NITROGEN, POTASSIUM, AND IRRIGATION EFFECTS ON WATER RELATIONS OF KENTUCKY BLUEGRASS LEAVES

Transpiration and expansive growth in leaves are turgor-dependent processes. Solute concentration and osmotic potential are inextricably linked to turgor maintenance. An empirical equation predicting stomatal resistance (R(s)) from bulk leaf turgor (psi-t) would be useful in developing computer simulations for turfgrass management. A growth-chamber study was conducted to quantify the relationship between R(s) and psi-t in Kentucky bluegrass (Poa pratensis L. cv. A-34). The effects of N, K, and irrigation frequency on bulk leaf osmotic potential at full turgor (psi-pi-100) and apoplastic water fraction (AWF) also were examined, using psychrometric techniques. Treatments consisted of two rates of N and K and two irrigation frequencies. An exponential model was used to describe the relationship between R(s) and psi-t. The least squares prediction equation was R(s) = 581 + 2500 exp(-6.99-psi-t), r2 = 0.69, where psi-t and R(s) are expressed in units of MPa and s m-1, respectively. Increasing the time between rewatering containers to -0.02 MPa from 1 to 5 d did not influence psi-pi-100. For plants watered daily, increasing the amount of N supplied every 30 d from 35 to 175 kg ha-1 increased psi-pi-100 0.22 MPa, while increasing K from 17.5 to 175 kg ha-1 for the same application interval caused psi-pi-100 to decline 0.20 MPa. Altering the supply of N or K did not affect psi-pi-100 when the containers were watered every 5 d. Increasing the irrigation interval from 1 to 5 d caused AWF to decline from 22 to 12%; however, AWF estimates were highly variable. Results indicate the combined influences of N and K fertility practices can have a significant impact on the concentration of osmotically active solutes within Kentucky bluegrass bulk leaf tissue.