Canopy Gas Exchange Response to Moisture Stress in Old and New Maize Hybrid

Grain yield improvemenotf 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) were grown under controlled‐environment conditions until tassel emergence and, subsequently, plants were transferred to enclosures, each containing the aboveground parts of four plants. Canopy photosynthesis, respiration, transpiration, and stem water potential were measured continuously for 7 d. Plants were either well watered or exposed to a water‐deficit stress by withholding water supply until net canopy photosynthesis 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 potential. In contrast, hybrids responded differently to a water‐deficit stress. Cumulative canopy photosynthesis and transpiration during the drying 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 transpiration, was lower for Pioneer 3902 than for Pride 5. During the recovery day, canopy photosynthesis was 53% higher and canopy transpiration was 31% higher in Pioneer 3902 than in Pride 5. Respiration per unit CO 2 fixed was lower in Pioneer 3902 than in Pride 5 in all conditions.