This study examined the developmental and diurnal variation of transpiration and Hill reaction activity in the leaf lamina of Rumex maritimus infected with the smut fungus Ustilago parletoreii. Transpiration rates varied with developmental stage and time of day. The pre-sporulation stage showed the highest morning transpiration (24.9 ± 0.1 mg cm⁻² hr⁻¹ at 9:00 A.M., increasing to 27.0 ± 0.1 mg cm⁻² hr⁻¹ at 11:00 A.M.), declining at noon (14.0 ± 0.2 mg cm⁻² hr⁻¹). Pre-flowering and flowering stages had moderate rates, while young- and mature-sporulation stages remained low throughout (6.5–14.5 mg cm⁻² hr⁻¹).

Hill reaction activity peaked during flowering (0.88 ± 0.02 µg DCPIP hr⁻¹ g⁻¹), followed by pre-flowering (0.70 ± 0.03) and pre-sporulation (0.69 ± 0.02), with lower values in later sporulation stages (0.54–0.40). These results suggest that fungal infection influences both water loss and photosynthetic efficiency, with maximal physiological activity occurring before sporulation, reflecting interactions between developmental stage, pathogen, and diurnal environmental factors.