Effects of Low Water Potentials on Transpiration and Photosynthesis in Mitchell Grass (Astrebla lappacea)

Abstract

Gas-exchange studies were carried out on potted Mitchell grass plants raised in the glasshouse and in growth cabinets. Provided that nutrition was adequate, the low irradiance of the growth cabinets did not impair the photosynthetic capacity at levels approaching full sunlight. The optimum temperature for net photosynthesis was in excess of 40ºC. Close linear relationships were established between leaf water potential and both transpiration and net photosynthesis. Most of the variation in gas exchange could be attributed to changes in stomatal diffusive resistance. Although there was an increase in mesophyll resistance as leaf water potential decreased, the proportion of the total resistance attributable to the mesophyll became substantially smaller. Gas exchange exhibited a remarkable recovery after brief but severe drought, during which leaf water potentials fell to about -48 bars. There was some inhibition of both transpiration and photosynthesis for about one week following rewatering. It is concluded that A. lappacea demonstrates a high degree of true drought tolerance.