Foliar water supply of tall trees: evidence for mucilage-facilitated moisture uptake from the atmosphere and the impact on pressure bomb measurements

Abstract

The water supply to leaves of 25 to 60 m tall trees (including high-salinity-tolerant ones) was studied. The filling status of the xylem vessels was determined by xylem sap extraction (using jet-discharge, gravity-discharge, and centrifugation) and by (1)H nuclear magnetic resonance imaging of wood pieces. Simultaneously, pressure bomb experiments were performed along the entire trunk of the trees up to a height of 57 m. Clear-cut evidence was found that the balancing pressure (P(b)) values of leafy twigs were dictated by the ambient relative humidity rather than by height. Refilling of xylem vessels of apical leaves (branches) obviously mainly occurred via moisture uptake from the atmosphere. These findings could be traced back to the hydration and rehydration of mucilage layers on the leaf surfaces and/or of epistomatal mucilage plugs. Xylem vessels also contained mucilage. Mucilage formation was apparently enforced by water stress. The observed mucilage-based foliar water uptake and humidity dependency of the P(b) values are at variance with the cohesion-tension theory and with the hypothesis that P(b) measurements yield information about the relationships between xylem pressure gradients and height.

Distribution and function of epistomatal mucilage plugs

“…All the investigated species of the other orders showed mucilage plugs. Even though the sample size was rather small several interesting conclusions could be drawn from the evaluation of the images of alcian bluestained leaves of the 67 species investigated here and previously (Zimmermann et al 2007a). …”

“…A sigmoid-shaped dependency of the P p -R.H. relationship was also found for mucilage plug-rich leaves of E. pilularis using both the leaf patch clamp pressure probe (data not shown) and the Scholander pressure bomb (see Fig. 5 in Zimmermann et al 2007a). Both techniques measure changes in turgor pressure in the low pressure range as shown recently (Westhoff et al 2009).…”

“…Drought and salinity stress enforce xylem-bound mucilage formation (Zimmermann et al 1994(Zimmermann et al , 2002Zimmermann et al 2007a). Xylem-bound mucilage is most probably involved in water lifting against gravity (Plumb and Bridgman 1972;Pollack 2001;Zimmermann et al 2004;Yeo and Flowers 2007).…”

“…Xylem-bound mucilage is most probably involved in water lifting against gravity (Plumb and Bridgman 1972;Pollack 2001;Zimmermann et al 2004;Yeo and Flowers 2007). Recent studies have shown (Zimmermann et al 2007a) that transpirational water loss and moisture uptake from the atmosphere are also modulated by mucilage. In some species (e.g.…”

“…Epistomatal cavities filled with mucilage are chararacteristic for the leaves of the salt-tolerant trees A. fraxinifolium and Bulnesia sarmientoi as well as for the salt-intolerant trees Populus nigra, Eucalyptus pilularis and Nothofagus dombeyi. Zimmermann et al (2007a) gave a large body of evidence that epistomatal mucilage plugs and acid mucilage layers are not only able to buffer changes in leaf water status when transpiration is high, but also facilitate moisture uptake from the atmosphere (including fog, rain, ascending transpirational water from lower leaves and evaporation from the soil). Via foliar moisture uptake apical leaves of tall trees are immediately supplied with water, whereas water uptake from the roots needs many days (up to 4 weeks; Woodward 2004).…”