The Forgotten Component of Plant Water Potential: A Reply - Tissue Pressures are not Additive in the Way M. J. Canny Suggests

Tyree, Mt

doi:10.1055/s-2007-978560

Cited by 4 publications

(4 citation statements)

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“…Canny (1995b) has recently introduced a new unifying theory to explain water lifting in tall trees. This so‐called ‘Compensating‐Pressure Theory’ has been challenged in a series of strenuous, partly impertinent critiques (Comstock, 1999; Tyree, 1999; Stiller & Sperry, 1999). We have also criticised Canny's theory for some thermodynamic reasons (Zimmermann et al , 1995b).…”

Section: The ‘Multi‐force’ or ‘Watergate’ Theorymentioning

confidence: 99%

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

et al. 2004

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Summary The Cohesion Theory considers plant xylem as a ‘vulnerable pipeline’ isolated from the osmotically connected tissue cells, phloem and mycorrhizas living in symbiosis with plant roots. It is believed that water is pulled exclusively by transpiration‐induced negative pressure gradients of several megapascals through continuous water columns from the roots to the foliage. Water under such negative pressures is extremely unstable, particularly given the hydrophobicity of the inner xylem walls and sap composition (lipids, proteins, mucopolysaccharides, etc.) that prevents the development of stable negative pressures larger than about −1 MPa. However, many plant physiologists still view the Cohesion Theory as the absolute and universal truth because clever wording from the proponents of this theory has concealed the recent breakdown of the Scholander pressure bomb (and other indirect methods) as qualified tools for measuring negative pressures in transpiring plants. Here we show that the arguments of the proponents of the Cohesion Theory are completely misleading. We further present an enormous bulk of evidence supporting the view that – depending on the species and ecophysiological context – many other forces, additional to low tensions, can be involved in water ascent and that water can be lifted by a series of watergates (like ships in staircase locks).

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Section: The ‘Multi‐force’ or ‘Watergate’ Theorymentioning

confidence: 99%

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

et al. 2004

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“…The plant LWP would be affected by soil moisture, temperature, air humidity, photosynthetically active radiation and leaf transpiration rate and so on. The LWP is usually positively related to soil water content, and largely affected by transpiration rate [12,30] . In this study, the grape LWP decreased linearly with soil water content after irrigation until watered again.…”

Section: Discussionmentioning

confidence: 99%

Effects of regulated deficit irrigation on the growth and berry composition of Cabernet Sauvignon in Ningxia

Wang

Yan

Sun

et al. 2019

International Journal of Agricultural and Biological Engineering

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The adoption of water-saving irrigation strategies is required particularly for wine grape variety, which has been widely cultivated in arid and semiarid areas. To assess vine response to regulated deficit irrigation (RDI), the grape growth and berry composition under five treatments that irrigated at a certain percentage of the crop evapotranspiration (ET c) were evaluated over a 3-year period in a vineyard with the grape variety of Cabernet Sauvignon. The results indicated that RDI had a significant effect on the grape berry size and yield. The largest berry size (12.20 mm) was obtained under the T50 in 2014, while the smallest berry size (9.83 mm) one was obtained under the CK treatments in the same season. The highest individual yield occurred in the T50 treatment, with an average of 1.99 kg, followed by the T25-50 treatment. However, both weights were significantly larger than that of the CK treatment. Compared with the T50 treatments, the individual grape vine yield in the T50-25 treatments were slightly less by 16.9% for 2013, 15.3% for 2014 and 18.1% for 2015. Compared to control (CK) treatment, the soluble solid and reducing sugar contents decreased, the total acid content increased, and the sugar/acid ratio basically showed a downward trend. The treatment irrigated at 50% ET c until veraison and 25% thereafter (T50-25) increased the phenolic compound content in grape skins. The treatment received only rain water during the grape growing season (CK) and the one irrigated at 25% of the ET c crop evapotranspiration (T25) caused defoliation and negatively affected the yields and grape composition during all 3 years. Therefore, the RDI not only inhibited the vine vegetative growth but also improved the fruit quality. In terms of productivity and grape composition, the Cabernet Sauvignon grape variety was most sensitive to water stress post-veraison. Over the comprehensive consideration of yield, water-use efficiency and berry composition, the T50-25 treatment was the most efficient irrigation strategy in this area.

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“…A possible role of phloem Of the proposed positive pressure scenarios, the 'tissue pressure' model of Canny (1995Canny ( , 1998 was the only one to directly invoke a role of water flow from phloem, which is typically pressurised in the order of 1 MPa. Canny attempted to explain all plant water transport with this model and was proven wrong in doing so (Comstock, 1999;Stiller & Sperry, 1999;Tyree, 1999), but the fact remains that phloem in roots and vascular bundles borders directly on xylem, and would be the most immediate source of water for xylem at sites of phloem unloading, such as sugar sinks in roots or storage tissues (Münch, 1930;Woodhouse, 1933;Hölttä et al, 2006;Nardini et al, 2011;Sevanto et al, 2011;Wegner, 2014). Water from phloem therefore could contribute to positive xylem pressure, including in stems and rhizomes.…”

Section: Reviewmentioning

confidence: 99%

Positive pressure in xylem and its role in hydraulic function

2021

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Summary Although transpiration‐driven transport of xylem sap is well known to operate under absolute negative pressure, many terrestrial, vascular plants show positive xylem pressure above atmospheric pressure on a seasonal or daily basis, or during early developmental stages. The actual location and mechanisms behind positive xylem pressure remain largely unknown, both in plants that show seasonal xylem pressure before leaf flushing, and those that show a diurnal periodicity of bleeding and guttation. Available evidence shows that positive xylem pressure can be driven based on purely physical forces, osmotic exudation into xylem conduits, or hydraulic pressure in parenchyma cells associated with conduits. The latter two mechanisms may not be mutually exclusive and can be understood based on a similar modelling scenario. Given the renewed interest in positive xylem pressure, this review aims to provide a constructive way forward by discussing similarities and differences of mechanistic models, evaluating available evidence for hydraulic functions, such as rehydration of tissues, refilling of water stores, and embolism repair under positive pressure, and providing recommendations for future research, including methods that avoid or minimise cutting artefacts.

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The Forgotten Component of Plant Water Potential: A Reply - Tissue Pressures are not Additive in the Way M. J. Canny Suggests

Cited by 4 publications

References 1 publication

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

Water ascent in tall trees: does evolution of land plants rely on a highly metastable state?

Effects of regulated deficit irrigation on the growth and berry composition of Cabernet Sauvignon in Ningxia

Positive pressure in xylem and its role in hydraulic function

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