Resistance to water flow through leaves of Coffea arabica is dominated by extra-vascular tissues

The leaf hydraulic conductance (Kleaf) of Coffea arabica L. was measured for shoots exposed to non-lethal temperature stress or to a freeze–thaw cycle, and compared with Kleaf of non-stressed samples (controls). Exposure to temperatures below 6°C for 1 h caused measurable damage to the functional integrity of cell membranes as shown by increased membrane leakiness to electrolytes. A 1 : 1 relationship was found to exist between relative electrolyte leakage and relative Kleaf suggesting that membrane damage caused Kleaf to increase. Low temperatures did not cause membrane disruption as shown by the comparison of chilled samples with frozen–thawed ones. In frozen leaves, membranes were extensively disrupted and both electrolyte leakiness and Kleaf increased 5-fold. Low temperatures did not induce alterations of the hydraulic properties of the leaf vascular system, as revealed by measurements of Kleaf after up to 500 cuttings of minor veins were made in the leaf blade of control and chilled leaves. Calculations showed that 62–75% of leaf hydraulic resistance resided in the extra-vascular water pathway. Results are discussed within the frame work of our current understanding of leaf hydraulic architecture as well as in terms of plant adaptation to extremes in temperature.