Figure 1. Corn showing the effects of extreme heat and drought stress in central Iowa in 2012.
Figure 2. Vapor pressure for water by relative humidity and temperature. As temperature rises, the difference in vapor pressure between the interior of plant leaves and the ambient air increases.
Vapor Pressure Deficit vs. Relative Humidity
How does VPD differ from relative humidity?
Relative humidity refers to the amount of water vapor in the air versus what it can hold; however, the amount of water air can hold varies with temperature.
If you think of the atmosphere as a container holding water, that container gets bigger as temperature increases so it takes more water to fill it.
Vapor pressure deficit is a more accurate way of expressing the evaporative demand the atmosphere exerts on plants.
Figure 3. Stomatal pores and stomatal chambers. Stomatal pores allow for the exchange of water and CO2 between the atmosphere and leaf internal structures. Stomatal chambers serve as locations where liquid water converts to water vapor and escapes into the atmosphere. There are approximately 36,000 stomates/in2 on the upper surface and 50,000 stomates/in2 on the lower surface of a corn leaf
Figure 4. Air temperature, atmospheric vapor pressure deficit, and leaf photosynthetic rate in irrigated corn over the course of a day (Hirasawa and Hsiao, 1999).
Figure 5. Leaf photosynthetic rate by time of day for irrigated and non-irrigated corn (Hirasawa and Hsiao, 1999).
Hirasawa, T. and T.C. Hsiao. 1999. Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field. Field Crops Research 62:53-62; Lobell, D.B., G.L. Hammer, G. McLean, C. Messina, M.J. Roberts, and W. Schlenker. 2013. The critical role of extreme heat for maize production in the United States. Nature Climate Change 3:497-501; Roberts, M.J., W. Schlenker, and J. Eyer. 2013. Agronomic weather measures in econometric models of crop yield with implications for climate change. Am. J. Agricult. Econom. 95:236-243. The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.
Vol. 15 No. 5 July 2023 Author: Mark Jeschke