We evaluated the effects of photosynthetic photon flux (PPF) on optimal fertilizer concentration, water !use efficiency (WUE), and whole !plant gas exchange of subirrigated plants. Our results indicate that although WUE increased, optimal fertilizer solution concentration [expressed as electrical conductivity (EC)] of subirrigated plants did not vary with increasing PPF. The EC of the fertilizer solution in the optimal range was 0.65 to 1.71 dS . m -1 in wax begonia (Begonia semperflorens !cultorum Hort.) and 1.18 to >2.77 dS . m -1 for petunia (Petunia xhybrida Hort. Vilm-Andr.). When the growing medium contained a starter fertilizer, a low fertilizer EC (0.5 to 0.9 dS @m -1 ) was sufficient for growing wax begonias. Whole !plant gas exchange measurements indicated that growth rate (measured as the amount of carbon accumulated in plants per day) increased linearly with PPF, and at lower PPF treatments, growth rate of plants decreased due to a smaller fraction of carbohydrates available for growth and growth respiration after meeting the maintenance needs.