Abstract
BackgroundPlants have been hypothesized to maintain strong control over xylem tension by closing stomata and to operate at a water potential above or near the critical potential at which cavitation commences. An alternative hypothesis holds that cavitation temporarily relieves water stress and stomatal closure is insufficient to prevent short term “run-away” cavitation.MethodsThe objectives of this study were to investigate the leaf conductivity loss at noon (Loss) of 13 woody species differing in leaf phenology at two sites on karst topography in the wet season in southwestern China; the hydraulic architecture of woody species has rarely been reported previously. Loss was predicted from minimum field leaf water potentials (Ψmin) and laboratory-generated vulnerability curves. We also measured the maximum quantum efficiency of photosystem II using chlorophyll a fluorescence (Fv/Fm) and other associated leaf traits.ResultsLoss in the field varied substantially, from 1.39% in evergreen Itea chinensis to 90.07% in deciduous Sapium sebiferum. However, the Loss did not significantly decrease the efficiency of photosystem II. The water potential at which a 50% loss in leaf conductivity occurred (Ψ50) was not correlated to Ψmin. The co-occurring evergreen and deciduous species differed significantly in some stem hydraulic and associated leaf traits. Deciduous species had higher hydraulic conductance, photosynthetic rate, stomatal conductance, lower cavitation-resistance and minimum water potential than co-occurring evergreen species.ConclusionsThere was no sign that karst woody species in southwestern China could control xylem tension above the threshold to avoid substantial xylem cavitation in the wet season. There was no association between Loss and Fv/Fm among the studied species. This “isohydric” regulation behaviour, as well as abundant rainfall in the wet season, may explain why large variations of Loss existed across karst woody species in southwestern China.
Highlights
Cavitation-induced decrease in the hydraulic conductance of plant stems, roots, and leaves has long been suggested to lead to stomatal closure, thereby preventing the increase in xylem tension that can induce run-away xylem cavitation (Jones and Sutherland 1991; Nardini and Salleo 2000; Domec et al 2006; Guyot et al 2012; Daniela et al 2016; Xiong et al 2018)
(2018) 5:40 average loss of hydraulic conductivity corresponding to minimum leaf water potential was about 20% in the wet season (Markesteijn et al 2011b)
Values of Ψmin ranged from − 0.96 MPa in Stachyurus obovatus to − 1.61 MPa in Mallotus japonicus, a difference of only 0.65 MPa (Table 2)
Summary
Cavitation-induced decrease in the hydraulic conductance of plant stems, roots, and leaves has long been suggested to lead to stomatal closure, thereby preventing the increase in xylem tension that can induce run-away xylem cavitation (Jones and Sutherland 1991; Nardini and Salleo 2000; Domec et al 2006; Guyot et al 2012; Daniela et al 2016; Xiong et al 2018). Because embolized xylem cells can reduce hydraulic conductivity and (2018) 5:40 average loss of hydraulic conductivity corresponding to minimum leaf water potential was about 20% in the wet season (Markesteijn et al 2011b). The functional association between minimum water potential and hydraulic safety does not prove that plants can control embolisms, because the loss of hydraulic conductivity is not a linear function of water potential. A detailed analysis of published data has shown that there exists a large variation among species at the same site in native embolism rates during a day (Pockman and Sperry 2000; Markesteijn et al 2011a, 2011b). An alternative hypothesis holds that cavitation temporarily relieves water stress and stomatal closure is insufficient to prevent short term “run-away” cavitation
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