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Effect of Water Stress and Infection by Ustilago striiformis or Urocystis agropyri on Leaf Turgor and Water Potentials of Kentucky Bluegrass¹

Plant tolerance to water stress can be measured, in part, by calculating the plants ability to maintain turgor during periods of water stress. Kentucky bluegrass (Poa pratensis) systemically infected by either Ustilago striiformis (Stripe smut) or Urocystis agropyri (Flag smut) exhibits greater mortality than noninfected plants during periods of water stress suggesting decreased drought tolerance with infection. The effects of water stress and systemic infection by the smut producing pathogens, Ustilago striiformis or Urocystis agropyri, were evaluated for their effects on leaf turgor and water potentials of Kentucky bluegrass ‘Merion’ grown in nutrient solution. Infection by either pathogen decreased leaf turgor and water potentials during light and dark periods as compared with noninfected controls. In addition, noninfected plants maintained higher leaf turgor and water potentials than infected plants as nutrient solution osmotic potentials were lowered with polyethylene glycol. Moisture release curves for single leaves from noninfected, U. striiformis-, and U. agropyri-infected plants were established to evaluate plants for osmotic adjustment by estimating leaf osmotic potentials and relative water contents at zero turgor as plants were grown in nutrient solution and nutrient solution amended with polyethylene glycol to increase water stress. Turgid weight/dry weight ratios also were calculated for noninfected and diseased leaves before and after water stress. Water stress lowered leaf osmotic potentials and relative water contents at zero turgor and decreased the turgid weight/dry weight ratios of noninfected and infected leaves. Noninfected plants exhibited lower osmotic potentials at zero turgor and maintained smaller turgid weight/dry weight ratios after water stress than infected plants, suggesting that infection by either pathogen inhibited osmotic adjustment and cell wall thickening in leaves of Kentucky bluegrass in response to water stress.

Key Words: Disease • Leaf osmotic potential • Moisture release curve • Nutrient solution • Osmotic adjustment • Polyethylene glycol • Pressure-volume technique • Turf

² Postdoctoral research associate, Agronomy and Soils Dep., Washington State Univ., WWREC, Puyallup, WA 98371; and professor of horticulture, agronomy, and plant pathology, Iowa State Univ., Ames, IA 50011.

Received for publication January 19, 1984.