Hot vegetables: Heat and sun damage

Complaining about the heat? Unfortunately, increasingly hot growing seasons are becoming the norm as the effects of unmitigated climate change wreak havoc on our ecosystems. Farmworkers feel the heat and need to learn the signs of dangerous heat-related health conditions, be more vigilant about skin cancer risks and cope with other hazards – think new wildlife, pests and diseases that thrive in the changing climate.

The crops are feeling the heat too. What crop damage can farmers expect with increased temperatures during the growing season? And what can be done to mitigate and prevent issues impacting crop health?

Heat Damage

Plants keep cool via transpiration, the evaporation of water from the plant. Ninety percent of this occurs via the stomata. But when there is water stress (like hot, dry conditions), the stomata close. Without transpiration, the plant has to utilize other means of cooling, and these also are compromised when the weather is hot and dry.

When soils are too dry, abscisic acid, a plant hormone, signals stomata to close, thus increasing internal plant temperatures. Windy conditions on hot days exacerbate plant tissue heating, as leaves will lose water faster than the plant can take it in via roots. Under high temperatures, radiated heat builds up in the atmosphere around leaves, forcing heat to be retained in plant tissues. Extreme heat causes the water vapor pressure deficit in the air to increase, causing the plant to lose water.

Extreme temperatures also cause harm by reducing the amount of photosynthesis a plant performs. Temperatures greater than 94º F cause a reduction. And nighttime temperatures matter too. If they remain above 75º, cell respiration will increase, and sugars and seed production will decrease as a result. Pollen production can be impacted, and fruit set can be aborted.

Even if there is rain during hot weather, and soils aren’t dry, damage to fruit can still occur. Too much rainfall can cause excessive leaf growth, which can then become defoliated due to diseases which thrive in hot, wet weather. Defoliation would then expose the maturing fruits to sudden intense sunlight, causing sunburn.

Sun Damage

When the crop is exposed to too intense or significantly increased periods of sunlight, sunburn or sunscald can occur.  Leaves can be affected, as can fruit. Damaged areas look bleached and dried out. Direct sunlight – particularly when the intensity of light increases rapidly, without time for plants to acclimate – is the culprit. Periods of intense heat exacerbate the problem, as plant tissue desiccation increases due to retained heat in plant tissue.

Sun damage occurs when the plant’s temperature elevates beyond its tolerance. A plant’s normal temperature is slightly above that of the air temperature. There are three types of sun damage: necrosis, browning and photooxidative. The conditions under which each occurs varies.

Necrosis occurs when plant cells are damaged and membranes leak. This happens on the side of the crop exposed to direct sunlight, and the temperature at which damage occurs varies by crop. The damaged tissue is white or brown in color. Browning occurs if the cells remain alive, but their pigments are impacted by direct sunlight. This occurs at slightly lower temperatures than necrosis.

Photooxidative damage happens when the plant is suddenly exposed to increased sunlight. Transplanting from the greenhouse to the field without acclimating, storm or disease damage suddenly destroying the leaf canopy or harvest damage exposing remaining fruit to the sun are reasons this damage occurs. Bleached white areas signify tissue death.

Sunburn can occur even after harvest. If the picked crop is left in the sunlight for enough time, damage will occur, making the fruit less desirable or unmarketable.

Other Heat Concerns

Cell membrane disruption occurs in distressed plants, and the plants will produce toxins which further damage cellular processes. Defensive chemicals are no longer produced as needed, and pest and disease pressures increase. Leaf drop, scorching of plant stems and leaves and wilting are all signs of heat distress. Plants can rapidly dry out and die. Some pesticides can cause crop damage when applied during times of intense sunlight or heat.

Temperatures on black plastic surfaces can be extremely hot (around 150º), and this heat will be reflected onto plants. Hot roots, growing in heated soils under the plastic, can reduce plant growth and cause issues, and heat lesions can occur on overheated plant tissues, particularly those close to the plastic row cover and receiving the most reflected sunlight.

If heat and sunlight don’t directly impact crops, ozone damage just might. Hot, humid weather with little air flow favors development of damage. In cucurbits, damage appears as flecks of tan, black, brown or white on the leaves. Small pigmented areas that appear dark, as well as bronzing and reddening of the leaves, can occur as well. In squash and melons, ozone-affected areas first appear yellow before bleaching or turning black, depending on crop. On beans, small bleached spots on pods and leaves turn bronze. Pathogens can attack the damaged tissues and cause further damage and loss.

Ozone damage is not always symmetrical, as exposure to this pollutant can vary from area to area on the plant. Older leaves are typically most impacted, and upper leaf surfaces normally bear the brunt of the damage. Although research is limited, ozone damage has been shown to cause significant yield losses in various crops, sometimes as much as 30%. Cultivars are not uniformly impacted.

Mitigation

Proper fertilization and pruning, to develop a healthy leaf canopy, can protect against sun and heat damage. Preventing diseases which damage leaves is crucial to protecting developing fruit from receiving too much sunlight.

Mulches which reflect heat, or low-density organic mulches such as straw which retain soil moisture levels, can reduce heat around plants and help with water management.

Providing overhead irrigation can help plants cool down, via evapotranspiration, lowering plant tissue and soil temperatures. The water vapor pressure deficiency is also reduced with overhead irrigation, so plants can also remove heat via convection, releasing it to the surrounding atmosphere. Proper drying of the canopy when using overhead watering is needed to prevent diseases.

The use of shade cloth in high-value crops or small-scale plantings can reduce sun damage to fruits and also keep plants cooler by reducing water stress, allowing transpiration to continue, and keeping ambient temperatures cooler so convection cooling can occur as well. Quality loss – bitterness, bolting, sunburn browning – can be reduced with the use of shade covers.

There are also substances which can be surface-applied, such as kaolin and calcium carbonate-based products, which reflect sunlight, reducing sun damage and keeping fruit temperatures lowered. But studies of products in some vegetable crops have shown adverse effects. Removing these films after harvest may also be a concern. Cultivar selection can play a role in sunscald susceptibility.

The continued accumulation of greenhouse gases in the atmosphere, and the resulting climate change, will have detrimental effects on crop production worldwide. Heat, sun intensity and ozone damage, reduced rainfall with periods of drought alternating with flooding and the changes in growing seasons which result from climate change will amplify existing crop concerns – and create new ones.

by Tamara Scully

2022-09-06T14:54:32-05:00September 6, 2022|Grower, Grower West|0 Comments

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