From a global perspective, water circulation in plants is an integral part of the natural water cycle, which is vital to the functioning of ecosystems. An analysis of the chemical and physical properties of water, as well as basic information about water uptake, transport, and transpiration in plants is beyond the scope of this resviearchw, and readers are encouraged to consult specialized reviews for this purpose ^[1][2][3][4][1,2,3,4].

Usually, the broad term “plant water status” is used when quantitatively describing the plant-water relationship. Plant water status has several various interrelated components or functional aspects, each describing different parts of this relationship: water potential, water movement, and water content [5]. In practice, one of the most widely used indices of the plant-water relationship is water potential, a complex parameter describing energy-related aspects of water status [6]. The water potential refers to the ability of water in the system to perform physiological functions and depends on the hydrostatic pressure in particular tissues, cells, or cellular compartments (pressure potential); the amount of dissolved solutes (solute potential); interactions with solid surfaces (matrix potential); and the effects of gravity (gravitation potential). Water movement largely depends on interactions between soil (water availability) and the atmospheric environment (air humidity, wind, etc.) through the soil-plant-atmosphere continuum, which can be particularly characterized by measuring the root pressure or transpiration rate. Within the present resviearchw, the focus will be on the third component of plant water status: water content.

Saturation with water is a critical concept in understanding plant water status, as precisely formulated by N.C. Turner: “living cells need to be more or less saturated with water to function normally, but they are usually incomplete in this desirable condition” [7]. To quantify the degree of water insufficiency or unsaturation, the actual water content of tissues is expressed relative to that at full saturation or full turgor, denoted as “relative water content” (RWC). Wide use of RWC has been associated with the fact that this parameter shows a certain coherence with experimentally measured water potential.

Full saturation with water is expected to occur when particular tissues reach a state of full turgor. In practice, during measurement of RWC, this is usually achieved by exposing detached tissues to water and allowing unlimited water uptake until saturation, as shown by weight stabilization. However, no special attention has been paid to the fact that different tissues of various plant species can have different values of absolute water content (in grams per mass unit) even at full saturation. However, some physiological and pathological disorders are associated with plant tissues becoming oversaturated with water, as in the case of hyperhydricity in plant tissue culture or water-soaking in plant pathogenesis.

Several papers in the 1980s compared various methods for measuring plant water status ^[5][7][8][5,7,8]. These studies most commonly indicated that measurement of absolute water content either on a dry or fresh mass basis was “generally unsatisfactory because neither is stable” [5], pointing to possible diurnal and seasonal changes in both dry mass and water content. Indications of the potentially erroneous nature of absolute water content measurements found in earlier works seem to have been fully accepted, leading to the current situation in which RWC is the only water content parameter usually determined and analyzed in different functional studies with plants. There is no doubt that each analytical method has its limitations, which must be clearly stated in any case. However, the objection that “because the dry weight can change diurnally and/or seasonally, comparisons of water content on a dry weight basis are unsatisfactory” [7] loses any meaning when performing relatively short-term comparative studies where sampling is carried out at the same time of the day. Nevertheless, widely used measurements of RWC have another possible problem because of experimental manipulation with plant materials to determine water content at “full saturation” or “full turgidity.” This problem has been previously discussed [5], and several more recent studies have provided experimental evidence that measurement of RWC can lead to underestimated results. Thus, it has been shown that in situations leading to internal osmotic adjustment, as in the case of both salt-affected and dehydrated plants, excess water is absorbed during the measurement procedure to obtain a “fully turgid state” [9]. As a result, measured RCW values are anomalously low.

Sometimes, dry matter content, as an inverse parameter to tissue water content, has been used as indicator of functional differences between species with fast growth rates vs. species exhibiting nutrient conservation strategies [10] or to characterize yield quality, as in the case of potato production [11]. However, in order to avoid possible differences in leaf water content caused by variations in soil moisture, samples are usually rehydrated Therefore, it appears that leaf dry matter content is derived from and represents an inverse parameter for relative water content, and it is susceptible to the same technical problems as described above for the measurement of RWC.

One of the problems related to absolute water content measurements and the use of the obtained results is the expression of the measurement data in a relative manner, either as a percentage of the amount of water either on a dry mass basis or on a fresh mass basis. It has been previously argued that, because of the extremely high proportion of water in fresh biomass, both types of expression are difficult to relate to any functional concept, as water content differences on a fresh mass basis tend to be extremely low (only a change in a few percentages when the actual water content changes by 30%), while differences on a dry mass basis tend to be extremely high (typical values for herbaceous plants being 500–850%) [5]. However, water content on a dry mass basis can be also expressed in absolute units, as grams of water per gram of dry mass. It can be argued that the visibility of the differences (and functional meaning of the results) increases significantly when water content is expressed in grams of water per gram of dry biomass (DM) compared to expressing it as a percentage of fresh or dry biomass. In practice, for example, changes in leaf water content from 85.22% to 80.43% were evaluated as “slight” while significant, as they corresponded only to a 5.6% decrease [12], but conversion to g H₂O g⁻¹ DM resulted in a decrease from 4.8 to 3.1, or 35.4%. In some studies, the same parameter (water content in grams of water per gram of dry mass) has been designated as “succulence” [13]. Alternatively, the ratio between fresh mass and dry mass, also showing the relative proportion of water, has been designated as “degree of succulence” [14].

2. Structural Organization of Water in Plant Tissues

Water molecules in tissues of all living beings, including plants, can be part of macromolecular structures, thereby forming a network of interfaces with different properties and diverse functional roles that generally determine the structural and functional properties of macromolecules [3]. Interfacial water (“bound” water) has different properties than bulk or “free” water. Bound water has been estimated to represent approximately 30% of the total water content of plants [15]. A significant proportion of water at any particular moment in time can be attributed to the water being transported by the xylem to be transpired through the stomata, or by the phloem to ensure circulation flow through the plant. Both types of transported water enable solute transport between plant parts. In addition, water can be transported through the apoplast and symplast as well as by the transmembrane pathway [16]. While the water potential in all separate water-containing compartments (xylem, cell wall, cytoplasm, and vacuole) at equilibrium is identical, the particular components of the water potential can differ significantly. In particular, the osmotic potential is usually high in both the vacuole and apoplast, but the turgor pressure potential is extremely important in the case of the vacuole [17]. In contrast, the gravitation potential is only relevant in tall trees. It can be expected that the relative proportion of water aimed to be transpired through the leaves at any particular point is significant, given the fact that the amount of transpired water per gram of synthesized organic matter can be as high as 500 g. However, in reality, due to the high proportion of water mass in the total fresh mass of the plant, relatively fast xylem flow velocity, and high transpiration intensity, transpiration water is only a relatively small proportion of the total water content of the plant organism and can usually be ignored. The results of direct measurements are not widely available, but the amount of water transpired by individual plants of Eichhornia crassipes within an hour was calculated to be equal to 0.33–0.58% from the total amount of water in these plants [18]. Consequently, approximately 70% of water in plants can be designated as “utilizable water”. The mechanical properties of plant organs are affected not only by their chemical composition and structure but also by maintenance of water-dependent turgor and rigidity. In this respect, plant tissues represent hydrostatic materials [19] and their resistance to mechanical stress is highly dependent on their water content [20]. Differences in the strength of water binding have been studied mostly from the point of view of desiccation tolerance of recalcitrant seeds ^[21][22][21,22]. Variations in stem water content in woody plants with respect to their drought tolerance is another relatively frequently assessed aspect of water content studies in plant biology [23]. Mostly methods based on infrared and Raman spectroscopy, isothermal sorption measurement, dielectric relaxation techniques, and nuclear magnetic resonance spectroscopy have been used to study water properties in plants. Many of these techniques are rather non-specific or require complex and bulky equipment [24]. Recent developments in the field of portable hardware for non-destructive measurement of water content by means of nuclear magnetic resonance have opened up new experimental possibilities, allowing for continuous water content measurements in growing leaves and other relatively small parts of intact plants [25].

3. Water Content of Different Plant Parts

3.1. Water in Leaves

Initial assumptions that “the weight of leaves is largely water and therefore the leaf blade is composed mostly of nothing than water” [26] still seem to be valid, as only a small number of studies have addressed differences in leaf water content among different plants or their changes under the effects of variable environmental conditions. However, more information is available regarding leaf succulence with respect to drought adaptation and in response to salinity, and these aspects will be analyzed further. In different grass species, leaf water content is positively related to the proportion of the total volume occupied by mesophyll plus epidermal cells in their leaves [27]. In addition, the size of mesophyll cells can also positively affect leaf water content. Water content per unit of dry matter increases in all vegetative parts with increasing genetically determined plant growth rate, expressed as the relative growth rate [28]. In a study with 24 wild plant species cultivated under controlled conditions, plant species with the lowest relative growth rate (100–120 mg g⁻¹ day⁻¹) had whole plant water contents of 4.8–5.7 g g⁻¹, but the fastest growing plants (relative growth rate > 300 mg g⁻¹ day⁻¹) had whole plant water contents of 8.1–10.1 g g⁻¹ [29]. These differences most likely resulted from higher rates of both mineral ion uptake and water absorption in fast-growing species. When two inbred lines of Plantago major with different growth rates were compared, the line with 25% higher growth rate appeared to have higher water contents in both leaves and roots (Table 1) [30]. It is reasonable to suggest that light conditions (intensity of photosynthetically active radiation, spectral characteristics, photoperiod) can also have a pronounced impact on water content in addition to developmental and growth effects. A study with the stoloniferous plant Potentilla reptans, adapted to high light environments, showed that shading conditions resulted in decreased leaf dry mass, increased water content from 4.95 to 8.52 g H₂O g⁻¹, and petioles grew taller and thinner as a result of the shade avoidance response [46].

3.2. Water in Fruits

Similar to other plant products, the quality of fleshy fruits is critically dependent on their water content, affecting both storage and suitability for food processing [47]. The functional aspects of water status in fleshy fruits are largely affected by structural differences in the surface as compared to those in leaves: stomata are nonfunctional if present and the cuticle is highly differentiated but usually more water-permeable [48]. Together with an increase in solute content in developing fruits, more water accumulates, resulting in increased fruit volume [49]. It can be supposed that during growth, increased water content occurs through cellular vacuolization, but additional water is accumulated in the pectin fraction of cell walls. During the early stages of maturation, water content still increases [40]. However, the timing and intensity of changes in fruit water content are highly genotype-dependent. During the final phases of maturation and senescence, loss of cellular integrity occurs due to high activity of polysaccharide-depolymerizing enzymes, leading to fruit softening [50]. This directly results in the loss of water compartmentalization, which affects the mechanical properties of the fruit, basically changing the fruit from being crunchy to juicy. The functional aspects of water transport and accumulation during fruit development have been recently reviewed, and readers are encouraged to seek further details ^[47][51][47,51]. While the chemical composition of fruits is related to their relative growth rate and the climacteric/non-climacteric character of maturation [52], no comparative study involving fruit water content has been performed. Purely intuitively, one would think that the water content of mature fruits would be related to their type. Thus, berries and citrus fruits seem to be fleshier than pomes, but these organoleptic characteristics are affected mostly by chemical composition and structure instead of water content. Examples of water content values in different fruits are given in Table 2 ^{[53][54][55][56]}[53,54,55,56]. It is evident that watermelons, melons, strawberries, and citrus fruits have the highest values, but bananas have among the lowest. Especially interesting with respect to water content and storage is the case of fruits of the coconut palm, Cocos nucifera, known as coconuts. Being a typical drupe, a coconut has three layers—exocarp, mesocarp, and endocarp—of which the first two layers form husk, but the hollow endocarp contains a multinucleate liquid endosperm, known as coconut water [57]. Coconut water is rich in sugars, minerals, vitamins, amino acids, etc., with an actual water content of approximately 15.2–16.0 g H₂O g⁻¹ DM (Table 1) [34]. During storage, bulbs of onions and garlic lose water through transpiration, which reduces their shelf life and quality. Agrotechnical measures during cultivation affect water loss from onion bulbs during storage. For example, increasing the nitrogen fertilizer rate from 100 to 150 kg ha⁻¹ increased the water loss from 36% to 57% during 150 days of bulb storage [65]. The time of harvesting and topping as well as the duration of the drying period after harvesting also significantly affects the water content of bulbs and water loss during storage [66]. Similarly, postharvest practices have significant effects on garlic bulb quality, as indicated by changes in the water content of the peel (Table 1) ^[33][67][33,67].

3.5. Water in Vegetables

To facilitate a comparison, the water content of different types of vegetables is given in Table 3 ^[68][69][68,69]. In many cases, there is no doubt that the domestication process of crops has selected for traits associated with increased water content compared to their wild ancestors. Unfortunately, more extensive comparative studies on the functional meaning of differences in water content, especially in relation to storage functions, are not available. However, these results are of key importance in the practical context of vegetable storage, food processing, etc. In general, moisture loss during storage is a critical factor that negatively affects the quality of stored vegetable products. Both high temperature and low air humidity facilitates water loss through evaporation and cuticular transpiration, which are highly genotype-dependent characteristics [70]. DM, dry mass. In some cases, the results may be relatively inaccurate because they were read from graphs with the closest possible accuracy. If the original results were not in units of mass of water per unit of dry mass, they were converted accordingly. A similar relationship has also been established for woody species. When 30 Mediterranean woody species with different post-fire regenerative strategies from a coastal shrubland were compared, the leaves of resprouting species appeared to have lower water contents, slower growth rates, and longer leaf lifespans compared to the leaves of species regenerating from seeds [45].

DM, dry mass. In some cases, the results may be relatively inaccurate because they were read from graphs with the closest possible accuracy. If the original results were not in units of mass of water per unit of dry mass, they were converted accordingly.

3.3. Water in Seeds

Highly controlled changes in water content are important during plant generative reproduction. In seeds, changes in water content during development are parts of physiological changes that lead to the formation of mature seeds. Seed moisture content decreases throughout its development, mostly due to a disproportionately larger rate of assimilate accumulation in comparison to that during the seed-filling phase followed by active water loss during the maturation phase [58]. The opposite process occurs during the imbibition of quiescent seeds before germination, but this initially relies entirely on physical processes, while further changes are under tight internal control [59]. The water uptake rate of dry seeds largely depends on the structure and chemical composition of different seed parts [60]. From a practical point of view, the amount of water in seeds or “seed moisture content” is an important indicator of their expected storage life and resilience. Seeds, detached from a plant, have limited means for controlling their internal water content, which largely depends on the relative humidity of the surrounding air. When the air humidity increases (or decreases), the seed water content slowly balances accordingly, reaching so-called “equilibrium moisture content”. It is important to note that seeds of a particular taxon have genotype-specific values of equilibrium moisture content for particular air humidity levels, but they also depend on temperature. Seed chemical composition has a significant effect on equilibrium moisture content, and starch-containing seeds usually have higher water sorption abilities compared to oil-containing seeds [61]. An increase in seed moisture as a result of storage in a humid atmosphere significantly reduces the seed’s viability and preservation of its germination capacity, leading to a shortened expected storage period of seed material [62]. For example, even an increase in relative humidity from 20% to 30% (resulting in increase in seed moisture only from 4.4% to 5.6%) can reduce seed longevity approximately two-fold. In contrast to the majority of plant species (about 90%) for which reducing the seed moisture content and decreasing the temperature will increase seed resilience and maintain viability (aka orthodox seeds), seeds of some species do not survive dehydration or low temperatures (aka recalcitrant seeds) [62]. Due to these differences, the viability of recalcitrant seeds is best preserved when stored at high relative humidity (98–99%) and positive temperature (7–17 °C for tropical species and 3–5 °C for temperate species).

3.4. Water in Vegetative Propagules

Underground storage organs of geophytes, bulbs, tubers, and corms act as vegetative propagation organs, and several crop species with bulbs and tubers are essential food plants. Similar to generative propagules, i.e., seeds, the water content of vegetative propagules, such as tubers and bulbs, changes during development and maturation and has immense practical importance with respect to storage and processing. The water content of potato tubers, often expressed as an inverse parameter, dry matter content, is an important feature during potato storage as well as further for food processing [11]. For example, a dry matter content above 22% (or water content below 2.55 g g⁻¹ DM) is necessary to gain product yield and profitability in potato chip production. There are characteristic biochemical changes during the growth of potato tubers, such as increased starch content at the expense of decreased sugar concentration, and water content decreases during potato tuber filling ^[41][42][41,42]. In mature tubers, water is not uniformly distributed, with lower levels in the outside than in the inside of the tuber and also lower levels at both the apical and stem ends (Table 1) [11]. Similar to genotype-dependent variability in chemical composition, water content also differs among potato cultivars [63]. In addition, agrotechnical measures significantly affect the water content of potato tubers. For example, excessive application of nitrogen fertilizers increased tuber water content, thereby reducing their quality [64]. At harvest, bulbs of onions and garlic have a relatively uniform distribution of water, including in the fleshy outer layers. To increase the shelf life, the water content of the outer layers needs to be significantly decreased, leading to the formation of several desiccated layers, i.e., the peel. As a result, the total water content decreases, for example, from 5.09 to 4.26 g g⁻¹ DM while the water content of the outer layers is only 0.3 g g⁻¹ DM (Table 1) [31].

DM, dry mass. If the original results were not in units of mass of water per unit of dry mass, they were converted accordingly.

3.6. Water Storage in Woody Plants

Water storage in trees is a rather specific case, mostly due to significantly different anatomical and physiological features of woody plants in comparison to herbaceous species. Water is stored mainly in xylem conduits and extracellular spaces of living vascular tissues (aka elastic water), but capillary water can also be stored in highly lignified or dead xylem cells ^[71][72][71,72]. In addition, succulent trees develop fleshy tissues adjacent to sapwood—outer parenchyma layers—that act as a water storage compartment, and parenchymatous pith and cortical tissues also can act as water reservoirs [73]. The anatomical characteristics of woody stems, such as the proportion of dead and living cells, largely affect water availability during events of decreased water potential [74]. Water storage in stems tissues of woody plants acts as a buffer to compensate for variations in leaf transpiration demands [75], but capillary water mostly protects the viability of the cambium [76].