title: A vapor pressure deficit effect on crop canopy photosynthesis.
abstract: Canopy CO2-exchange rates (CER), air temperatures, and dew points were measured throughout ten days during the 1987 growing season for cotton (Gossypium hirsutum L.), grain sorghum [Sorghum bicolor (L) Moench], and five soybean [Glycine max (L) Merr.] cultivars, and throughout seven days in 1988, on maize (Zea maize L.). The objective was to determine if the decline in CER per unit light during the afternoon is associated with a vapor pressure deficit (VPD) increase. Some of the soybean and maize plots were kept as dry as possible. A VPD term significantly contributed (P≤0.05) to a canopy CER regression model in 54 of 80 data sets in 1987. Grain sorghum was less sensitive than the well-watered soybean genotypes to an increasing VPD (P≤0.05) on three of the ten measurement days and less sensitive than cotton (P≤0.05) on only one day. Cotton demonstrated less VPD sensitivity than soybean (P≤0.05) on one day. The moisture stressed soybean plots showed a greater CER sensitivity to VPD (P≤0.05) than the well-watered soybean plots. In 1988, the frequently irrigated maize plots were less sensitive to VPD (P≤0.05) than the rain-fed plots early in the season, before the rain-fed plots were excessively damaged by moisture stress. These results indicate that the afternoon declines in canopy CER found in a number of different species are associated with increases in the VPD; recent work of others suggests that this may be due to partial stomatal closure.
authors.affiliation: USDA-ARS, P.O. Box 345, 38776, Stoneville, MS, U.S.A..
authors.forename: W T
authors.initials: WT
authors.lastname: Pettigrew
authors.forename: J D
authors.initials: JD
authors.lastname: Hesketh
authors.forename: D B
authors.initials: DB
authors.lastname: Peters
authors.forename: J T
authors.initials: JT
authors.lastname: Woolley