Comparative Water‐Use Rates and Efficiencies, Leaf Diffusive Resistances, and Stomatal Action of Healthy and Stripe‐Smutted Kentucky Bluegrass 1

Systemic infection by Ustilago striiformis (stripe smut) results in pathogen‐induced water stress of Kentucky bluegrass ( Poa pratensis L.). Such pathogen‐induced water stress may arise, in part, from increases in host transpiration due to decreases in cuticular resistance or an inhibition of stomatal movement. This study was conducted in controlled‐environment chambers to compare Kentucky bluegrass ‘Merion’ systemically infected by Ustilago striiformis with healthy plants for water‐use rate (WUR) and efficiency (WUE), leaf diffusive resistance (LDR), and critical leaf‐water potentials for stomatal closure. Leaves of infected plants with immature sori and no visible rupturing of the epidermis did not differ in WUR(mL) day −1 ) or WUE (g L −1 ) from healthy plants. Leaves of infected plants with mature sori with moderate to heavy sporulation and subsequent epidermal damage showed sharp decreases in WUR and WUE compared with healthy plants or infected leaves with immature sori and a visibly intact epidermis. No differences were observed in LDR between healthy plants and infected plants with mature or immature sori in the light. The LDR of infected plants in the dark was reduced only slightly by immature sori, but the presence of mature sori and subsequent epidermal rupturing lowered LDR to values similar to those exhibited by both healthy and infected plants in the light. Stomatal closure on infected leaves with immature sori occurred at lower leaf‐water potentials than on healthy leaves, suggesting a degree of osmotic adjustment in response to pathogen‐induced water stress.