Several microclimate and plant variables of field grown wheat plants, subjected to 2 irrigation regimes, were monitored to observe their combined effect on chamber measurements of AP . Dirunal AP and evapotranspiration (ET) measurements were taken on Olaf spring wheat (T. aestivum L.) on 5 days from heading through late milk growth stages. Plants were irrigated either weekly (W) or biweekly (BW). A sequential analysis of covariance of the total data set showed ATP was significantly associated with flag leaf stomatal resistance and water potential, canopy air temperature, leaf area index (LAI), irradiance, date and time of day. The consistently higher AP rates of treatment W were attributed mostly to higher values of LAI and water potential, and lower stomatal resistances, compared to treatment BW. Although reductions in leaf area and increases in stomatal resistance resulted in less ET for treatment BW, AP was reduced proportionately more by the effects of biweekly irrigation. Thus, water use efficiency was always greater for W treatment. Maximum AP rates always occurred in the morning and earlier than maximum rates of ET. This observation was attributed partly to warmer afternoon temperatures, which increased ET and, in combination with lowered leaf water potentials, decreased AP. As a result, photosynthetic capacity was always greater in the morning than the afternoon. However, time of day still affected AP significantly after accounting for all of the variability associated with these and other measured variables which were known to affect AP. Based on measurements taken for both treatments, stomata appeared to be generally more open after than before anthesis. On the final day of measurements, stomata of treatment BW were closing, apparently in response to leaf aging.