CANOPY GAS-EXCHANGE RESPONSE TO MOISTURE STRESS IN OLD AND NEW MAIZE HYBRID

Grain yield improvement of maize (Zea mays L.) hybrids in Ontario has been associated with increased stress tolerance. Effects of shortterm water-deficit stress on canopy gas exchange of an old and a new maize hybrid were studied. Plants of Pride 5 (old) and Pioneer 3902 (new) we re grown under controlled-environment conditions until tassel emergenc e and, subsequently, plants were transferred to enclosures, each conta ining the aboveground parts of four plants. Canopy photosynthesis, res piration, transpiration, and stem water potential were measured contin uously for 7 d. Plants were either well watered or exposed to a water- deficit stress by withholding water supply until net canopy photosynth esis declined to zero and, subsequently, plants were rehydrated. Under well-watered conditions, the two hybrids did not differ significantly in terms of canopy photosynthesis, transpiration, and stem water pote ntial. In contrast, hybrids responded differently to a water-deficit s tress. Cumulative canopy photosynthesis and transpiration during the d rying cycle were reduced by 21 and 31%, respectively, in Pioneer 3902 and by 34 and 31%, respectively, in Pride 5. The threshold stem water potential related stomatal closure, and thus, photosynthesis and trans piration, was lower for Pioneer 3902 than for Pride 5. During the reco very day, canopy photosynthesis was 53% higher and canopy transpiratio n was 31% higher in Pioneer 3902 than in Pride 5. Respiration per unit CO2 fixed was lower in Pioneer 3902 than in Pride 5 in all conditions .