oil
moisture deficits have been a concern for our neighbors in
Illinois and parts of Ohio since earlier in June. Most of Indiana's
corn crop was spared until recently due to more generous spring
rainfall plus a good shot of rain from Tropical Storm Arlene in
mid-June. Within the past week or so, however, the consequences
of mounting soil moisture shortages are becoming increasingly
evident in cornfields throughout Indiana, primarily in the form of
dramatic leaf rolling during daytime hours.
Top soil and subsoil moisture estimates reported by the Indiana Ag. Statistics Service declined sharply in today's report (6/27/05); only 35% and 54% Adequate to Surplus, respectively, contrasted to last week's estimates of 80% and 78% Adequate to Surplus. Another indicator of the worsening soil moisture deficits was the decrease in crop condition rated Good to Excellent from 66% of the corn acres last week to 56% reported today (6/27/05). Last year at this time, 73% of the state's corn crop was rated Good to Excellent.
Given that leaf rolling is an early symptom of drought stress, how does one assess the yield consequences once it appears in a field? An accurate estimate is difficult to give, but we can identify the high risk situations. Obviously, yield losses are more likely the more hours of a day and the more consecutive days that leaves are rolled tightly because of the overall reduction in photosynthetic energy capture and carbon fixation. Yield losses are also more likely when severe drought stress occurs shortly before, during, and shortly after pollination than at any other time of the growing season.
The most commonly cited data on expected yield loss due to severe drought stress
at varying growth stages come from a summary of five drought stress studies
(Shaw, 1988). 
Those
data suggest potential yield losses range from 2 to 4 percent per day during
the four weeks preceding pollination, 3 to 8 percent per day during pollination,
and 3 to 5 percent per day during the four weeks following pollination. These
estimates need to be tempered with the fact that overall hybrid tolerance to
stress in general, including drought, has greatly improved during the past 15
to 20 years. The effects of drought stress on today’s hybrids, while severe,
are undoubtedly much less than for hybrids used many years ago.
The effects of extended periods of drought stress and reduced photosynthesis prior to pollination include shorter plants, smaller leaves, and smaller potential ears (especially ear length or kernels per row). Plants are already noticeably stunted in the more severely stressed areas of many fields.
The effects of severe drought stress during pollination include the hastened onset and shortened duration of pollen shed, delayed silk emergence, and/or desiccated silks not receptive to pollen germination. The effect of drought stress and reduced photosynthesis shortly after pollination is primarily in the form of the higher risk of abortion of the young developing kernels.
Leaf rolling typically occurs first in fields where soil moisture or root development is already restricted: e.g., sandy soils, field edges near tree lines, end rows (compacted soil), other areas of compacted soil, nutrient deficient areas, nematode-damaged areas, rootworm-damaged areas, and areas of severe weed pressure. As drought conditions worsen, leaf rolling eventually spreads throughout entire fields.
 Leaf rolling occurring in sandy soil, but not heavier
textured soil. |
 Closer view of leaf rolling. |
 Even closer view of leaf rolling. |
 Very sparse crop canopy due to severe leaf rolling.
|
 More dense crop canopy of healthier plants. |
 Very sparse crop canopy due to severe leaf rolling.
|
 Severe leaf rolling throughout entire field. |
 More severe leaf rolling in compacted end rows of
field. |
Hybrids often vary for severity of leaf rolling. Side-by-side in the same field, one hybrid may exhibit severe leaf rolling while the other shows no such effects. Which is most affected by the soil moisture deficit? Some argue that the hybrid that rolls its leaves earlier may be better off because of lower transpiration rates. Others argue that the hybrid that doesn’t roll its leaves as easily may simply be more drought tolerant. In practice, the leaf-rolling characteristic is only one of several drought tolerance tactics breeders can exploit (Bänzinger et al., 2000).
Research cited by Bänzinger et al. (2000) suggests that leaf rolling is not simply a form of wilting, but rather is a response to a growth regulating hormone (absciscic acid or ABA) produced in the roots and translocated to the leaves during the early stages of soil moisture deficits before leaf turgor loss and wilting actually occur. As such, the initial onset of leaf rolling may well be a protective mechanism to minimize excessive transpiration.
An associated response to ABA transport to the leaves during the early stages of drought stress is the closure of the leaf stomata (openings in the leaves that allow carbon dioxide into the leaf for photosynthesis and water out of the leaf for transpiration). Closing the stomata helps reduce the transpiration rate of the leaves and temper the loss of leaf cell turgor pressure, but occurs at the cost of lower rates of photosynthesis (carbohydrate production or carbon fixation) due to restricted carbon dioxide movement into the leaves.
As severe heat and drought stress continues, lower transpiration rates due to leaf rolling and stomata closure can unfortunately increase leaf temperatures to potentially damaging levels that may eventually result in leaf death. Low plant turgor pressure also limits cell expansion and, over extended periods of drought stress, can result in smaller leaves and shorter stalk internodes (i.e., a smaller photosynthetic factory).
Bänzinger, M, G.O. Edmeades, D. Beck, and M. Bellon. 2000. Breeding for drought and nitrogen stress tolerance in maize. From theory to practice. [Online] Mexico D.F.: CIMMYT. Available at http://www.cimmyt.org/Research/Maize/map/guides_tools/BredDrougMaize/BredDroug.htm [URL verified 6/25/05].
Indiana Ag. Statistics Service. 27 June 2005. Indiana Crop & Weather Report. United States Dept. of Agriculture. Available online at http://www.nass.usda.gov/in/cropweat/2005/we2605.pdf [URL verified 6/27/05].
Lauer, Joe. 2003. What Happens Within The Corn Plant When Drought Occurs? Wisconsin Crop Manager. Univ. of Wisconsin. Available online at http://corn.agronomy.wisc.edu/WCM/2003/W137.htm [URL verified 6/27/05].
Nafziger, Emerson. 2005a. Is it drought yet? Illinois Pest & Crop Bulletin. Univ. of Illinois. Available online at http://www.ipm.uiuc.edu/bulletin/contents.php?issueNumber=11&issueYear=2005 [URL verified 6/27/05].
Nafziger, Emerson. 2005b. Hanging on. Illinois Pest & Crop Bulletin. Univ. of Illinois. Available online at http://www.ipm.uiuc.edu/bulletin/contents.php?issueNumber=14&issueYear=2005 [URL verified 6/27/05].
Nielsen, R.L. (Bob). 2005. Ear size determination in corn. Corny News Network, Purdue Univ. Available online at http://www.agry.purdue.edu/ext/corn/news/articles.05/EarSize-0523.html [URL verified 6/27/05].
Shaw, Robert H. 1988. Climate Requirement. In Corn and Corn Improvement.(3rd Ed., ASA Monograph No. 18) G.F. Sprague and J.W. Dudley, editors. American Soc. of Agronomy.