Capacitive water release and internal leaf water relocation delay drought-induced cavitation in AfricanMaesopsis eminii

Abstract

The impact of drought on the hydraulic functioning of important African tree species, like Maesopsis eminii Engl., is poorly understood. To map the hydraulic response to drought-induced cavitation, sole reliance on the water potential at which 50% loss of xylem hydraulic conductivity (ψ50) occurs might be limiting and at times misleading as the value alone does not give a comprehensive overview of strategies evoked by M. eminii to cope with drought. This article therefore uses a methodological framework to study the different aspects of drought-induced cavitation and water relations in M. eminii. Hydraulic functioning of whole-branch segments was investigated during bench-top dehydration. Cumulative acoustic emissions and continuous weight measurements were used to quantify M. eminii's vulnerability to drought-induced cavitation and hydraulic capacitance. Wood structural traits, including wood density, vessel area, diameter and wall thickness, vessel grouping index, solitary vessel index and vessel wall reinforcement, were used to underpin observed physiological responses. On average, M. eminii's ψ50 (±SE) was -1.9 ± 0.1 MPa, portraying its xylem as drought vulnerable, just as one would expect for a common tropical pioneer. However, M. eminii additionally employed an interesting desiccation delay strategy, fuelled by internal relocation of leaf water, hydraulic capacitance and the presence of parenchyma around the xylem vessels. Our findings suggest that exclusive dependence on ψ50 would have misdirected our assessments of M. eminii's drought stress vulnerability. Hydraulic capacitance linked to anatomy and leaf-water relocation behaviour was equally important to better understand M. eminii's drought survival strategies. Because our study was conducted on branches of 3-year-old greenhouse-grown M. eminii seedlings, the findings cannot be simply extrapolated to adult M. eminii trees or their mature wood, because structural and physiological plant properties change with age. The techniques and methodological framework used in this study are, however, transferable to other species regardless of age.

Implementing Plant Hydraulics in the Community Land Model, Version 5

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Sugars from woody tissue photosynthesis reduce xylem vulnerability to cavitation

“…To reduce noise interference from leaves and the exposed end, sensors were installed in the middle of the leaf-free section of the branches. The distance between the AE sensor and the dendrometer was fixed at 10 cm via a custom-built holder to ensure an unbiased link between AE and diameter shrinkage (Epila et al, 2017). The AE signals were amplified by 35.6 dB with an amplifier (AMP-1BB-J, KRN Services, Richland, WA, USA) and waveforms of 7168 samples length were acquired at 10 MHz sample rate.…”

Stem hydraulic capacitance decreases with drought stress: implications for modelling tree hydraulics in the Mediterranean oak Quercus ilex

“…Accordingly, a trade-off between hydraulic capacitance and structural traits involved in xylem resistance to cavitation has been observed across a wide range of woody species (Meinzer et al , 2009). Nevertheless, the role of hydraulic capacitance in plant hydraulics has been traditionally overshadowed by the study of drought-induced cavitation and the vulnerability of xylem to changes in water potential (Meinzer et al 2009;McCulloh et al 2014;Epila et al 2017).…”