Plant's Natural Tolerance to Environmental Stress and How to Improve it!

We all know how hard it is to maintain optimal temperatures in the middle of the summer. Once the temperatures start creeping into the mid-80’s and beyond, plants often begin to suffer… and so do we! Although it’s not always possible to prevent heat stress, there are a few simple steps that can be taken to improve your plant’s tolerance to stress before the heat wave arrives. It’s all about conditioning. If your plants have a robust root system, strong cell walls and an extra reserve of anti-oxidants, they will be better prepared to handle stress when the summer heat arrives.

Benefits of Stress on Plants

Stress isn’t all bad. In fact, when plants are subject to moderate stress, they often produce better colors and aromas, and the vitamin content of the fruit improves. That’s why hydroponic tomato growers often raise the EC of the nutrient solution during the fruiting and flowering stage. The “saltiness” at the root zone makes it harder for the plant to take up water, so sugars and organic acids condense in the fruit. By manipulating EC, it is possible to double the lycopene content (red coloring) of tomatoes, and increase vitamin C content by up to 50%. There is also a direct proportional relationship between EC and sugar content in the fruit: the higher the EC, the sweeter the tomatoes. Also, by gradually increasing EC, the plants will begin to accumulate more sugars and dissolved solutes in their roots, making them more tolerant to the effects of future salt stress.

So a little stress is good, but too much stress will put you in the hospital! If the plant experiences too much heat or salt stress, it can’t take up enough water to meet its basic needs. The edges of the leaves will start to curl back and turn brown, and plants may start to show signs of calcium deficiency such as tip burn in lettuce or blossom end rot in tomatoes. High heat and humidity makes the problem worse. So as the temperature increases above optimal levels, it is important to begin to dilute the EC of the nutrient solution to make it easier for the plant to take up water and nutrients. It is also important to have plenty of air movement in the garden so that plants can continue to transpire and naturally cool themselves.

Biostimulants & Microorganisms Reducing Plant Stress

Plants have the uncanny ability to produce their own plant protection agents and cope with a wide variety of environmental stressors. But sometimes plants need a little help. In nature, plant-growth-promoting microorganisms in the root zone make compounds that improve the plant’s tolerance to diseases and stimulate the plant to become more resistant to environmental stress. By studying the interactions between plants and their microbial guests, plant scientists are beginning to learn how to harness the power of natural biostimulants to provide new and improved plant protection agents. Among the most promising organic biostimulants are seaweed extracts, humic acids and l-amino acids.

Virginia Tech did a ten-year study on biostimulants and they found that a combination of seaweed extracts and humic acids had a profound effect on the stress tolerance of plants. Seaweed extracts are rich in natural plant hormones called cytokinins. Cytokinins stimulate cell division. So when applied to the root zone, seaweed extracts stimulate increased lateral root growth and root mass, providing a healthy root mat for the uptake of water and nutrients. If a gardener can help plants develop greater root mass before the plant begins to experience stress, the plant will be much better prepared to efficiently take up water when the summer heat arrives.

The scientists at Virginia Tech discovered even greater benefits when seaweed extracts were combined with humic acid. The best combination was 5 parts humic acid to 2 parts seaweed extracts (5:2 ratio). The combination stimulated the plants to produce 50% more of a powerful plant protection agent known as superoxide dismutase (SOD). SOD neutralizes harmful free radicals before they can accumulate in plant cells and do damage to cellular membranes. Under normal conditions plants produce all of the SOD that they need, but during times of excessive stress plants can’t keep up. The extra SOD levels stimulated by humic acid and seaweed extracts protect the plants against damaging free radicals, and help keep the plants green during the summer heat.

Humic Acid and Fulvic Acid Increase Iron Uptake

Humic and fulvic acids are also rich in beneficial trace elements, and they are particularly good at helping the plant take up iron. During times of stress, plants need iron and other trace metals to activate important enzymes. For example, SOD is activated by either a zinc/copper complex or an iron/manganese complex. If the trace elements are not available, the enzymes are turned “off” and they won’t protect the plant. But if the enzymes are turned “on”, a single molecule of SOD can perform over 1000 chemical reactions per second in the cell! That’s a lot of plant protection from a very small amount of humic acid.

Seaweed extracts/humic acids also contain vitamins and amino acids that can help strengthen the plant and provide an additional level of plant protection. Amino acids are intermediate chelators. In other words, amino acids help keep trace metals soluble and available to the plant. In addition, some amino acids also dramatically improve the uptake of calcium. Certain amino acids, such as glutamic acid and glycine, stimulate root cells to open up calcium ion channels, allowing calcium to be taken up many times faster than simple osmosis. Calcium forms the glue that glues cell walls together, strengthening the plant tissues against temperature stress. So when temperatures and humidity start to rise, the enhanced calcium availability provided by amino acids will continue to have a protective effect on the plant.

9 Tips to Prevent Heat Stress in Plants

To grow healthy plants, gardeners should pay close attention to both plant nutrition and the growing environment. So here are a few practical tips to help prepare plants to beat the heat:

1. Supplement plants with a 5:2 ratio of humic acid and kelp extracts. The combination works 50% better than either product alone.
2. Provide good drainage. Stagnant water can cause root rot which can ultimately reduce the uptake of water and nutrients.
3. Maintain nutrient temperatures between 68-75 degrees. As water temperature increases, dissolved oxygen in the water decreases. Plants need plenty of oxygen for healthy roots.
4. Avoid large swings in temperature and humidity. Try to allow no more than a 10 degree difference between day and night temperatures.
5. Provide adequate air movement. All of the leaves in the garden should be moving gently to keep the transpiration stream flowing.
6. Don’t let pH drift too high. As pH becomes more alkaline (above 6.5), iron and other trace metals start to become unavailable to the plant.
7. Provide carbon dioxide supplementation. Higher CO2 levels allow plants to grow at higher temperatures (up to 5 degrees higher) without negative side effects.
8. Don’t apply too much nitrate-nitrogen. Excessive nitrates promote lush top growth, but restrict root growth. Plants become more susceptible to heat stress.
9. Use silica supplements. Silica strengthens plants and helps alleviate heat stress.

Remember, healthy plants are naturally resistant to stress and disease. There are, of course, limits to the amount of heat stress that even healthy plants can tolerate, so we should do everything that we can to provide the best environment possible. But if a plant has a strong root system, thick cell walls, abundant trace minerals, and a reserve of antioxidants and plant protection agents, it will be better prepared to beat the heat. Don’t wait until it’s too late! Give your plants a boost with kelp and humic acid before the summer comes on strong.

By Harley Smith