How Does Adding Salt to a Bean Plant Affect the Growth?

Overview

Every year, commercial farmers in the United States plant about 2 million acres of bean plants. Beans are a popular backyard crop in home gardens as well. They're nutritious, low in carbohydrates, and can be eaten fresh or dried and stored without refrigeration almost indefinitely. Because they're are such an important crop, much research has been done on beans and how they're affected by different environmental conditions, including soil and water salinity.

Salt and Bean Plants

Salt accumulation in cultivated soil is a big problem for the agriculture industry. Up to half of all irrigated farmland in the U.S. suffers some form of salt build-up serious enough to affect yields. The two forms of salt most prevalent in soil are sodium chloride (table salt) and sodium sulfate. Salts are naturally in soil and groundwater, but reach toxic levels over time as other minerals are taken up by crops and removed, leaving the salts behind. Bean plants are especially sensitive to salt.

Beans and Sodium

Salts readily dissolve in water. Sodium separates in water with a positively-charged ion that is chemically close to potassium (K). Cellular sites in bean plants need potassium, but instead bind to the sodium, causing nutrient deficiencies in the plant.

Beans and Water Scarcity

Salt also increases osmotic pressure in the cells of bean plant roots that drive water out of the plant. The bean plant responds to the dehydration by closing little holes in its leaves, called stomata, that allow for gas exchange inside the plant, and which also release water vapor. Closing the stomata slows this rate of water loss through transpiration. Beans also ramp up production of an osmolyte called proline, which stabilizes cell membranes and prevents water from passing through the the roots by osmosis to where the salt is.

How Salt Affects Photosynthesis

These protective measures on the part of the bean plant against salt come at a price. With stomata closed, the plant can't absorb carbon dioxide. Salt also causes the plant to lose water. In photosynthesis, plants convert water and carbon dioxide into glucose and oxygen. The sugar serves as food energy for the plant.

How Beans Are Affected by Salt

Water pressure in plant cells makes their membranes rigid and able to hold up the plant. This is especially true for a long, thin-stemmed plant such as a pole bean. Water loss due to salt in the soil causes beans to shrivel dramatically. As photosynthesis is affected and the plant is robbed of its food source, bean plants break down their stores of the photoreceptive molecules called chlorophyll, which causes the plant to yellow in appearance. The nutrient loss from potassium displacement by sodium stunts the plant's growth as well. The plant is further robbed of resources by the need to make proline to try to fight off water loss. All of these affects are directly proportional to the salt concentration in the soil. If the salt problem isn't rectified, the plant will yield less harvest, or could die.