Abstract Detail

Ecophysiology

Klein, Tamir [1], Uni, Daphna [1], Winters, Gidon [3], Sheffer, Efrat [4].

One tree species, three extremes: Resilience to high temperature, aridity, and solar radiation of Acacia tortilis in the Arava desert, Israel.

Drought-induced tree mortality has been increasing globally and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe’s deserts, little is known about their growth dynamics and water-use in situ.
Stem diameter dynamics, leaf phenology, leaf gas exchange, and sap fow were monitored during 3 consecutive years in ten Acacia tortilis trees in the arid Arava Valley, southern Israel (annual precipitation 20–70 mm, restricted to October–May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or fashfood events.
Unexpectedly, cambial growth was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure defcit of up to 9 kPa. Summer growth was correlated with peak sap fow in June, with almost year-round activity and foliage cover. Photosynthesis measurements showed increasing rates at midday and mid-summer, with no sign of light-saturation. Using an artificial light source and chlorophyll fluorescence, we showed that photosynthesis in these Acacia trees is sustained at levels higher than Earth’s maximum (2200 µmol photons m^-2 s^-1), with little or no photoinhibition.
To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. Our study also demonstrates the edge of light-use efficiency and indicates the existence of a yet unknown photoprotection mechanism. Pinpointing this mechanism will require further research at the molecular and biochemical levels. Summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

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1 - Weizmann Institute Of Science, 234 Herzl, Rehovot, 76100, Israel
2 - Weizmann Institute Of Science, 234 Herzl, Rehovot, 76100, Israel
3 - The Dead Sea-Arava Science Center, Tamar Regional Council, Neve Zohar
4 - Faculty of Agriculture, Food and Environment, The Hebrew University of

Keywords:
Hyper-arid
drought resistance
phenology
photo-inhibition
vapor pressure deficit
Acacia.

Presentation Type: Oral Paper
Number: EPH3002
Abstract ID:159
Candidate for Awards:None