Title THE EFFECT OF SOIL MOISTURE SUCTION AND SOIL TEMPERATURE ON TRANSPIRATION, PHOTOSYNTHESIS AND RESPIRATION OF MONTEREY PINE (PINUS RADIATA D. DON) Abstract approved Redacted for Privacy

A special apparatus was developed in which the soil moisture levels of several soil cells could be maintained independently by various osmotic solutions at a selected temperature. The effect of four soil moisture suctions and four soil temperatures on the rates of transpiration, photosynthesis and respiration of monterey pine seedlings was studied. Transpiration was determined by a constant water level device which recorded the total amount of water used by the plant. Leaf resistances to CO2 and water vapour were calculated. In general, the rates of photosynthesis, respiration and transpiration decreased with increasing soil moisture suction. Photosynthesis and transpiration decreased sharply from 0. 35 bars to 0. 70 bars soil moisture suctions. The rates of transpiration increased faster than the rate of photosynthesis with increasing soil temperature. The rates of respiration fluctuated with soil temperature. Where the ratio of transpiration to photosynthesis is constant as between 600 F and 800 F at all soil moisture suctions except 0.35 bars, the same process was assumed to be limiting (probably stomatal control). At 0.35 bars soil moisture suction, physiological and viscosity factors are to be considered. The activity of the root is temperature dependent and suction dependent. At high suctions, the temperature effect is small, at low suction the temperature effect is large. At suctions higher than 0. 70 bars, the diffusion process is stomatal controlled