SOIL COMPACTION AND MOISTURE STRESS PRECONDITIONING IN KENTUCKY BLUEGRASS .2. STOMATAL-RESISTANCE, LEAF WATER POTENTIAL, AND CANOPY TEMPERATURE

In this greenhouse study we investigated the effects of soil compaction and moisture stress preconditioning on stomatal diffuse resistance (Rs), leaf water potential (.PSI.1), and canopy minus air temperatures (.DELTA.T) of Kentucky bluegrass (Poa pratensis L. ''Ram I''). The compaction treatments were: (i) NC = no compaction, (ii) LT = long-term compaction over a 99-day period, and (iii) ST = short-term compaction for 9 days. The compactive treatment was equivalent to 720 J energy. Irrigation regimes were initiated at the same time as LT compaction and were: (i) well-watered = irrigation at -0.045 MPa and (ii) water-stressed = irrigation at =0.400 MPa. Ninety-nine days after initiation of preconditioning treatments, a dry-down cycle was started by watering each treatment to saturation. At this time, we monitored on a daily basis Rs, .PSI., and .DELTA.T. Under low soil O2, Rs remained low for 2 days and then increased over a 5-day period for all treatments, even though .PSI.1 did not change until the fifth day after irrigation (DAI). By DAI 9, Rs declined but then increased between DAI 10 to 13 as soil water potential (.PSI.s) and .PSI.1 decreased. As soil water deficits increased, plants preconditioned to LT compaction or water-stressed exhibited lower .PSI. (0.2 to 0.4 MPa), higher Rs and higher .DELTA. (1 to 2.degree. C) compared with uncompacted or well-watered plants. Regardless of the cause for higher Rs (i.e., low soil O2, LT compaction or water-stress preconditioning), the result would be lower photosynthesis and greater higher-temperature stress.