The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUEeco), is widely expected to increase because of the rising atmospheric carbon dioxide concentration (Ca). However, little is known about the interactive effects of rising Ca and climate change on WUEeco.

Figure 1. Global water use efficiency saturation due to increased vapor pressure deficit

Global water use efficiency saturation due to increased vapor pressure deficit is shown in Figure 1. On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUEeco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.[1]

Water use efficiency (WUE) is defined as the amount of carbon assimilated as biomass or grain produced per unit of water used by the crop. One of the primary questions being asked is how plants will respond to a changing climate with changes in temperature, precipitation, and carbon dioxide (CO2) that affect their WUE At the leaf level, increasing CO2 increases WUE until the leaf is exposed to temperatures exceeded the optimum for growth (i.e., heat stress) and then WUE begins to decline.

Leaves subjected to water deficits (i.e., drought stress) show varying responses in WUE. The response of WUE at the leaf level is directly related to the physiological processes controlling the gradients of CO2 and H2O, e.g., leaf:air vapor pressure deficits, between the leaf and air surrounding the leaf. There a variety of methods available to screen genetic material for enhanced WUE under scenarios of climate change.

When we extend from the leaf to the canopy, then the dynamics of crop water use and biomass accumulation have to consider soil water evaporation rate, transpiration from the leaves, and the growth pattern of the crop.

Enhancing WUE at the canopy level can be achieved by adopting practices that reduce the soil water evaporation component and divert more water into transpiration which can be through crop residue management, mulching, row spacing, and irrigation. Climate change will affect plant growth, but we have opportunities to enhance WUE through crop selection and cultural practices to offset the impact of a changing climate.[2]

Vapor Pressure Deficit, also known as VPD is a very useful tool for growers. However it is a variable growers sometimes forget, do not know about its existence or do not understand.

VPD is a variable related to humidity. Humidity in growing systems can be expressed as: Relative humidity, one of the most common variables used in greenhouses and is expressed as percentage; Vapor concentration, a variable used to express the amount of water vapor in a specific volume of air and Vapor pressure deficit.

Vapor pressure deficit can be defined as the amount of vapor that can still be stored in the air until reaching saturation point, under the same temperature. This variable can be calculated as the difference between the actual vapor pressure and the saturation vapor pressure.[3]

Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense out to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important for greenhouse regulation. If a film of water forms on a plant leaf, it becomes far more susceptible to rot. On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.

In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.[4]

References:

1. https://www.science.org/doi/10.1126/science.adf5041
2. https://www.frontiersin.org/articles/10.3389/fpls.2019.00103/full
3. https://hortamericas.com/blog/science/understanding-vapor-pressure-deficit-vpd
4. https://en.wikipedia.org/wiki/Vapour-pressure_deficit#

Cite this article:

Gokula Nandhini K (2023), Global water use efficiency saturation due to increased vapor pressure deficit, AnaTechmaz, pp.466