Nitrogen, Potassium, and Irrigation Effects on Water Relations of Kentucky Bluegrass Leaves

Abstract

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 (Rs) from bulk leaf turgor (ψt) would be useful in developing computer simulations for turfgrass management. A growth‐chambers study was conducted to quantify the relationship between Rs and ψt 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 (ψπ100) 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 Rs and ψt. The least squares prediction equation was Rs = 581 + 2500 exp(−6,99ψt), r2 = 0.69, where ψt and Rs. 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 ψπ100. For plants watered dally, increasing the amount of N supplied every 30 d from 35 to 175 kg ha−1 increased ψπ100 0.22 MPa, while increasing K from 17.5 to 175 kg ha−1 for the same application interval caused ψπ100 to decline 0.20 MPa. Altering the supply of N or K did not affect ψπ100 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.