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 nonlethal temperature stress or to a freeze - thaw cycle, and compared with K-leaf of non-stressed samples ( controls). Exposure to temperatures below 6degreesC 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 framework of our current understanding of leaf hydraulic architecture as well as in terms of plant adaptation to extremes in temperature.