Grain sorghum [Sorghum bicolor (L.) Moench. cvs. TX430 and KS82] was grown in a Haynie very fine sandy loam (coarse-silty, mixed, superactive, calcareous, mesic Mollic Udifluvents) under constant 47 % shade or full irradiance in a greenhouse under two watering regimes to see the combined and individual effects of low irradiance (LI) and low water (LW) on the sorghum genotypes. Under the high-irradiance (HI) and high-water (HW) treatment (control) and the LI-HW treatment, TX430 grew taller than KS82. Both LI and LW reduced several times the fresh and dry masses. Under the control conditions, TX430 reached its maximum net photosynthetic rate (PNmax) of 28.93 μmol m-2 s-1 at a photosynthetic photon flux density (PPFD) of 1 707 μmol m-2 s-1, and KS82 reached its PNmax of 28.32 μmol m-2 s-1 at a PPFD of 2 973 μmol m-2 s-1. The fact that TX430 had PNmax under a lower PPFD than KS82 may relate to its taller growth under LI conditions. Hence genotypes of sorghum might be selected for low irradiance using curves relating PN to PPFD. and Qingzhang Xu, M. B. Kirkham.