. (2008): Data of root density, matric flux potential M and soil hydraulic parameters. Simulate the dry period with the CO 2 assimilation model of Jacobs et al. (1996) to identify the midday depression in photosynthesis in plants of both plots: Meteorological data. Data of D s (specific humidity difference between atmosphere and leaves) and LAI. Field Experiment Field experiment was performed in Brazil (UTM 253.300E, latitude 153.400N) from June/2010 to September/2010. Fig. 1: Experimental site. 02/07/201019/07/2010 1 - Plants of non irrigated plot were water stressed. T canopy and T wb Pressure head h - non irrigated plot Fig. 2: ∆T canopy-air x VPD for irrigated plot (above) and non irrigated plot (below). 4 - Water stress decreased in 16-Aug as VPD reduced. 5 - Even with a high VPD difference in 25-Aug, water stress reduced as soil water content increased. Fig. 3: Difference of ∆T canopy-air and VPD between non irrigated (nir) and irrigated (ir) plots Fig. 4: Pressure head h for both observation points of non irrigated plot Fig. 5: Difference between T canopy and T wb for non irrigated and irrigated plots. 3 - Water stress started on 5-Aug (comparing both plots and considering plants in irrigated plot were non water stressed). 9 - Although atmospheric demand was the same for both plots, hydrological parameters differed significantly between them. 10 - Pressure head dropped down to -150.0 m in non irrigated plot and at this time T canopy presented its maximum values (~ 38.0°C). 6 - T canopy is higher than T wb for both plots. 7 - Δ T canopy-wb is approximately constant during whole period for irrigated plot but increases for non irrigated plot. 8 - At the end of the month, T canopy of non irrigated plot becomes even higher than T wb indicating an increment in stomatal resistance to transpiration. 2 - Plants of irrigated plot were not water stressed (linear relationship). ∆T canopy-air and VPD between plots ∆T canopy-air x VPD (02-Aug to 02-Sep) Preliminar conclusion: Sometimes observed plant water stress was a combined effect of soil and atmosphere, on other ocassions it has been a single effect of soil or atmosphere.