Theory is presented for a differential mass-volume technique to measure non-destructively gas volume (Vg) changes, based only on the initial and final masses and volumes of an organ. Volume was measured using Archimedes' principle, but a non-invasive image analysis procedure could be an improvement. A reduction in Vg during the ripening of `Kada' tomato (Lycopersicon esculentum Mill) fruits, and irreversible Vg changes of 0.02, 0.29, 0.66, 1.2, and 1.3 ml for mature-green fruits compressed by 0, 2.5, 5.0, 7.5. and 10 mm for 5 minutes indicates the potential of this procedure. The method was compared with other methodologies using sweetpotato (Ipomea batatas L.) root segments subjected to vacuum water infiltration. The results were similar to the pycnometric method. The gasometric method underestimated Vg for roots in which the intercellular air volumes where blocked by the water used for infiltration, and large overestimation occurred with the traditional infiltration technique without correction for water absorption. Absolute Vg values were also estimated by semi-pycnometry (defined as the difference between the organ volume measured by water immersion and the organ volume without Vg measured with a pycnometer, after its maceration and elimination of gas bubbles with vacuum). Semi-pycnometry applied to tomato and bell pepper (Capsicum annuum L.) fruits, where the use of tissue segments limits the pycnometric method, and in sweetpotatoes, where the gasometric method overestimates Vg, generated results that were consistently similar to the differential mass-volume method.
Constant-pressure manometry, previously designed to study O2 and CO2 gas exchange in small pieces of tissue, cells, and organelles, was adapted to study bulky organs. According to this new procedure, a near-zero-volume Devaux chamber connects a manometer to the internal atmosphere volume (VG) of a plant organ covered by a layer of epoxy, submerged in unstirred water, kept at constant temperature, and kept at the same VG pressure. Equations, based on CO2 and O2 solubility at equilibrium with VG, were used to follow O2 consumption as a function of reduced internal O2 pressure over time [for organs with VG < 0.1 (v/v) and respiratory quotient (RQ) of 0.7 to 1.3] to observe the transition between aerobiosis and anaerobiosis and to measure CO2 evolution during the anaerobic phase. For those measurements, bulky-organ manometry performed consistently in tomato [VG = 6.41% (v/v)], sweetpotato [VG = 8.57% (v/v)], and potato [VG = 0.34% (v/v)]. The results indicate that constant-volume manometry is sufficiently precise to detect differences in respiratory metabolism as a function of intercellular O2 concentration in intact plant organs.