Abstract A pair of field-portable, humidity-controlled, whole-plant enclosures were used to study the effect of increased transpiration rates (TS) on whole plant photosynthesis (PS). Four experiments are reported: two on highly fertilized, well-watered plants grown in sandy soil, and two on plants grown hydroponically out of doors. Photosynthesis in the hydroponically grown plants was less sensitive to changes in TS than that of the soil-grown plants, even though soil water potentials never dropped below −30 kPa during the experiments. Regression analysis indicated that a 1 g dm −2 hr −1 increase in TS resulted in a 2 mg dm −2 hr −1 decrease in PS in the soil-grown plants, while a similar increase in TS in the hydroponically grown plants resulted in only a 0.9 mg dm −2 hr −1 decrease in PS. It is postulated that reductions in PS during periods of high TS may be important in determining the final yield of plants grown in drier soils.