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Unseen process for local/global water footprint and water balance estimates in grapevines.

Understanding the dynamics of nighttime transpiration and water rehydration of plants and grapevines has benefited from recent research, with the aim of obtaining amelioration strategies to maximize water-use efficiency for crops in challenging climates.

Recent research on grapevines has challenged the paradigm that C3 and C4 plants do not transpire at night due to complete stomata closure. Grapevine water loss by transpiration at night can reach up to 50% of daytime transpiration, depending on the level of aridity and water stress. (19.20)

C3 plants (including grapevines), which account for more than 95% of earth's plant species, use rubisco to make a three-carbon compound as the first stable product of carbon fixation. C4 plants possess biochemical and anatomical mechanisms to raise the intercellular carbon dioxide concentration at the site of fixation; this reduces (and sometimes eliminates) carbon losses by photorespiration. Both C3 and C4 plants were thought to close stomata at night.

Since nighttime transpiration is not coupled with photosynthesis, it contributes to decreased water-use efficiency. Furthermore, when plants are not subjected to water stress, nighttime transpiration is highly correlated to vapor-pressure deficit.18 Therefore, considering climate change models have forecasted that nighttime temperatures will increase at a higher rate compared with diurnals, nighttime transpiration might be exacerbated in future global-warming scenarios."

Respecting new insights from research, it is a concern that nighttime transpiration has not been considered in small- scale evapotranspiration models (irrigation scheduling of crops), nor for large-scale models (catchments and forest water-use estimations). This creates a problem for water footprint, water balance and evapotranspiration calculations that could potentially affect growers, irrigation practitioners, water modeling for catchments and government policy.

Implications for irrigation/water use

Climate-change predictions have prompted research into water conservation, since important reductions in rain events in most agricultural areas have been forecasted. Australia is a water-deficit country, and many areas of the continent have experienced long periods of drought during the past two decades.

Australia's agriculture and horticulture industries depend on water supplied by irrigation and use around 70% of the fresh water available. Therefore, much research has explored how to improve water-use efficiency (WUE) at the farm level.

In the 1990s in Australia, two major irrigation strategies were developed: regulated deficit irrigation (RIX) and partial root zone drying (PRD). These techniques can help to increase WUE by exploiting plant physiology-manipulation strategies according to the spatial and temporal distribution of water application within the root zone. These techniques rely heavily on pressurized irrigation systems.

Growth and wood thickening of trees and shrubs are enhanced with increased atmospheric CO2 concentration due to higher photosynthesis and decreased growth and refill cavitated xylem conduits to maintain plant hydraulic conductivity. However, Enight is not coupled with photosynthesis, and this contributes to decreased WUE.

Plants have different strategies to respond to water stress, and these influence shape and dimensions of the parabolic curve presented in Figure 1. In general, for horticultural plants it is considered 0 = -1.0 MPa as a threshold between non-water stress and water-stressed plants. (17,22,24)

Grapevine cultivars that are more conservative in water use (isohydric) tend to show a displacement of the parabolic curve to the right, closing stomata at night with higher 0, measured the previous day. Non-water conservative grapevine cultivars (anisohydric) present peaks shifted toward the left of -1.0 MPa in the parabolic relationship (Fuentes et al., under review).

There is an opportunity for adaptation strategies, in the case of grapevines, defined first by cultivar selection and, second, by management strategies through reduced or controlled water application that will help minimize Enight and maximize rehydration and WUE.

Implication of nighttime transpiration on modeling and water footprint estimations

In general, evapotranspiration (ET) models do not take nighttime water uptake into consideration for small-scale calculations (irrigation scheduling) or larger scale (global modeling). The main problem to account for this previously unseen factor was the requirement of accurate instrumentation to record low flows.

The micrometeorological approach (Bowen ratio, Eddy covariance) has been the most used method to validate ET models in crops and forests. However, low wind velocity and the lack of eddies at night makes it difficult to account for low flows using these techniques. The alternative could be the use of lysimeters, which are sensitive enough to register flows at night.

However, it is not possible to uncouple transpiration from evaporation of water directly from the soil using the latter. It is important to notice that Enight will be associated with water extraction by deeper roots and likely that these methodologies to estimate ET and water footprint need to be revisited.


There is significant value in understanding and characterizing the dynamics of nighttime water uptake and transpiration processes by plants and trees to obtain insights into the adaptation strategies that will contribute to increased WUE in a climate-change scenario. Further research is required to understand the differences in strategies according to different species and cultivars.

Modeling and water footprint methodologies need to be revisited in light of the latest research about nighttime water consumption and transpiration by plants and trees.

Edited and reprinted from Wine & Viticulture Journal July/August 2012 with Approval of the publisher, Winetitles. photorespiration. In these conditions, WUE is expected to increase, since more CO2 will also decrease stomatal conductance by about 20%

In the past, it was a general consensus that C3 and C4 plants closed stomata at night. Any nocturnal water uptake would mainly correspond to stem and organ rehydration and cuticular transpiration. Under these conditions, WUE is not affected by nocturnal water uptake, which only corresponds to plant recovery from water stress endured the previous day.

However, new research has demonstrated that nocturnal transpiration (Enight) due to stomatal opening at night (Gnight) can be as high as 30%-60% compared to diurnal transpiration for arid and semi-arid regions in a variety of eco-systems. (2,9,11,19,20,23,24 )

Considerable Enight has significant implications for the whole-tree water budget and WUE.

Recent studies have characterized nighttime water uptake (Sn) dynamics using sap-flow sensors and gas-exchange measurements and have offered insights into the relationship between nighttime transpiration (Enight) and rehydration.

Literature review/recent research

Sensitive sensors can pick up nighttime water flow

Sap-flow probes used in the past were not sensitive to low flow (typically during nighttime) and gave "noisy" data that added to the paradigm of stomatal closure during the night and led scientists to disregard nighttime sap-flow values and force them to zero for analysis.

However, continuous measurements of sap flow using probes sensitive to low flow has been possible in the past 10 years through the compensated heat-pulse sap-flow system.10,11,12,13,14,15

Figure 1 shows the total Sn measured during nighttime (from sunset to sunrise) by the sap-flow sensors. The total Sn response to water application (either by irrigation or rainfall) shows that S is sensitive to the dynamics of soil-water availability and plant-water status associated with the previous day.

Similar results have been obtained for grapevines (Fuentes et al., submitted).

Parabolic relationships between Sn and plant-water status

Factors such as hormonal signals, mainly abscisic acid (ABA), are related to lower diurnal stomatal conductance (G) at increased levels of water stress.L346 This has been the basis for the development of irrigation techniques such as RDI and PRD that help to increase WUE.

Based on the hypothesis that the same internal factors affecting G, (ABA) will affect Gnight, it is expected that the latter will be responsive to the level of water stress endured by the plant or tree the previous day that can be measured as stem-water potential (Os) using a pressure chamber. (5,21)

Recent research has shown a parabolic relationship between S, and Os measured the previous day for a variety of crops and fruit trees such as almonds," grapevines (Fuentes et al., under review) and citrus trees (Fuentes et al., unpublished).

Figure 2 shows the relationship from which two areas can be identified: non-water stress conditions to mild water-stress conditions (0>Os> -1.0 MPa [megapascalsp, and from mild water-stress conditions to severe water-stress conditions (-1.0>os> -2.5 MPa). The peak of this relationship corresponds to the maximum S.

Physiological data helps explain nighttime behavior

Since Sn data is obtained using sap-flow sensors inserted in the trunk, it is not possible to know the partitioning of Sn between rehydration (water that remains in wood and organs), cuticular transpiration (smallest component) and transpiration through stomata within leaves. An indirect method to characterize this partitioning has been proposed by S. Fuentes et al. comparing S data with the atmospheric demand for water as nighttime vapor-pressure deficit (VPD). (11)

When comparing Sn data from the first zone (Figure 2) with nighttime VPD and the corresponding values for the second zone of the curve with nighttime VPD, it was found that there were high correlations between S and VPD in Zone 1, but no correlations at all for Zone 2. (11,18) The observations have been supported by collecting gas-exchange measurements of Enight and Gnight with accurate and sensitive instrumentation at night.'

Furthermore, Zone 2 of the curve is characterized by high ABA levels in the sap (Fuentes et al., unpublished). Therefore, it could be speculated that Gnight and Enight in Zone 1 is more influenced by hydraulic signals (VPD and Os) and Zone 2 by hormonal signals (ABA).

Implications of nighttime transpiration and rehydration in the whole-plant water budget

Rehydration at night helps the plant recover from water stress endured the previous day to maintain cell turgor for

Figure 2. Nighttime water uptake (S) response to plant water status measured the previous day for fully irrigated grapevines cv. Shiraz (Benalla, Victoria. Australia). Two zones are characterized for non-water stress to mild water stress (Zone 1) and from mild water stress to severe water stress (Zone 2), (Fuentes et al. under review).


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Sigfredo Fuentes," Roberta De Bei' and Stephen Tyermare 'University of Melbourne, Melbourne School of Land and Environment. Victoria. Australia: 'School of Agriculture. Food and Wine and Waite Research Institute, University of Adelaide. Plant Research Centre. Waite Campus, Glen Osmond, SA, Australia: 'Corresponding author: sfuentes@ BY
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Comment:Unseen process for local/global water footprint and water balance estimates in grapevines.(NIGHTTIME PLANT WATER LOSS)
Author:Fuentes, Sigfredo; Bei, Roberta De; Tyerman, Stephen
Publication:Wines & Vines
Geographic Code:8AUST
Date:Jun 1, 2014
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