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

The leaf hydraulic conductance (K_leaf) of Coffea arabica L. was measured for shoots exposed to non-lethal temperature stress or to a freeze-thaw cycle, and compared with K_leaf 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 K_leaf suggesting that membrane damage caused K_leaf 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 K_leaf increased 5-fold. Low temperatures did not induce alterations of the hydraulic properties of the leaf vascular system, as revealed by measurements of K_leaf 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.