Differences In Water Potential Research Articles

Vapour pressure deficit affects crop water productivity, yield, and quality in tomatoes

A high atmospheric vapour pressure deficit (VPD) hinders calcium absorption in tomatoes (Solanum lycopersicum L.), which severely reduces tomato yield, crop water productivity (WPcrop), and quality. Although reducing the VPD is effective for increasing tomato yield and water productivity, regulating the VPD is costly. Therefore, this study aimed to explore the effects of VPD regulation (high VPD=2.22 kPa, low VPD=0.95 kPa) during different growth stages on tomato WPcrop, yield, and quality to identify a VPD management method that saves costs and enhances the above parameters. The results showed that the yield was significantly positively correlated with fruit calcium absorption. Decreasing the VPD during the fruit expansion stage and increasing it during the flowering stage significantly increased calcium accumulation in the fruits, thereby improving tomato WPcrop, yield, and quality. Additionally, a high VPD during the flowering stage increased stomatal conductance and mesophyll conductance, thereby increasing intercellular and chloroplast CO2 concentrations and enhancing plant photosynthetic capacity, and reduced the stem water potential, leaf water potential, and leaf relative water content, thereby increasing the water potential difference, leaf hydraulic conductance, and calcium absorption. By integrating game theory and the technique for order of preference by similarity to ideal solution method, a comprehensive analysis of tomato appearance, nutrients, and flavour quality revealed that growing plants under a high VPD during the seedling, flowering, and fruit ripening stages and a low VPD during the fruit expansion stage not only enhanced the comprehensive tomato quality value but also maximally reduced the humidification cost. Conclusively, controlling the environmental VPD at 2.22 kPa during the seedling, flowering, and fruit ripening stages and at 0.95 kPa during the fruit expansion stage can effectively increase tomato WPcrop, yield, and quality. This study provides a theoretical basis for the environmental regulation of high-quality, efficient, and water-conserving cultivation of greenhouse tomatoes.

Development and application of an inexpensive open-source dendrometer for detecting xylem water potential and radial stem growth at high spatial and temporal resolution.

There is currently a need for inexpensive, continuous, non-destructive water potential measurements at high temporal resolution (<1min). We describe here the development and testing of an entirely open-source dendrometer that, when combined with periodic Scholander pressure chamber measurements, provides sub-minute resolution estimates of water potential when placed on tissues exhibiting little or no secondary growth (petioles, monocotyledon stems). The dendrometer can also be used to measure radial growth of stems and branches when placed on dicotyledon and gymnosperm species. The dendrometer can be interfaced directly with a computer in real time in the lab or greenhouse, or connected to a datalogger for long periods of use in the field on batteries. We tested this device on a herbaceous dicotyledon (Helianthus annuus) (petioles and stems) and a monocotyledon (Zea mays) species (stems) for 1 week during dehydration and re-watering treatments under laboratory conditions. We also demonstrated the ability of the device to record branch and trunk diameter variation of a woody dicotyledon (Rhus typhina) in the field. Under laboratory conditions, we compared our device (hereafter 'contact' dendrometer) with modified versions of another open-source dendrometer (the 'optical' dendrometer). Overall, contact and optical dendrometers were well aligned with one another, with Pearson correlation coefficients ranging from 0.77 to 0.97. Both dendrometer devices were well aligned with direct measurements of xylem water potential, with calibration curves exhibiting significant non-linearity, especially at water potentials near the point of incipient plasmolysis, with pseudo R2 values (Efron) ranging from 0.89 to 0.99. Overall, both dendrometers were comparable and provided sufficient resolution to detect subtle differences in stem water potential (ca. 50 kPa) resulting from light-induced changes in transpiration, vapour pressure deficit and drying/wetting soils. All hardware designs, alternative configurations, software and build instructions for the contact dendrometers are provided.

Crop Coefficient of Sunflowers under Drip Irrigation with Plastic Film in Different Hydrological Years in the Hetao Irrigation District

An accurate assessment of crop water requirements during the crop growth period can help organize irrigation schedules and investigate the hydrological environments in irrigation districts, especially in shallow groundwater districts. In irrigation scheduling, crop coefficients, representing three development stages, play a critical role in modeling evapotranspiration. Therefore, in this study, the crop coefficient (Kc) of sunflowers under drip irrigation in different hydrological years in the Hetao irrigation district with shallow groundwater is determined. Based on the analysis of rainfall frequency, the experimental situations of 2012, 2013, and 2014 are adopted as high, normal, and low flow years. Using the water balance method, groundwater recharge and crop evapotranspiration in different hydrological years were investigated. The results showed that the groundwater recharge in a high-flow year was larger than that in normal and low-flow years with values of 67.47, 66.75, and 42.61 mm, respectively. Crop evapotranspiration is directly related to the irrigation amount at the same growth stage. The Kc of sunflowers in a high-flow year is higher by 0.1 and 0.15 than that in normal and low-flow years. The Kc of sunflowers under drip irrigation is lower than that under surface irrigation by 8.9% in the initial and late stages of sunflower growth, but higher than that by 10.6% in the mid-stage, which is due to differences in small water potential differences and less evapotranspiration in the initial and late stages under drip irrigation. The study is of great significance for formulating a reasonable schedule of drip irrigation with shallow groundwater and improving the field environment.

Linking water‐use strategies with drought resistance across herbaceous crops

AbstractWoody plants minimize xylem embolism formation during drought essentially by closing stomata at higher water potentials and/or by increasing the xylem resistance to embolism. Both of these mechanisms result in a higher stomatal safety margin (SSM), which is the water potential difference between stomatal closure and embolism formation. Here, we investigated whether increasing SSM represents a drought resistance mechanism for herbaceous plants and how the different water‐use strategies impact their survival. For that, we exposed four herbaceous crops with contrasting drought resistance to severe water deficit to assess drought‐induced damage and mortality. Unlike woody species, SSM was not associated with plant survival for herbaceous crops. Soybean, which presented the largest SSM across the four crops (1.67 MPa), exhibited the earliest mortality of leaves and whole plants as well as the highest rate of plant mortality (100%) at the end of the drought period. Cowpea, with an SSM of 0.63 MPa, was the most drought‐resistant species, with the latest leaf damage and the highest plant survival (100%). The most effective traits ensuring survival in herbaceous crops under severe drought were those related to drought avoidance mechanisms such as (1) early stomatal closure, (2) very low residual transpiration post‐stomatal closure, and (3) high capacitance pre‐ and post‐turgor loss.

How are physiological responses to drought modulated by water relations and leaf economics’ traits in woody plants?

Species’ drought resistance is determined by a combination of multiple traits and their plastic response. However, a clear understanding of how these traits are coordinated and modulate plant responses to drought is still lacking.Here we used a water exclusion experiment on 20 Mediterranean woody species to evaluate a new framework to study plant drought responses, in which relatively constant functional traits modulate the physiological responses to a given drought exposure. In particular, we assessed how the response of stomatal conductance (Gs), leaf relative water content (RWC) and leaf water potential difference (ΔΨ) to leaf predawn water potential (Ψpd) were modulated by commonly used functional traits. The latter included hydraulic and water relations attributes such as turgor loss point in leaves (Ψtlp), vulnerability to xylem embolism in stems (Ψ50), hydraulic conductivity per unit sapwood (Ks), hydraulic conductivity per unit leaf area (Kl, hydraulic sufficiency) and Huber value (Hv), as well as specific leaf area (SLA) as a proxy for the leaf economics spectrum.Our results show that functional traits are highly coordinated, defining two main axes: the first related to drought tolerance and resource-use strategies and the second characterising the hydraulic sufficiency of leaves. These two axes, as well as many of the underlying functional traits, showed a significant interaction with Ψpd in explaining physiological responses to drought, thus modulating Gs, RWC, and ΔΨ responses. Drought-tolerant species showed a less stringent regulation of water use and leaf water potential but were more effective at regulating leaf RWC. Also, the leaf water potential at turgor loss (Ψtlp) emerged as an important trait orchestrating plant responses to drought.This study highlights that a clear separation between slow-varying traits (Ψtlp, Ψ50, Ks, Kl, Hv, SLA), physiological responses (Gs, RWC, ΔΨ) and tissue-level exposure to drought (Ψpd) improves our understanding of plant drought resistance strategies.

Arbuscular Mycorrhizae Alter Photosynthetic Responses to Drought in Seedlings of Artemisia tridentata.

The establishment of Artemisia tridentata, a keystone species of the sagebrush steppe, is often limited by summer drought. Symbioses with arbuscular mycorrhizal fungi (AMF) can help plants to cope with drought. We investigated this possible effect on A. tridentata seedlings inoculated with native AMF and exposed to drought in greenhouse and field settings. In greenhouse experiments, AMF colonization increased intrinsic water use efficiency under water stress and delayed the decrease in photosynthesis caused by drought, or this decrease occurred at a lower soil water content. In the field, we evaluated the effect of AMF inoculation on colonization, leaf water potential, survival, and inflorescence development. Inoculation increased AMF colonization, and the seedlings experienced water stress, as evidenced by water potentials between -2 and -4 MPa and reduced stomatal conductance. However, survival remained high, and no differences in water potentials or survival occurred between treatments. Only the percentage of plants with inflorescence was higher in inoculated than non-inoculated seedlings. Overall, the greenhouse results support that AMF colonization enhances drought tolerance in A. tridentata seedlings. Yet, the significance of these results in increasing survival in nature remains to be tested under more severe drought than the plants experienced in our field experiment.

Insights into the dwarfing mechanism of pear (Pyrus betulaefolia) based on anatomical and structural analysis using X-ray scanning

The lack of a suitable rootstock to control scion growth has limited the development of high-density plantations in pear production, which is partly attributed to poor understanding of the dwarfing mechanism. In the present study, the rootstock of the dwarf-type pear (Pyrus betulaefolia) ‘PY-9’ was identified and used as the material for anatomical analysis. ‘PY-9’ grew to half the tree height of the normal cultivar ‘Zhengdu’, along with fewer internodes and shorter length. Significant differences in growth rate between ‘PY-9’ and ‘Zhengdu’ were detected at approximately 30 days after full bloom, which corresponded with the time of the greatest difference in water potential between the dwarf and normal cultivar. ‘PY-9’ showed a higher photosynthetic rate than ‘Zhengdu’. Anatomical analysis showed that ‘PY-9’ had higher area ratios of both phloem and xylem and more developed vascular tissues than ‘Zhengdu’. The three-dimensional reconstructed skeleton of the xylem from X-ray computed tomography scanning revealed greater intervessel connectivity in ‘Zhengdu’ than in ‘PY-9’, which could contribute to the more vigorous growth of ‘Zhengdu’. This study thus provides the first comparison of the microstructural properties of xylem elements between a dwarfing-type and vigorous-type pear rootstock, providing new insights into the dwarfing mechanism in pear and facilitating breeding of dwarf pear rootstocks to increase crop productivity.

Hydric Behavior: Insights into Primary Metabolites in Leaves and Roots of Cabernet Sauvignon and Grenache Grapevine Varieties under Drought Stress

The classification of grapevine cultivars into isohydric and anisohydric categories depends on their ability to close stomata under conditions of low soil water availability or high atmospheric demand. This study aimed to compare the responses of Grenache, classified as isohydric, and Cabernet Sauvignon, classified as anisohydric, both grafted onto Richter 110 rootstock, and subjected to severe drought stress. Three cycles of drought stress were applied, followed by watering, while a well-watered treatment served as the control. Stomatal conductance and stem water potential were measured at predawn and midday during the drought cycles, and primary metabolites were analyzed in leaves and roots using gas chromatography. We found that Grenache had significantly higher stomatal conductance than Cabernet Sauvignon under both well-watered and water-stressed conditions. There were no significant differences in stem water potential between the two varieties, but the control treatment maintained a higher stem water potential at predawn and midday for both varieties. Primary metabolite analysis showed that both varieties accumulated sugars and polyols in their leaves and roots under drought stress, while organic acids were more abundant in leaves than in roots. Overall, the results suggest that the hydric behavior of grapevines depends on the intensity and duration of drought stress. In this study, both varieties exhibited near-isohydric behavior by regulating stomatal closure under drought stress. The metabolites identified in this study may serve as potential biomarkers of water drought stress in Grenache and Cabernet Sauvignon grapevines under the conditions of this experiment.

Hydrological–Thermal Coupling Simulation of Silty Clay during Unidirectional Freezing Based on the Discrete Element Method

A hydrological–thermal coupling discrete element model depicting the unidirectional freezing process of unsaturated silty clay was developed in order to investigate the migration law of unfrozen water in unsaturated silty clay under unidirectional freezing circumstances. The model uses the contact heat transfer equation to calculate the heat transfer process while taking into account the latent heat of phase transition. To obtain the silty clay’s freezing characteristic curve, the model combines the unfrozen water content curve with the Clausius–Clapeyron equation. The water migration from the unfrozen zone to the frozen zone was calculated using Harlan’s model and the frozen fringe hypothesis. The discrete element application MatDEM 3.0 was used to incorporate the mathematical model for computation, and the output was compared to the result of indoor unidirectional freezing tests. The soil closest to the stable freezing front had the largest water content, according to the findings of numerical modeling and laboratory testing, and unfrozen water in the soil would move from the unfrozen zone to the frozen zone under the action of water potential difference. The results of laboratory tests and numerical simulations can accurately describe the temperature variation and water migration of soil during freezing, demonstrating the accuracy of the established discrete element model and proving the viability of the discrete element method in the study of frozen soil.

Xylem conduit deformation across vascular plants: anevolutionary spandrel or protective valve?

The hydraulic system of vascular plants and its integrity is essential for plant survival. To transport water under tension, the walls of xylem conduits must approximate rigid pipes. Against this expectation, conduit deformation has been reported in the leaves of a few species and hypothesized to function as a 'circuit breaker' against embolism. Experimental evidence is lacking, and its generality is unknown. We demonstrated the role of conduit deformation in protecting the upstream xylem from embolism through experiments on three species and surveyed a diverse selection of vascular plants for conduit deformation in leaves. Conduit deformation in minor veins occurred before embolism during slow dehydration. When leaves were exposed to transient increases in transpiration, conduit deformation was accompanied by large water potential differences from leaf to stem and minimal embolism in the upstream xylem. In the three species tested, collapsible vein endings provided clear protection of upstream xylem from embolism during transient increases in transpiration. We found conduit deformation in diverse vascular plants, including 11 eudicots, ginkgo, a cycad, a fern, a bamboo, and a grass species, but not in two bamboo and a palm species, demonstrating that the potential for 'circuit breaker' functionality may be widespread across vascular plants.

Unraveling the Physiological Mechanisms Underlying the Intracultivar Variability of Water Use Efficiency in Vitis vinifera “Grenache”

Selecting genotypes with a better capacity to respond and adapt to soil water deficits is essential to achieve the sustainability of grapevine cultivation in the context of increasing water scarcity. However, cultivar changes are very poorly accepted, and therefore it is particularly interesting to explore the intracultivar genetic diversity in water use efficiency (WUE). In previous studies, the cultivar "Grenache" has shown up to 30% variability in WUE. This research aimed to confirm the intracultivar variability and to elucidate the traits underlying this variability in the response to a water deficit by analyzing the growth rates, water relations, osmotic potential, leaf morphology, leaf gas exchange and carbon isotope discrimination in nine "Grenache" genotypes grown in pots during two seasons. The results showed lower differences in WUE and carbon isotope ratio than in previous field studies, but fairly good consistency in genotype ranking. Leaf mass area and osmotic potential did not underlie differences in stem water potential and in stomatal conductance. Overall, stomatal regulation and photosynthetic capacity seem to underlie differences in WUE among genotypes with an important environmental influence. These results confirm the ability to select clones with higher WUE and present an opportunity for the genetic improvement of WUE in grapevines.

Herbivory Amplifies Adverse Effects of Drought on Seedling Recruitment in a Keystone Species of Western North American Rangelands.

Biotic interactions can affect a plant’s ability to withstand drought. Such an effect may impact the restoration of the imperiled western North American sagebrush steppe, where seedlings are exposed to summer drought. This study investigated the impact of herbivory on seedlings’ drought tolerance for a keystone species in this steppe, the shrub Artemisia tridentata. Herbivory effects were investigated in two field experiments where seedlings were without tree protectors or within plastic or metal-mesh tree protectors. Treatment effects were statistically evaluated on herbivory, survival, leaf water potential, and inflorescence development. Herbivory occurrence was 80% higher in seedlings without protectors. This damage occurred in early spring and was likely caused by ground squirrels. Most plants recovered, but herbivory was associated with higher mortality during the summer when seedlings experienced water potentials between −2.5 and −7 MPa. However, there were no differences in water potential between treatments, suggesting that the browsed plants were less tolerant of the low water potentials experienced. Twenty months after outplanting, the survival of plants without protectors was 40 to 60% lower than those with protectors. The percentage of live plants developing inflorescences was approximately threefold higher in plants with protectors. Overall, spring herbivory amplified susceptibility to drought and delayed reproductive development.

Unexpected Responses of Bean Leaf Size to Elevated CO2.

CO2 is currently a growth-limiting resource for plants with C3 metabolism, and elevated CO2 also often reduces stomatal conductance, reducing plant water stress. Increased photosynthesis and improved water status might be expected to result in increased leaf size. It is therefore unexpected that leaf size is in some cases reduced in plants grown at elevated CO2, and also unexpected that elevated CO2 applied only during darkness can increase leaf size. These experiments compared leaf size responses to day and/or night elevated CO2 in six cultivars of Phaseolus vulgaris grown with either constant or varying temperature in controlled environment chambers. Diverse responses of leaf size to elevated CO2 were found among the cultivars, including increased leaf size with elevated CO2 applied only during darkness in some cultivars and temperature regimes. However, leaf size responses to elevated CO2 and cultivar differences in response were unrelated to differences in leaf water potential or turgor pressure.

Cowpea Physiological Responses to Terminal Drought-Comparison between Four Landraces and a Commercial Variety.

Cowpea (Vigna unguiculata) is a robust legume; nevertheless, yield is always affected by drought, especially when it occurs during reproductive growth and seed filling. Considered a key crop in the effort to attain food security, and a suitable crop for a scenario of climate change, modern disregard for cowpea landraces is particularly detrimental as it causes genetic variability loss, compromising breeding efforts. To contribute to the evaluation of the cowpea germplasm, four Portuguese landraces (L1, L2, L3, L4) were compared with a commercial variety (CV) to evaluate their physiological responses to terminal drought and their inter-variation on productivity, under semi-controlled conditions. Despite no differences in relative water content (RWC) between the CV and the landraces under water deficit (WD), differences in leaf water potential (Ψ) defined the CV as having an isohydric control of stomata in contrast with anisohydric control for landraces. There was an identical decrease in the photosynthetic rate for all plants under stress, caused by both stomatal and non-stomatal limitations, namely, damages at the level of photosystem II as indicated by fluorescence measurements. Instantaneous water use efficiency (iWUE) was improved with stress in L1 and L3. Maintenance of higher relative chlorophyll content for longer periods in the CV revealed a stay-green phenotype. The slim differences observed in terms of stomatal control, iWUE and progression of senescence between the CV and the landraces under WD led to quite important differences in terms of productivity, as inferred from improved yield (number of pods and number of grains per plant). This is a clear result of pragmatic on-farm selection. On one hand it shows that small differences in stomatal responses or water saving strategies under WD may lead to desirable outcomes and should therefore be considered during breeding. On the other hand, it suggests that other traits could be explored in view of drought adaptation. These results highlight the need to preserve and characterize as many genetic pools as possible within a species.

Hydrotropism: Analysis of the Root Response to a Moisture Gradient.

Plant adaptation to environmental stress generated by low water availability requires continuous search for moisture niches in the soil. Thus, roots have evolved a hydrotropic response to sense differences in water potential of the soil, and through asymmetric growth, roots can bend to avoid lower water potential sites. Different experimental systems have been devised for hydrotropism assays, which usually rely on air moisture or split agar assays. This latter system uses plates containing an osmolyte only in a region of the plate in order to generate a water potential gradient. Seedlings are placed on the agar plate containing normal medium (NM) so that their root tips are near the junction between NM and the region supplemented with the osmolyte. As a result,a hydrotropic responseis elicited to avoid the low water potential medium,whichis reflected inthe root curvature angle.

Root Growth Dynamics and Structure in Seedlings of Four Shade Tolerant Mediterranean Species Grown under Moderate and Low Light

Specific functional traits such as shade tolerance or leaf habits can enhance root growth dynamics and structure of planted seedlings in the understory of planted forests. We assessed how low and moderate light levels (17 and 33% of full sunlight, mimicking after-thinning stocking) affect the root growth dynamics and structure of four late successional trees, three deciduous (Acer monspessulanum L., Quercus pyrenaica Willd and Sorbus torminalis (L.) Crantz) and one evergreen (Quercus ilex L.) species. Rooting depth, dynamics and structure were mainly explained by species functional differences. Roots of deciduous trees elongated faster and deeper and were larger than the roots of the evergreen Q. ilex. Among deciduous trees, S. torminalis had the lowest root growth. Specific leaf area and nutrient concentration were positively related to root growth, highlighting the importance of traits related to the plant economic spectrum, as determinants of species root growth differences. Moderate light level slightly enhanced root growth and decreased the specific leaf area (SLA). Species differences in water potential under drought were positively related to rooting depth, evidencing the importance of its role in overcoming drought stress during seedling establishment. These findings can guide the selection of late successional, shade tolerant tree species for underplanting thinned Mediterranean plantations and provide insights into their ecology.

A computational model of the cereal leaves hydraulics

Environmental factors and plant architectonics significantly determine its water regime, namely, the water content and its movement through the tissues forced by the difference in water potentials, turgor pressure in cells, etc. In turn, the cumulative water regime affects the functioning and growth of cells, photosynthesis, and, as a result, the plant’s growth. The leaf contributes to the formation of the water regime and characterizes the contour of the plant’s adaptive system. Nevertheless, the data on the contribution of leaves to the total resistance to water transport in the plant and the structure of the hydraulic resistance of the leaf itself is still contradictory. This paper presents the formulation and justification of the monocots leaf hydraulics model based on Darcy’s law. The model was tested in computational experiments in the Comsol 4.3b package on idealized geometric models of leaf blades. Simulations showed the dependence of water potential distribution in xylem vessels and leaf mesophyll on the permeability of these tissues and on microclimatic parameters around the leaf. The adequacy of the model parameters selected as a result of testing is discussed.

Consistently lower sap velocity and growth over nine years of rainfall exclusion in a Mediterranean mixed pine-oak forest

Mediterranean forests face an intensification of droughts caused by ongoing climate change. To improve our understanding of tree and forest responses to increasing drought, we explored over nine years, the effects of a 30% rainfall exclusion experiment on the water potential, sap velocity and primary and secondary growth of two co-occurring species (Quercus ilex and Pinus halepensis) in a French Mediterranean forest. In addition, native embolism was measured after six and nine years of exclusion onset. Water potentials decreased earlier during summer drought for both species in the rainfall exclusion plot, and to a higher extent during the drought peak for Q. ilex, involving earlier stomatal closure and reduced sap velocity. Sap velocity reduction persisted throughout the years in the exclusion plot. Outside summer the water potential difference between predawn and midday was similar between treatment which indicate that reduced water transport efficiency may be linked to decrease hydraulic conductance. Such differences were neither related to differences in xylem embolism, that remained similar between treatments, nor to change in secondary growth. In contrast primary growth measurements indicate that P. halepensis trees, and Q. ilex to a lesser extent, experienced reduction in total leaf areas in response to the rainfall exclusion. Globally, our results suggest that increase drought lead, for both species, to a decrease in sap velocity not mediated by increase embolism but rather by a reduction in primary growth. So far little conclusion can be drawn regarding the competitive advantage of one species over the other in the context of increasing drought related to climate change.

Anatomies, vascular architectures, and mechanics underlying the leaf size-stem size spectrum in 42 Neotropical tree species

The leaf size-stem size spectrum is one of the main dimensions of plant ecological strategies. Yet the anatomical, mechanical, and hydraulic implications of small versus large shoots are still poorly understood. We investigated 42 tropical rainforest tree species in French Guiana, with a wide range of leaf areas at the shoot level. We quantified the scaling of hydraulic and mechanical constraints with shoot size, estimated as the water potential difference (ΔΨ) and the bending angle (ΔΦ), respectively. We investigated how anatomical tissue area, flexural stiffness and xylem vascular architecture affect such scaling by deviating (or not) from theoretical isometry with shoot size variation. Vessel diameter and conductive path length were found to be allometrically related to shoot size, thereby explaining the independence between ΔΨ and shoot size. Leaf mass per area, stem length, and the modulus of elasticity were allometrically related to shoot size, explaining the independence between ΔΦ and shoot size. Our study also shows that the maintenance of both water supply and mechanical stability across the shoot size range are not in conflict.

The Nature and Measures of Osmotic Pressure

Introduction In our first abstract titled “The Concept of Osmotic Pressure: Two Common Misunderstandings and Resolutions” in EB2021, we addressed that osmotic pressure should be defined in the context of a simple osmosis system (i.e., S-m-H2O, with a solution compartment (S) and a water compartment separated by a selectively permeable membrane (m)), not in the context of a composite osmosis system (i.e., S1-m-S2), which can instead be deconstructed into two mirrored simple osmosis systems: S1-m-H2O and H2O-m-S2. In this presentation, we 1) use a new set of terminology to differentiate the osmotic pressure (π) defined in three common ways in physiology and biology as well as in chemistry and physics; 2) address which of these ways represents the nature of π; and 3) illustrate the osmotic pressure difference/gradient in S1-m-S2. Method Logical reasoning as shown in Fig 1 and 2. Results 1) In S-m-H2O (Fig 1a to 1b), osmosis occurs and reaches equilibrium (1). We define the gravitational hydrostatic pressure exerted by the resulting liquid column as πachieved. 2) We define the π applied to S to prevent osmosis as πapplied (Fig 1c). 3) According to van't Hoff's Law, πachieved or πapplied = RTOC0 (OC0 is the initial osmotic concentration in S). 4) In S-m-H2O, a transmembrane pressure drives osmosis (illustrated using the blue arrow in Fig 1c), which can be viewed as exerted by either the water potential difference (Δμ0, not shown) or the OC0 difference (ΔOC0, the red gradient in Fig 1a and 1c) across the m: during osmosis, Δμ0 pushes or ΔOC0 pulls water toward S. We define this pressure to be πTM (TM refers to transmembrane). In physical chemistry, π (our πTM) = const×Δμ01. Hence, πTM = -RTOC0 = const×Δμ0. Conclusion 1) Among the three common ways to define π (πachieved, πapplied, πTM), πTM is thenature ofosmotic pressure; 2) πachieved and πapplied are two different ways to measure πTM. 3) In S1-m-S2, it is the osmotic pressure difference (ΔπTM) that drives osmosis, which can be measured using either Δπachieved or Δπapplied (Fig 2). In our third abstract, titled “The Pressure Profiles of the Equilibrium States of Osmosis Across Plant and Animal Cell Membranes”in EB2021, we will apply the concepts of ΔπTM to analyze the pressure profiles that characterize the equilibrium states of osmosis (after water enters a cell) across plant and animal cell membranes. 1Chang R. Physical Chemistry for the Chemical and Biological Sciences. Sausalito, CA: University Science Books, 2000, p. 235.