BUSINESS crop stress

Transpiration stress during hot weather can cause wilting, leaf scorch, fruit drop, and disruptions to the movement of nutrients, minerals, sugars, and metabolites throughout the plant. Damage to plant cells caused by transpiration stress can also favour disease development and impact fruit quality.

Words Helen Newman, WA Berry Industry Development Officer, Agricultural Produce Commission and Graeme Smith, Graeme Smith Consulting

WHILE mild stress in some crops can trigger processes that improve plant health and fruit quality, prolonged or acute stress is often associated with poor outcomes. Maintaining an environment where plants can transpire as needed throughout the day provides optimum conditions for growth.

Good irrigation system design and scheduling are essential for minimising transpiration stress. This article assumes good irrigation and instead examines additional strategies that can be used to limit transpiration stress that occurs even in the presence of optimum irrigation.

Plants can (to an extent) open and close their stomata depending on water levels in the plant, light intensity, and CO2 levels. Stomata open to allow CO2 to enter a leaf and water vapour to leave (see Figure 1).

FIGURE 1: Close-up view of the plant transpiration process.

Source: Unknown — widely circulated online

Transpiration stress occurs when a plant cannot transport enough water from the root zone to the stomata to match the transpiration demand. This can occur, even with good root zone moisture content, in very sunny conditions.

Figure 2 shows an example of a plant under transpiration stress and how this can be managed using shading. The happy plant on the left is getting 50% less light, so its leaves are cooler, and the transpiration demand is 50% lower, despite the warmer ambient temperature that can occur under screens. The stressed plant on the right is getting full light, so its leaves are hotter, and the transpiration demand is 100% despite the cooler ambient temperature. This plant cannot take up enough water to match its transpiration needs, so its stomata have closed to protect from further water loss, and it has wilted. This plant has stopped transpiring so leaf temperature will continue to rise. The plant on the left continues to transpire, so it will stay cooler and will use more water than the plant on the right.

FIGURE 2: How shading can help manage transpiration stress.

Source: Greame Smith Consulting

Light drives leaf temperature leaf temperature drives transpiration.

It takes about a minute for a plant to respond to a rapid rise in leaf temperature (Figure 3).

FIGURE 3: Air and plant temperature patterns after significant changes in light levels (e.g. sunrise).

Source: Greame Smith Consulting

FIGURE 4: Ambient and leaf temperature pattern according to radiation.

Source: Greame Smith Consulting

At 200W/m2 radiation, plant, and air temperature are basically the same. With each extra 100W/m2 (above 200W/m2 ), plant leaf temperature can increase by 0.6ºC. So, at 800W/m2 (600W/m2 higher), plant leaf temperature can be 3.6ºC higher.

A linear increase in plant temperature over air temperature is generally okay with a linear increase in radiation intensity. A non-linear increase is not okay and indicates transpiration stress (Figure 4).

Shade options to manage transpiration stress

Shading can be achieved in several ways including screens, netting, reflective films sprayed on polytunnels or glasshouses, and kaolin or mineral-based particle films applied directly to the foliage.

When selecting a shading option, consideration should be given to how it will influence ventilation, pollination, IPM, crop physiology, and yield. An understanding of the light spectrum, and how it is impacted by the different shading options is also important (Figure 5).

FIGURE 5: Light spectrum.

• UVC is blocked by the ozone layer and can only reach plants if applied through artificial lights.

• UVB assists insect navigation (including bees), has sterilisation properties (including powdery mildew), and enhances colour in plants.

• UVA also influences pigmentation and plant physiology and may also influence insect populations.

• Visible Light is Photosynthetic Active Radiation (PAR) and is used by plants for photosynthesis. Within the visible light spectrum, the different colours (violet, indigo, blue, green, yellow, orange, red) trigger multiple physiological processes in plants.

• Infrared Radiation heats objects and structures.

Screens

Shade screens come in a variety of formats and materials. High-tech shade screens respond autonomously to environmental conditions, providing shade when needed and retracting to allow maximum light when conditions are favourable. These screens are normally installed within an overhead structure. More durable screening fabrics can be placed on top of structures and moved manually or autonomously each day or seasonally as required. Pale-coloured (so-called diffuse) materials are often selected as they uniformly reflect light and don’t absorb as much heat as darker materials. A range of products offer shading from 30% up to almost total blackout, and screens can be either open, with gaps that allow air exchange, or closed to retain more warmth when needed.

Netting

Netting is usually more driven by the need for bird or hail protection, however its positive impact on light, especially in brighter areas, is becoming more understood. White and black are the most used netting colours in the Australian blueberry industry. Black netting absorbs the most light and therefore provides the most shade.

Autonomous shade screens used in a Cravo system.

Drone applying reflective paint to polytunnels.

Light that passes through the holes in black netting remains spectrally unchanged. Light that hits fibres of white and coloured netting scatters and changes spectrally.

The modified light spectrum alters plant physiological processes, while scattered (diffuse) light penetrates deeper into the canopy between leaves. Most netting sold in Australia has an 18–20% shade factor, which may not be sufficient to avoid transpiration stress in some locations.

Reflective paint

Where polytunnels or glasshouses are used, reflective white paint applied over the plastic/glass before summer heat is a popular and effective option. This can be done by hand on elevated platforms on lower tunnels or by drone. A potential advantage of whitening over shade screens and netting is that it doesn’t affect ventilation. It also significantly increases the fraction of diffuse light. The paint may need to be removed seasonally as light levels fall.

Kaolin-based clay spray applied to citrus leaves.

Particle film sprays

Kaolin clay and mineral-based particle film sprays applied to plants during vegetative stages may be another tool in an integrated heat-management approach. The particles reflect ultraviolet (UV) and infrared radiation (IR) and a portion of photosynthetically active radiation (PAR), reducing leaf temperature and transpiration stress. Reflection of IR alone can reduce canopy temperature by up to 5ºC (Glenn, 2021).

Permanent and seasonal shading options may reduce yield as they cannot be readily adjusted for changing conditions from day to day or during a day. This results in reduced light levels even when it’s not needed.

Humidity and temperature

The difference in moisture content between the plant and ambient air also impacts transpiration. Combined with temperature, ambient relative humidity can be used to calculate Humidity Deficit (HD) and/or Vapour Pressure Deficit (VPD). Both are a good indication of plant comfort.

Research suggests that the optimum HD for plants is 3.5 and that they are comfortable over the range of 2–8 g/ m3 (VPD 0.3–1.5kPa) and temperature of 16–28ºC (Figure 6).

Plants generally stop growing and transpiring when the HD is less than 2 (VPD <0.3). In these situations, the plant leaves are potentially cooler than the ambient air. HD’s over 8 (VPD >1.5) can lead to transpiration stress. Plant leaves are often increasingly hotter than the ambient air in these conditions.

Shading and ventilation are the most economical methods of improving HD and VPD.

Studies have shown that shading lowers the ambient temperature, increases transpiration (as plants are no longer in transpiration stress), and increases humidity in hot and dry conditions.

Ventilation replaces warmer air inside a structure with cooler air from the outside. If the outside temperature is low enough, and the temperature inside the structure is not too high, warm air can be exhausted passively (naturally) through vents. If air circulation is low, HAF fans can be used to gently push excess heat out of the structure. HAF fans can also be useful where low air movement has allowed humidity to build up around leaves to a point where plants are suffering a self-induced reduction in transpiration, which can also be detrimental.

FIGURE 6: A Humidity Deficit Diagram can be used to estimate plant comfort based on ambient air temperature and relative humidity.

When ambient temperatures are high and humidity is low, fogging systems or sprinklers to water interrows may also be useful to bring the HD/VPD back into the comfortable zone. These can be used strategically in the most challenging part of the day to limit the time plants are under stress. This strategy is less useful in windy conditions.

Other strategies to increase humidity under high ambient temperatures include closer plantings, vegetated interrows, and windbreaks.

Note: strong winds can create situations where humidity is continuously stripped away from leaves, increasing HD/VPD and sometimes outstripping a plant’s ability to keep up with transpiration losses.

An integrated approach

Shading is the most effective means of reducing crop stress during extreme heat. It can also save a crop in an emergency where the irrigation schedule is disrupted. Ventilation is also critical for crops growing under protective structures. Other practices and infrastructure that decrease ambient temperature and increase humidity can be considered as part of an integrated approach to maintaining a comfortable growing environment.

Critical temperatures and light levels (trigger points) that cause crop stress and fruit damage vary with the cultivar and fruit development stage. The interventions (e.g. shading) needed to manage crop stress will also vary with location. On-farm experimentation is needed to determine the best solution for your situation.

It’s important to remember that under somewhat normal conditions, plants are very resilient and can manage themselves very well, however there is a range of technologies to relieve a plant of stress as required. Overuse of some technologies can encourage a weaker, less resilient plant. It is important to find a balance.