A MODIFICATION OF THE WARBURG RESPIROMETER TO MEASURE THE RESPIRATION RATE OF TOMATO LEAF DISCS

In connection with a physiological study of six tomato varieties, the need for a method of measuring simultaneously the respiration rate of the leaves of several replicates of these varieties arose. BONNER and WIDMAN (1) have reported on the use of the Warburg apparatus for measuring the respiration of spinach leaves. They obtained highly reproducible rates of gas exchange from 200 milligram samples of fresh spinach leaf fragments 5-10 mm2 in area, when mixed with two milliliters of a phosphate buffer and measured in the Warburg respirometer. Preliminary attempts were made to measure respiration of tomato-leaf discs by the oxygen absorption method discussed by UMBREIT, BURRIs and STAUFFER (3). Twenty discs were cut from a tomato leaf with a onequarter inch cork borer. Oxygen uptake measurements of these -discs were made in a darkened room in Warburg flasks. Two-tenths milliliter of 20% potassium hydroxide was used in the center well of each flask to absorb the carbon dioxide. All measurements were made at 250 C, using an M/15 potassium-dihydrogen-phosphate buffer with a shaker rate of 120 strokes per minute. Extremely variable rates of oxygen uptake were obtained between replicates of discs taken from the same leaf. It seemed reasonable -to suspect that the buffer might be the principal cause of these erratic rates of oxygen absorption. The buffer being in close contact with the leaf discs, would greatly impede the normal exchange of gases in respiration. This interference would be especially great in the absorption of oxygen since its solubility in water is quite low. From Lange 's Handbook of Chemistry (2) the a value for oxygen at 250 C is 0.0283; whereas the a value for carbon dioxide at 250 C is 0.759, or carbon dioxide is approximately 27 times more soluble in water than is oxygen. Subsequently a modification of this method was tried in which the buffer was eliminated and filter paper of uniform size was cut and fitted in the bottom of the Warburg flasks. One-tenth milliliter of water was pipetted into the bottom of each flask to moisten the filter paper. Twenty tomato leaf discs were then placed in regular order on the filter paper. The flask atmosphere was kept sufficiently moistened by the wet filter paper to maintain turgor of the leaf discs. Two-tenths milliliter of 20% potassium hydroxide was used in the center well to absorb the carbon dioxide. With the Warburg apparatus available for this work, simultaneous respiration determinations could be made of leaf discs from two complete replicates of six varieties each plus a thermobarometer. All measurements were made at 280 C in a darkened room. 1 Paper No. 331, Department of Vegetable Crops, Cornell University, Ithaca, New York. 354