The Importance of Vapor Pressure Deficit (VPD) in Crop Performance

Vapor Pressure Deficit (VPD) and its Impact on Almond Tree Performance

Vapor pressure deficit (VPD) is a crucial environmental factor significantly impacting almond tree performance. It represents the difference between the saturation vapor pressure (the amount of water vapor the air can hold at a specific temperature) and the actual vapor pressure (the amount of water vapor currently present in the air). Here’s a breakdown of its influence:

Stomatal Conductance and Transpiration:

• Stomata: Tiny pores on the underside of leaves that regulate gas exchange (CO2 intake and water vapor and oxygen release) between the plant and the atmosphere.
• Impact of VPD: High VPD creates a strong “pull” on water molecules within the plant, causing stomata to open wider to release water vapor through transpiration.

Water Stress and Plant Health:

• Increased Transpiration: As VPD rises, transpiration increases, leading to greater water loss from the plant.
• Water Stress: If the rate of water loss through transpiration exceeds the rate of water uptake from the roots, the plant experiences water stress.
• Impact on Almond Trees: Water stress in almond trees can negatively affect:
  • Photosynthesis: CO2 uptake is reduced due to partially closed stomata to conserve water.
  • Growth and Development: Growth slows down, potentially leading to smaller fruit size and reduced yield.
  • Nutrient Uptake: Water stress can hinder the ability of roots to absorb nutrients effectively.
  • Susceptibility to Disease: Water-stressed trees are more vulnerable to insect pests and diseases.

Optimal VPD Range for Almonds:

Almond trees are relatively drought-tolerant, but they still have an optimal VPD range for ideal growth and performance. This range can vary slightly depending on the specific growth stage, age and cultivar, but it generally falls between:

Day time:

• Lower Limit: Around 1 kPa (kilopascal)
• Upper Limit: Around 3 kPa

Nighttime:

• Lower Limit: Around 0 kPa (kilopascal)
• Upper Limit: Around 0.6 kPa

Strategies to Mitigate High VPD:

• Irrigation Management: Proper irrigation practices are crucial for maintaining adequate soil moisture and reducing water stress during periods of high VPD. To optimize tree health during periods of high upcoming VPD (day or night), consider strategic late afternoon or early evening irrigation for your almond trees. This timing can help them replenish water reserves before the increased evaporative stress hits.
• Canopy Management: Techniques like pruning to promote air circulation and shade the orchard floor can help create a more favorable microclimate around the trees.
• Selection of Drought-Tolerant Cultivars: Choosing almond varieties known for better drought tolerance can be beneficial in areas with high VPD conditions.

Monitoring VPD:

Monitoring VPD levels throughout the growing season allows growers to implement proactive measures to minimize water stress on their almond trees. Weather stations or dedicated sensors can be used for VPD measurement.

By understanding the relationship between VPD and almond tree performance, growers can optimize irrigation practices and orchard management techniques to ensure healthy trees and maximize yields.

Impact of High Nighttime Vapor Pressure Deficit (VPD) on Almond Trees

High nighttime VPD, even more so than high daytime VPD, can have significant negative impacts on almond tree performance. Here’s a breakdown of the specific concerns:

Reduced Photosynthesis:

• Stomatal Closure: Unlike daytime, when stomata open for CO2 uptake and transpiration, in most of the crops, stomata partially/totally close at night to minimize water loss. However, high nighttime VPD can lead to excessive stomatal closure, restricting CO2 intake needed for photosynthesis the following day.
• Limited Carbon Gain: Reduced CO2 uptake during the night directly translates to lower carbohydrate production, a crucial energy source for various plant functions, including growth, flowering, and fruit development.

Impaired Flower Bud Development:

• Flower bud differentiation: Almond trees initiate flower bud development during the fall and winter. High nighttime VPD can disrupt this process, leading to:
  • Reduced flower bud formation: Fewer flower buds develop, potentially resulting in lower fruit set and yield.
  • Weak flower buds: Even if buds form, they may be weaker and more susceptible to damage from frost or other environmental stresses.

Reduced Fruit Set and Yield:

• Pollination: While almond pollination primarily occurs during the day, high nighttime VPD can indirectly affect pollination success. Stressed trees with limited carbohydrate reserves might produce fewer or less viable pollen grains, reducing pollination potential.
• Fruit Development: If flower bud formation and development are compromised by high nighttime VPD, it ultimately translates to a lower number of fruits set and potentially smaller fruit size, leading to lower overall yield.

Additional Considerations:

• Water Stress: High nighttime VPD can exacerbate water stress even if irrigation is adequate. The increased “pull” on water molecules at night can deplete plant water reserves, further impacting daytime performance.
• Increased Susceptibility to Disease: Water-stressed trees are more vulnerable to fungal diseases, and high nighttime VPD can create a favorable environment for fungal spore germination and infection.

Strategies to Mitigate High Nighttime VPD:

While controlling nighttime VPD directly is challenging, some strategies can help minimize its negative impacts:

• Irrigation Management: Ensuring adequate soil moisture before nightfall allows trees to access readily available water at night, reducing the stress caused by high VPD. If you’re expecting high night VPD values for your almond orchard, prioritize irrigation in the late afternoon or early evening. This will help them better cope with the upcoming evaporative stress.
• Canopy Management: Techniques like promoting good air circulation within the orchard can help reduce nighttime humidity buildup, leading to slightly lower VPD around the trees. However, nighttime canopy manipulation strategies are limited compared to daytime options.
• Selection of Drought-Tolerant Cultivars: Choosing almond varieties known for better water use efficiency and drought tolerance can be beneficial in areas with frequent high nighttime VPD.

Monitoring and Early Intervention:

Monitoring nighttime VPD levels along with other environmental factors like temperature and humidity helps growers identify potential risks and implement strategies to mitigate their impact on almond trees. By taking proactive measures, growers can improve tree health, flower bud development, fruit set, and ultimately, maximize their almond yields.

Yes, High Nighttime VPD Can Reduce Plant Tissue Rehydration

Here’s how high nighttime VPD hinders plant tissue rehydration in almonds and the concept of threshold values:

Mechanisms:

• Increased Transpiration: Even with stomata partially closed at night, high VPD can still create a strong pull on water molecules within the plant. This can lead to continued, albeit reduced, transpiration, causing some water loss throughout the night.
• Reduced Water Uptake: While water uptake from the soil can occur at night, high VPD can limit the rate of water movement within the plant. This is because the “driving force” for water movement (the pressure gradient between soil and plant) is partially reduced due to the drier conditions (higher VPD) within the plant.

Consequences for Rehydration:

• Limited Nighttime Recovery: Due to the factors mentioned above, high VPD can prevent the plant from fully rehydrating its tissues at night, following water loss during the day.
• Accumulated Water Stress: Over time, this incomplete nocturnal rehydration can lead to accumulated water stress in the plant, impacting its overall health and performance.

Threshold Values:

Unfortunately, there isn’t a single, universally accepted threshold VPD value for nighttime rehydration in almond trees. It can vary depending on several factors:

• Almond Cultivar: Different almond cultivars may have varying tolerances for nighttime VPD based on their inherent water use efficiency and drought resistance.
• Plant Age and Health: Younger or weaker trees might be more susceptible to rehydration limitations at lower VPD levels compared to mature, healthy trees.
• Environmental Conditions: Other factors like nighttime temperature and humidity can influence the impact of VPD on rehydration. For example, a high VPD combined with high nighttime temperatures would create a more significant rehydration challenge than high VPD with cool nighttime temperatures.

The use of foggers or sprinklers at night to address high nighttime VPD in almond trees:

Potential Benefits:

• Increased Humidity: Foggers and sprinklers directly add moisture to the air, raising the relative humidity (RH) around the trees. This reduces the VPD, lessening the “pull” on water molecules within the plant and minimizing water loss through transpiration at night.
• Improved Rehydration: By creating a more humid microclimate, foggers or sprinklers can potentially enhance nighttime rehydration of plant tissues, counteracting the drying effects of high VPD.

Limitations and Considerations:

• Disease Risk: High humidity at night can create favorable conditions for fungal spore germination and disease development. This is a major concern for almond trees, which are susceptible to fungal diseases like Botrytis blight.
• Water Management: Foggers and sprinklers can use significant amounts of water. Careful monitoring and application techniques are crucial to avoid overwatering and potential root rot issues.
• Leaf Wetness Duration: While some nighttime leaf wetness can be beneficial, prolonged periods can also promote disease growth. Balancing the rehydration benefit with minimizing wetness duration is important.
• Cost and Infrastructure: Setting up and operating fogger or sprinkler systems requires additional investment and ongoing maintenance.

Foggers vs. Sprinklers:

• Foggers: Generally considered a more targeted approach, creating a fine mist that increases humidity without excessive water application. However, foggers can be more expensive to install and operate compared to sprinklers.
• Sprinklers: Offer a simpler and potentially less expensive option for raising humidity. However, sprinklers can deliver more water than necessary and lead to longer leaf wetness periods, potentially increasing disease risk.

Overall:

Using foggers or sprinklers during high nighttime VPD can be a strategy to consider, but it requires careful evaluation of the potential benefits and drawbacks. Disease risk management and efficient water use are key concerns.

The optimal approach likely involves a combination of strategies depending on the specific needs of the orchard, environmental conditions, and water availability.

Research and Monitoring:

While specific threshold values are elusive, ongoing research is helping to refine our understanding of how VPD interacts with other environmental factors and plant characteristics to impact rehydration. Growers can benefit from monitoring nighttime VPD levels along with other environmental parameters to assess potential rehydration risks and implement strategies like irrigation management to mitigate them.

In conclusion, high nighttime VPD can hinder plant tissue rehydration in almond trees, but the specific threshold values for concern depend on various interacting factors. Ongoing research and monitoring practices are crucial for optimizing almond tree health and performance in the face of fluctuating VPD conditions.