Reducing Plant Heat Stress with Irrigation, Nutrients and Microbes

How excessive heat affects plants

Increased plant water demand

The hotter the temperature, the more water plants must transpire to keep themselves cool. Extreme temperatures can outstrip the plant’s ability to take up water, translocate it to the leaves and respire in sufficient quantities. When water translocation is insufficient, leaf stomata close and photosynthesis stops.

Plant species express this shutdown in different ways, but in general they droop or cup their leaves due to loss of cell turgor pressure. Transpiration shutdown typically occurs during the hottest afternoon hours. These hours are then lost to plant productivity, as the photosynthetic process is interrupted. This is like a plant holding its breath.

If you have a healthy plant with adequate available water, you will lose fewer of these plant productivity hours and avoid these direct impacts to crop quality and yield.

Rapidly changing effective plant moisture availability

Along with increased transpiration demand, there are also increased irrigation losses to surface evaporation. Plants pull shallow moisture more easily than water from deep roots.

Trees and vines waterlogged from the winter and spring rains are additionally disadvantaged: deep feeder roots and root hairs, which do most of the water collection, suffered necrosis from anaerobic conditions and will not pull deep water as efficiently.

Shallow root water mining efficiency can also be compromised by high temperatures — shallow roots can shut down and suffer necrosis in the heat. Be mindful of both constraints when evaluating your effective soil water availability during peak heat stress periods.

Reduced photosynthesis

Heat increases the formation of reactive oxidative species (ROS) in the plant. These chemicals reduce the efficiency of several photosynthesis-related transactions. Transpiration shutdown also reduces photosynthesis, as explained above. Protein synthesis is also reduced or inhibited by these conditions.

Photosynthesis that does occur during stomatal shutdown contributes to the accumulation of ammonium forms of nitrogen in the plant tissues, which encourages infestations of mite and thrips pests.

Premature ripening and senescence hormones

In addition to disrupting photosynthetic efficiency, ROS can also trigger formation of ripening- and senescence-related hormones such as abscisic acid and ethylene. These premature ripening hormones can cause abortion of bloom and young fruit, shatter, premature hull split, and a premature shift from fruit sizing into fruit maturation.

Extreme heat and transpiration shutdown of the fruit can also change the forms of acid from vegetative to ripening forms, and cause vascular system collapse and necrosis. Once the fruit starts to grow again after the shutdown, it may not be able to shift sufficient nutrients to prevent other fruit quality impacts, such as calcium deficiency triggering blossom end rot.

Four ways to reduce plant heat stress

1. 1

   Maintain adequate, freely available soil moisture

   Stay on the free-release side of your soil moisture release curves, and make sure the water is where the roots are. Adjust your irrigation schedule to more frequent irrigations with shorter durations, and factor in the increased water loss to surface evaporation. More frequent irrigation ensures there is more “easy” water for the plant to mine and helps protect shallow roots from cooking. If you have different water sources, use your best-quality water.

2. 2

   Avoid key minor nutrient deficiencies

   Nutrient imbalances can negatively impact water translocation, stomatal efficiency and cell wall turgor. Several nutrients are key for efficient translocation and transpiration; the most important for heat stress management are calcium, magnesium and silica. By avoiding these deficiencies, you optimize transpiration capacity and build strong plant cell walls that are more resistant to desiccation, osmotic pressure and electrolyte leakage.

3. 3

   Avoid foliar sprays

   Most foliar sprays increase leaf cell osmotic pressure stress. This is generally not a problem under normal conditions, but it can contribute additional stress under extreme heat, even when the foliar is applied at the cool time of day. This is particularly true of pesticides, which often stun the plant for a couple of days even under favorable conditions. Avoid stacking additional sources of stress on top of extreme heat — the combination of stressors is much worse than just additive.

4. 4

   Build healthy soils

   Healthy soils reduce all abiotic stressors (heat, drought, salt pressure, cold, pest, disease). Plants use microbial metabolites for stressor responses and to moderate the premature expression of fruit ripening and senescence hormones. That improved stress tolerance — and the resulting reduction in lost photosynthetic hours — partly explains MetaGrow performance. Our microbial programs result in higher fruit brix and earlier ripening. Healthy soils produce plants that are less vulnerable to stress and therefore less impacted by extreme heat.

SGS recommendation

Adjust your irrigation program, address nutrient imbalances that impact plant respiration, and build healthy soils that improve plant resiliency under extreme heat conditions.