Functions of Water in Corn Growth

Water, whether provided via rainfall or irrigation, is essential for corn production. In the Midwestern Corn Belt, a successful corn crop consumes approximately 25 acre-inches of water (680,000 gallons of water per acre) during its life cycle (Strachan and Jeschke, 2017). According to research at Iowa State University (Licht and Archontoulis, 2017), approximately 55-60% of this water (about 400,000 gallons per acre) transpires through the corn plant while the remainder evaporates from soil.

If the field yields 300 bushels per acre, corn plants transpire a little over 1,300 gallons of water for each bushel of grain. On a per plant basis, if the field population is 32,000 plants per acre, each corn plant transpires about 12.5 gallons of water between germination and maturity. If we also include the amount of water lost through evaporation from soil, each bushel of corn requires about 2,300 gallons (about 19,000 pounds) of water or a little over 21 gallons of water per corn plant. If we assume a 300 bushel per acre yield and a nitrogen conversion factor of 1.1 pounds of N per bushel of corn, the water to nitrogen use ratio is about 58:1 (19,000 pounds of water/330 pounds of nitrogen) (Table 1).

Although water is often viewed as a “resource”, corn producers may need to think of water more as a “nutrient” that should be managed. Climatologists are predicting more occurrences of extended periods of excessive rainfall and periods of dry and droughty conditions. Corn producers may need to adapt their water management programs to continue to produce corn under these more varied and stressful environments. A better understanding of what water does in the corn plant contributes toward making the correct decisions.

Water serves four main functions in the corn plant. These are:

1. Evaporative cooling to maintain plant temperature
2. Carrier for nutrient and sugar transport
3. Hydraulic force for cell growth, development, and expansion
4. Source of hydrogen for sugars, starches, and plant cell components

Temperature is a measure of the average speed of molecules in a system. The more heat that is applied to a system, the faster the molecules move, and the higher the temperature. As the faster-moving molecules escape from the system these molecules do two things – they extract heat from the system as they escape, and their leaving the system reduces the average speed of the molecules left behind in the system thus reducing the temperature.

Water has a tremendous ability to absorb heat. One gram of water removes 540 calories of heat energy as the water converts from liquid water to water vapor. Sunlight generates heat. Corn plants grow most rapidly at about 86°F (30°C). Their rate of growth slows dramatically as plant temperatures exceed 86°F (30°C). During those hot summer days, corn plants must transpire a lot of water to maintain optimal operating temperatures. As this water evaporates, the faster-moving liquid water molecules within stomatal enclosures convert to molecules of water vapor and escape into the atmosphere through stomatal openings (Figure 1).

Corn has a high capacity to exchange water and carbon dioxide with the atmosphere. There are approximately 36,000 stomates per square inch on the upper leaf surface and approximately 50,000 stomates per square inch on the lower leaf surface of a corn leaf (Dodd, 2020). As these molecules of water vapor exit through plant stomata, they remove heat from the system, reduce the average speed of water molecules remaining in the corn plant, and reduce the plant temperature (Figure 2).

Water carries and moves nutrients, sugars, and other plant products throughout the corn plant. How fast does water move in the corn xylem? There appears to be no literature reference to answer this question for corn. However, in trees, peak xylem velocity is about 10 to 30 inches per minute for trees with large xylem vessels and about 0.5 to 4 inches per minute for trees with small xylem vessels (Taiz et al., 2014). It is therefore reasonable to assume that maximum water velocity in corn xylem is likely in the range of 0.5 to 4 inches per minute. Nutrients that readily move with water could easily move from the corn root to the tassel or ear within a day. Nitrogen is a highly water-soluble nutrient. This explains why corn appears to “green up” relatively quickly after nitrogen fertilizer is applied as a side-dress treatment to emerged corn.

The driving force for water movement through the xylem is water evaporation through stomata. A corn’s vascular system permeates the entire corn plant, and many vascular bundles pass very closely to plant stomata (Figure 1). The vascular system consisting of xylem and phloem rapidly moves water and nutrients long distances in the corn plant. However, water movement from cell to cell is much slower because cellular membranes inhibit water movement.

Water in a plant cell behaves just like oil in a hydraulic cylinder. As the cell grows, the cell pulls in ionic nutrients, produces and consumes sugars, and generates many complex organic molecules and organelles during the growth process. All of these cellular components pull water into the cell through a myriad complex of ionic charge and hydrogen bonding interactions with water molecules. As water is pulled into the cell, this additional water creates hydraulic pressure that pushes outward against the cellular membrane and expands the membrane just like additional oil in a hydraulic cylinder pushes against the cylinder piston to extend the piston rod. Plant cells continue to expand until the call wall forms. The rigid cell wall defines the size and shape of a plant cell during the remainder of the plant life cycle. In dry or drought-stressed environments, less water is available to support cell growth and expansion. The consequence of this is small or severely stunted corn.

Hydrogen is an essential nutrient that comprises approximately six percent of the final corn weight. All of the hydrogen in a corn plant is derived from water. During photosynthesis, the water molecule (H₂O) is split to form hydrogen (H) and oxygen (O). The hydrogen atoms are first incorporated into simple sugars, and these sugars are subsequently modified and incorporated to form all of the organic molecules and cellular components in the plant. The corn plant uses some of the oxygen to support respiration, but most of the oxygen is released into the atmosphere as molecular O₂.

Water is essential for corn growth. Water: (1) helps to cool the corn plant to maintain temperatures supportive of rapid growth, (2) carries nutrients, sugars, and other essential molecules throughout the plant to support growth, (3) supplies the turgor pressure or hydraulic force for cell growth, development, and expansion, and (4) supplies hydrogen for incorporation into chemical compounds and cellular components.

A restriction in activity of any of these four processes reduces corn growth and grain yield. Water must therefore be managed. If excess water is present, this water must be rapidly removed because corn does not grow in flooded soil. Tiling fields and reducing tillage improves water permeation through soil. Reducing tillage allows soils to develop more structure and better retain naturally forming drain channels resulting from animal activity (for example: earthworms) and decaying plant roots. When water is limited, irrigation is often the first choice to supply water. For all corn producers, reducing water loss via evaporation from the soil surface also increases the amount of plant-available water. A management program that retains mulch or plant residue on the soil surface slows water loss via evaporation from the soil. Another management tool to retain water is to increase soil organic matter. Soil organic matter acts like a sponge in soil and can retain substantially more plant-available water than the soil mineral fraction.

• Dodd, J. 2020. Cornjournal.com Archives - February 25, 2020 - publication date.
• Heckman, J. R., J. T. Sims, D. B. Beegle, F. J. Coale, S. J. Herbert, T. W. Bruulsema, and W. J. Bamka. 2003. Nutrient removal rate by corn grain harvest. Agronomy J. 95:587-591.
• Latshaw, W. L. and E. C. Miller. 1924. Elemental composition of the corn plant. Journal of Agricultural Research 27:845-860.
• Licht, M. and S. Archonoulis. 2017. Corn water use and evapo-transpiration. Integrated Crop Management, June 26, 2017. Iowa State University extension, Ames, Iowa.
• Salisbury, F. B. and C. W. Ross. 1978. Plant Physiology 2nd ed. pp. 79- 92. Wadsworth Publishing Co., Belmont, California 94002.
• Strachan, S. D. and M. Jeschke. 2017. Water, soil nutrients, and corn grain yield. Crop Insights, 27 (7). Pioneer, Johnston, Iowa.
• Taiz, L., E. Zeiger, I. Møller, and A. Murphy. 2014. Plant Physiology 6th ed. Topic 4.3, Calculating velocities of water movement in the xylem and living cells. Oxford Press.