Mitigating heat stress

Our special feature continues with a look at cool‑season turfgrasses. They are highly vulnerable to heat stress, leading to yellowing and tissue decline. Their survival depends on complex cellular and physiological heat‑tolerance responses.

Heat stress is a constant challenge for turf managers, especially with cool‑season species like bluegrass, ryegrass, bentgrass, poa, and fescue, which are prone to yellowing and tissue decline during prolonged high temperatures. Their heat tolerance depends on a range of cellular, molecular, metabolic, and physiological responses.

Causes of heat stress

Protein Degradation - As turf plants age, proteins are broken down through proteolysis, where protease enzymes convert them into amino acids for redistribution. Heat stress accelerates senescence in cool‑season turf, reducing protein synthesis and increasing protein degradation. This leads to the loss of essential proteins, including enzymes involved in photosynthesis. Rubisco, one of the most abundant plant enzymes and essential for the Calvin cycle, is particularly vulnerable and is degraded under heat stress, contributing to early decline.

Carbohydrate Metabolism - Carbohydrates produced during photosynthesis fuel respiration and growth. Under heat stress, respiration rates increase, often exceeding the plant’s ability to produce carbohydrates - especially at low mowing heights. This imbalance weakens the plant and reduces its ability to recover.

Photosynthesis - Photosynthesis drives carbohydrate production, so any disruption affects turf quality and performance. Heat stress reduces chlorophyll content, leading to chlorosis. This may be due to reduced chlorophyll synthesis or accelerated breakdown, both of which impair light capture and carbon fixation.

Antioxidant Metabolism - Heat‑induced reductions in photosynthesis can cause reactive oxygen species (ROS) to accumulate. These damage cell membranes through lipid peroxidation, leading to oxidative stress and premature tissue death.

Lipid Metabolism - Lipids are vital for membrane structure, energy storage and signalling. Their role in maintaining membrane stability is closely linked to heat tolerance in cool‑season turfgrasses.

Nutritional measures to mitigate heat stress

Recent research has highlighted several nutritional and biochemical tools that can help cool‑season turf withstand heat stress.

Calcium

• Cellular structure: Strengthens cell walls, particularly outer cell walls, providing structural integrity to resist heat-induced damage and physical stress.
• Stomatal regulation: Helps control the opening and closing of guard cells, managing water loss during heat.
• Signal transduction: Acts as a key signal, triggering plant defenses and activating heat-shock proteins.
• Antioxidant defence: Reduces the harmful effects of reactive oxygen species (ROS) by boosting antioxidant activity and decreasing lipid peroxidation in membranes.
• Membrane stability: Maintains healthy cell membranes, crucial for proper protein function and nutrient regulation under heat.

Potassium

• Water regulation (Stomata): Potassium controls the opening and closing of stomata (pores on leaves) to manage transpiration (cooling), preventing excessive water loss in heat.
• Cell wall strength: It boosts cellulose production, making cell walls stronger and more rigid, which helps maintain turgor pressure (keeping blades upright) and reduces wilting and sun scorch.
• Energy and metabolism: Plants burn more energy (potassium) during high heat; K helps with respiration and carbohydrate production, providing energy for stress response systems.
• Nutrient Balance: K helps regulate nutrient flow and can fight off sodium accumulation from irrigation, preventing sodium-induced wilting.

Silicon

• Strengthens cells: Silicon deposits in cell walls and cuticles make them tougher, protecting against heat damage and physical stress.
• Improves hydration: It helps plants manage water better, reducing water loss (transpiration) and keeping cells turgid, preventing wilting.
• Cools leaves: Silicon-infused epidermal structures can help dissipate heat, acting as a physical cooling mechanism.
• Boosts antioxidants: It activates the plant’s natural defence system, increasing antioxidant enzymes (like SOD, catalase) to neutralise heat-induced oxidative stress.
• Enhances photosynthesis: Improved water management and less cellular damage allow photosynthesis to continue more effectively.

Seaweed Extracts

One of the major components of commercial liquid seaweed extracts are the long chain polysaccharides, or carbohydrates. These sugars promote plant growth and are elicitors of plant defences against fungal pathogens.

Brown seaweeds, such as Ascophyllum nodosum, are also rich in phenolic compounds, which are secondary metabolites synthesized when a plant is under stress and protect cells and cellular components by scavenging ROS. Seaweed extracts also help the plant to withstand stress through its phytohormone content; auxins, cytokinins, betaines, gibberellins, abscisic acid and brassinosteroids.

Seaweed has little nutritional value, although, they typically contain trace amounts of nutrients, and the real benefits of seaweed formulations lie in their antioxidant properties, elicitors of plant defences, and in helping the plant to uptake nutrients from the rootzone and moving them around the plant to where required.

Humic Acid

Humic acids provide antioxidant benefits and improve the availability of micronutrients, phosphate and potassium. They also enhance chlorophyll content, supporting photosynthesis under stress.

5‑Aminolevulinic Acid

In turf and plants, 5-Aminolevulinic acid serves as a precursor to chlorophyll, the molecule responsible for photosynthesis. It is synthesized in the chloroplasts and leads to the production of both chlorophyll and other essential molecules that regulate plant growth and development. 5-Aminolevulinic acid also plays an important role in plant stress responses, including tolerance to drought stress, salinity, shade and heavy metals through the production of antioxidants to combat free radicals.

Salicylic Acid

Salicylic acid influences a wide range of plant processes. It improves tolerance to abiotic stresses - including heat, drought, salinity and UV - through its antioxidant activity, and also enhances resistance to biotic stress.

Mannitol

Mannitol protects against damaging free radicals such as singlet oxygen and hydroxyl radicals. When present in chloroplasts, it helps prevent oxidative damage to key photosynthetic proteins, including the D1 protein of Photosystem II.

Vitamin B6

Vitamin B6 acts as a cofactor and antioxidant, protecting against ROS such as singlet oxygen. It also plays a key role in amino acid metabolism and supports the formation of proteins essential for chloroplast and thylakoid function.

Amino Acids

Applied amino acids support the formation of proteins required for Photosystem I, Photosystem II and thylakoid membranes. Lysine can donate a proton to stabilise Rubisco under high temperatures. Key proteins such as D1, D2, CP43, CP47 and cytochrome b559 rely on adequate amino acid availability, with D1 being particularly vulnerable to singlet oxygen.

CLICK HERE FOR PART ONE - End of season renovation to summer survival