Irrigation Water Management

Soil-Water-Plant Relationships

1-Plant Response to Water

Typical Plant Production Budget

Plants use lots of water, but few of us really understand why this water is needed. Here is a typical production budget for an agricultural crop:

Typical Plant Production Budget (relative units) Water Taken In    10,021 Water 10,000 Carbon Dioxide 20 Minerals 1 Matter Given Off    9,921 Transpired Water 9,910 Oxygen 11 Matter Retained in Plant   100 Dry Matter 10 Water 90

Most of the water taken in is just given off again - its not used or stored in the plant. What's going on here? Why does the plant take in water it doesn't use?

The answer is in the photosynthetic process.

Photosynthesis - Most Plants

Using chlorophyll and sunlight, green plants produce sugar, which serves as fuel for growing the plant and producing the parts we harvest. The chemical reaction goes like this:

6 H2O + 6 CO2 --[with sunlight & chlorophyll]--> C6H12 + 9 O2

The need for carbon dioxide [CO2] is the key to a plant's water needs.

In the underside on plant leaves (and for some plants on the top side too) are small openings called "stomata". There are millions of stomata on the leaves of agricultural plants. The following table shows the number of stomata in one square inch of leaf for the crops listed.

Stomata per square inch of leaf surface Crop Upper Leaf Surface Lower Leaf Surface Total Stomata  Orange  0  290,000  290,000  Apple  0  250,000  250,000  Pumpkin  18,000  175,000  193,000 Bean  26,000  160,000  186,000  Sunflower  55,000  100,000  155,000  Corn  39,000  64,000  103,000

During daylight, if the plants are not wilted, these stomata are open, allowing carbon dioxide from the air to enter the leaf. This provides the necessary ingredient for photosynthesis. Unfortunately, as carbon dioxide is entering, water vapor is leaving out these same openings. This water vapor is known as "transpiration." Without open stomata, which unfortunately results in transpiration, the plants couldn't get the carbon dioxide they need for production. If the plant ever runs out of water, it wilts, closing the stomata as a water conservation measure. This stops the water loss, but also stops the production, since carbon dioxide can no longer enter the leaf.

For a healthy, well watered plant, the stomata are open during daylight hours. Thus the water used ("lost") by transpiration is tremendous. Tabulated below is the Water Ratio for several agricultural crops. This numerical value of this ratio is the number of pounds of water required to produce one pound of harvested crop material. As we have learned before, a tremendous amount of water is needed to produce our food and fiber.

Water ratio for selected agricultural crops: Pounds of water required to produce a pound of harvested material Crop Water Ratio ------------------------------------- Wheat, grain only 1,325 Oats, grain only 1,250 Sorghum, grain only 1,250 Corn, grain only 865 Alfalfa, dry matter 847 Soybeans, dry matter 646 Potato, dry matter 600 Oats, dry matter 593 Cotton, dry matter 562 Barley, dry matter 525 Sugar beets, roots only 523 Wheat, dry matter 513 Millet, grain only 500 Sugar beets, dry matter 385 Corn, dry matter 357 Millet, dry matter 296 Sorghum, dry matter 284

In one sense, then, more transpiration means more yield - the more the stomata are open, the more photosynthesis can take place. In fact, researchers have found a strong relationship between crop ET and crop yield. Relative ET, that is, actual crop ET divided by potential crop ET is related to, and sometimes directly proportional to, relative yield (actual yield divided by potential yield).

Relative ET and relative yields can be related to soil moisture conditions, as shown in this graph:

The Unusual Pineapple

The pineapple and many cactus varieties have evolved as exceptions to the above. The pineapple is one of the most water-efficient producing agricultural crops. Pineapple stomata open at night! There is transpiration then too, but it is usually much lower than during the day time. Carbon dioxide enters the leaves at night, and is stored chemically by the plant. Then the next day, the stomata close, reducing transpiration, and the carbon dioxide is released from chemical storage in the presence of sunlight when it can be used in photosynthesis.