LACUNAR PRESSURES IN MYRIOPHYLLUM-HETEROPHYLLUM - MANOMETRIC MEASUREMENT AND DIURNAL GAS-DYNAMICS IN-FIELD POPULATIONS

A simple, inexpensive manometer was developed to measure lacunar press ures in submersed vascular plants. Manometers consisted of a sealed mi cropipet connected to a hypodermic needle and partially filled with ma nometer fluid. The accuracy and sensitivity of the manometer was deter mined by the amount of fluid and manometers were reliable over the ran ge of 80 to 120 kPa. Diurnal changes in lacunar gas pressure and compo sition were examined in intact plants of Myriophyllum heterophyllum Mi chx, exposed to different current velocities in the field. Lacunar pre ssures were about 1 kPa below atmospheric in the morning, and by early afternoon reached maximum levels of 3 kPa and 5 kPa above atmospheric at a high and low velocity site, respectively. Lacunar O-2 concentrat ions increased as pressures increased and ranged from 17 to 23% and 15 to 34% at these sites. Lacunar O-2 concentrations at the low velocity site were higher at the shoot apex than toward the base and cutting o f these stems lowered the O-2 concentration in basal segments, indicat ing that O-2 distribution may be influenced by photosynthetic differen ces and O-2 transport. The behavior of lacunar gases was examined by c omparing measured gas pressures with pressures predicted based on meas ured lacunar O-2 concentrations. Gas pressures measured at the high ve locity site were slightly higher than predicted, but in general follow ed the ideal gas law. Measured gas pressures at the low velocity site, however, became progressively lower than predicted throughout the aft ernoon, High photon irradiance and thick boundary layers at this site apparently enhanced lacunar O-2 accumulation, promoted the release of gas bubbles from the lacunar atmosphere, and led to non-equilibrium la cunar N-2 content and to lacunar O-2 enrichment without increased lacu nar pressure. Lacunar pressures measured in this field study were lowe r than those reported for laboratory studies. Factors which can influe nce lacunar pressurization and lacunar function are discussed.