CHILLING STRESS LEADS TO INCREASED CELL-MEMBRANE RIGIDITY IN ROOTS OFCOFFEE (COFFEA-ARABICA L) SEEDLINGS

Tropical and sub-tropical higher plant species show marked growth inhi bition when exposed to chilling temperatures. In root tip segments of coffee seedlings which were subjected for 6 days to temperatures of 10 , 15, 20 and 25 degrees C, in darkness, we have detected an increased amount of malondialdehyde formed in the 10 degrees C treatment, accomp anied by higher electrolyte leakage. The electron paramagnetic resonan ce (EPR) technique and the fatty acid spin probes 5-, 12- and 16-doxyl stearic acid were used to assess cellular membrane fluidity. At the de pth of the 5th and 16th carbon atom of the alkyI chains the nitroxide radical detected more rigid membranes in seedlings subjected to 10 deg rees C compared with 15 and 25 degrees C. At the C-12 position of the chains the probe showed very restricted motion and was insensitive to chilling induced membrane alterations. EPR parameters for intact tissu es and microsome preparations from root tips showed that the fluidity was essentially the same when evaluated at C-5 and C-16 positions of t he chains, and was considerably more fluid for microsomal membranes in the region of the C-12 position of the bilayers. The rotational motio n of the nitroxide at C-16 position of the chains experienced a phase transition at about 15 degrees C. The calculated energy barriers for r eorientational motion of the probe 16-doxylstearic acid were higher at temperatures of 5-15 degrees C than in the interval of 15-25 degrees C, suggesting that below the phase transition the membrane lipids assu me a more ordered and compacted array. Membrane rigidity induced by ch illing was interpreted as due to lipid peroxidation that could have be en facilitated by higher density of peroxidizable chains below the mem brane phase transition.