Is the coordination between leaf and stem hydraulic capacitance the “power bank” to delay embolism in olive?

Comunicación presentada en el XIM4 4th Xylem International Meeting Padua, 25-27 September 2019Key message: Both stem and leaf tissues act as coordinated capacitors releasing water into the transpiration stream to delay xylem embolism. The olive tree (Olea europaea L.) is becoming increasingly topical in plant hydraulics because of its physiological characteristics to cope with drought. Rodriguez-Dominguez et al. (2018) recently found that the coordination in olive of disparate organs like leaves and roots can significantly contribute to a marked stem xylem resistance reaching P50 values of ca. -6 MPa. Although such safety margin is well known, there is a lack of the knowledge about the effect of water that is released from surrounding tissues into the xylem (so-called hydraulic capacitance; kg MPa-1 m-3), which contributes to guarantee plant hydraulic performance under drought stress. Moreover, the gradient of water potential that transpiration path creates from the leaves throughout the plant hydraulic system suggests a coordination between the hydraulic capacitance of leaf and stem tissues. To this end, we will present the results of drought-induced embolism in two-year old olive seedlings submitted to bench-top dehydration. The hydraulic capacitance of the stems, derived from desorption curves, and that of leaves, quantified from pressure-volume curves, were compared with xylem embolism formation, estimated from stem vulnerability curves.N

Long-term versus daily stem diameter variation in co-occurring mangrove species: environmental versus ecophysiological drivers

High temporal resolution stem diameter variation (SDV) patterns have been widely recognized as a tool to study fundamental plant physiological mechanisms underlying whole-plant functioning and growth. As an integrative response to hydraulic and carbon related processes, SDV research has greatly improved insights in plant functioning of several herbaceous and woody species. Nevertheless, to date little detailed information on SDV and related physiological processes is available for mangrove species. By measuring continuous tree physiological variables such as stem diameter variations, sap flow and stem water potential in relation to the microclimatic conditions, the water use strategies of two co-occurring mangrove species, Avicennia marina (Forssk.) Vierh. and Rhizophora stylosa Griff. were investigated. Even though both species showed a similar long-term growth trend, closely linked to the environmental conditions, their daily SDV pattern was markedly different. While for Avicennia marina the SDV showed the standard daily pattern of morning decline and evening rise, the opposite daily SDV pattern was observed for Rbizophora stylosa. The contrasting patterns of SDV in both species thriving in the same environment indicates the importance and complexity of physiological endogenous mechanisms in addition to environmental conditions in controlling SDV and radial stem growth. (C) 2014 Elsevier B.V. All rights reserved

Sugars from woody tissue photosynthesis reduce xylem vulnerability to cavitation

Reassimilation of internal CO2 via woody tissue photosynthesis has a substantial effect on tree carbon income and wood production. However, little is known about its role in xylem vulnerability to cavitation and its implications in drought-driven tree mortality. Young trees of Populus nigra were subjected to light exclusion at the branch and stem levels. After 40 d, measurements of xylem water potential, diameter variation and acoustic emission (AE) were performed in detached branches to obtain acoustic vulnerability curves to cavitation following bench-top dehydration. Acoustic vulnerability curves and derived AE(50) values (i.e. water potential at which 50% of cavitation-related acoustic emissions occur) differed significantly between light-excluded and control branches (AE(50,light-excluded) = -1.00 +/- 0.13 MPa; AE(50,control) = -1.45 +/- 0.09 MPa; P = 0.007) denoting higher vulnerability to cavitation in light-excluded trees. Woody tissue photosynthesis represents an alternative and immediate source of nonstructural carbohydrates (NSC) that confers lower xylem vulnerability to cavitation via sugar-mediated mechanisms. Embolism repair and xylem structural changes could not explain this observation as the amount of cumulative AE and basic wood density did not differ between treatments. We suggest that woody tissue assimilates might play a role in the synthesis of xylem surfactants for nanobubble stabilization under tension