For proper growth and function plants require water. Water is used as building material (Figure 3.1 in §3.1), nutrient solvent, transport- and cooling agent. Water, moreover, plays an important role in photosynthesis. For most crops water is taken up by the roots. There are three main forces that drive water movement trough the plant: root pressure, capillary action and transpiration. Simply put, root pressure is an osmotic pressure in the root system that causes water to travel from outside the root inside the vascular tissue (xylem). This uptake causes a pressure in the xylem, which pushes the water upwards trough the xylem. However, when transpiration is high, xylem sap is usually under tension due to transpirational pull and not under pressure generated by the roots. Plant scientist refer to evaporation if water passively vaporizes from liquids on the outside surface of plant organs (e.g. leafs and stems) into a gaseous phase.  The term transpiration refers to the water that evaporates from within their tissue. If it is hard to discriminate between evaporation and transpiration, then the term evapotranspiration is used. Evapotranspiration is the cumulative amount of water that vaporizes from the substrate, the plant inside and the plant surface.

Regulation of transpiration is done by stomata. Several plant organs contain stomata, but the highest abundance is found at the bottom of the leaves. Stomata are small (~10 to 80 µm) kidney-shaped pores surrounded by guard cells. By regulating the turgor pressure in the guard cells, plant determines the pore opening (aperture). Opening and closing of stomata pores influence the amount of water that can escape from plant organs. The opening and closing of the stomata is influenced by many external factors such as light, temperature, relative humidity, CO2 concentrations, water availability and pathogens. Endogenous factors that regulate stomata movement are phytohormones and their interactions with secondary messengers. Generally, stomatal aperture changes over diurnal cycles. For CO2 uptake (assimilation) stomata stay open during the day especially in response to blue light and tend to be closed at night. Under optimal conditions CO2 assimilation is closely related to plant growth. Which makes gas exchange trough stomata an important plant trait.

As a rule of thumb greenhouse crops transpire ~90% of all the water that is taken up. In protected horticulture plant transpiration can be regulated by the Vapour Pressure Deficit, or VPD. 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. In other words how much water can the air hold before condensation takes place. “The VPD measure is an improvement over relative humidity (RH) measurement alone because VPD considers the effect of temperature on the water holding capacity of the air. Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation. 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 to 0.95 kPa” (Prenger and Ling 2001).

When air moisture levels rise beyond its holding capacity — or a surface is colder than the dew point — there will be deposition of liquid water somewhere in the growth system or on the plant. At high RH, condensation takes places at places where the surface temperature is lower than the air temperature. Liquid water on the plant surface is an important source of pathogen infections and should be prevented. In the morning, when the light intensity increases, the top leaves in the canopy start to transpire, increasing RH. However, the fruits and bottom leaves will still be cold and prone to condensation. Growers often gradually heat their greenhouse before sunrise to increase the VPD. Another important measure to control VPD is ventilation. Ventilation can remove the moist air from the plant growth environment and in closed systems this water can be condensated and reused.

Growing at low VPD (High RH) is advocated to conserve energy. The concept of ‘Next generation cultivation’ implies better insulation measures and adapted climate control, lower gas use, and CO2 emissions. These growth conditions usually lead to higher air humidity during periods of heat demand in the greenhouse (De Gelder and Dieleman 2012). A 5% higher humidity set-point would safe 75 MJ per year (De Gelder et al. 2012). This presents a big problem for growers, because a high humidity in the greenhouse can result in increased Botrytis growth, and stomata malfunctioning. The latter phenomenon affects many different species, from leafy vegetables to fruit bearing crops, and flowering plants. In plants grown for cut flowers, the malfunctioning of stomata caused by high air humidity during growth has been shown to be an important factor that reduces vase life, and that it is dependent on genotype (Nejad and Van Meeteren 2005; Fanourakis et al. 2012; Arve et al. 2013; Schouten et al. 2018).

In relation to influencing nutritional value of plants, Leyva et al. (2014) showed that the use of fogging systems to increase RH in screenhouses increases lycopene and organic acid content, and antioxidant activity of tomato fruit compared screenhouses without fogging or the open field. These findings were confirmed by Leonardi et al. (2000) and Rosales et al. (2011) who showed that low VPD increased lycopene content of cherry and salad tomato fruit. High VPD is often accompanied with high temperatures and solar radiation, inducing oxidative stress with on the one hand severe negative effects on marketable yield, carotenoid and mineral contents, and on the other hand increase in phytonutrients, antioxidant activity, and sugars contents (Rosales et al. 2011).

 CO2 effects on nutritional value of plants

Increasing CO2 levels during cultivation of C3 species often results in higher biomass accumulation. That is more sugars and higher fibre content. However, a meta-analysis by Loladze (2014) showed that increasing CO2 concentrations in C3 species also leads to lower nutrient concentrations (Figure 5.3). These trends are confirmed by a study Myers et al (2014) who found that iron, zinc and B vitamins will be reduced when atmospheric CO2 concentration reaches values of (550-590 ppm) by 2050. This is a disturbing notion as many low-income countries depend on C3 staple foods like wheat, rice and legumes for their day to day nutrition. Note that species that are predominantly C4 like maize and sorghum are less effected by CO2 increase and generally maintain their nutrient status.

One has to realise that the higher carbohydrate content per unit of dry weight dilute the nutrient content per unit of dry weight and this does not necessarily translate to a reduced nutrient content per eaten plant volume. That is, leaves can be heavier, i.e. have a higher density, but have the same dimensions.

Not only plant mineral content is affected by higher CO2 levels. Medek et al. (2017) found that the expected raise in CO2 concentration (500-700 ppm) by 2050 will significantly decrease crop protein content: rice (-7,6%), wheat (-7,8%), oat (-14,1%), potato (-6,4%), bean (-4,6%), chickpea (-13,5%). Additionally, C3 vegetables (-17,3%) and fruit (-22,9%) will also decrease in protein content if CO2 rises. Khan et al. (2013) observed a negative effect on tomato vitamin C, protein, organic acids, fat and ash contents.

Figure 5.3: Copied from Loladze (2014): the effect of CO2 on individual chemical elements in plants. Change (%) in the mean concentration of chemical elements in plants grown in eCO2 relative to those grown at ambient levels. Unless noted otherwise, all results in this and subsequent figures are for C3  plants. Average ambient and elevated CO2 levels across all the studies are 368 ppm and 689 ppm respectively. The results reflect the plant data (foliar and edible tissues, FACE and non-FACE studies) from four continents. Error bars represent the standard error of the mean (calculated using the number of mean observations for each element). The number of mean and total (with all the replicates) observations for each element is as follows: C(35/169), N(140/696), P(152/836), K(128/605), Ca(139/739), S(67/373), Mg(123/650), Fe(125/639), Zn(123/702), Cu(124/612), and Mn(101/493). An element is shown individually if the statistical power for a 5% effect size for the element is >0.40. The ‘ionome’ bar reflects all the data on 25 minerals (all the elements in the dataset except of C and N). All the data are available at Dryad depository and at GitHub.

In greenhouses it is common practice to increase CO2 concentration up to 1000-1200 μmol mol−1 this results in higher yield and early harvest (Chalabi 1992; Chalabi et al. 2002). Studies about CO2 enrichment in greenhouses focus mostly on fruit vegetables and lettuce. The general message is that sugar content rises, just as the flavonoid content, as sugars serve as precursors in flavonoids biosynthesis (Becker and Kläring 2016). Red lettuce cultivars appear more responsive to elevated CO2 than the green ones, showing a higher content in secondary metabolites and antioxidant activity (Sgherri et al. 2017).


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