The Impact Transpiration Has on Fruit Quality

• Fruit transpiration occurs through the fruit skin, lenticels, calyx, and stem scar.
• Preharvest fruit transpiration is good as it drives the accumulation of assimilated and dry matter to improve fruit quality.
• Postharvest fruit transpiration is undesirable, leading to shriveling, weight loss, wilting, loss of quality, shelf-life, and reduced profitability.

Fruit transpiration causes most weight loss in postharvest commodities and drives the engineering of storage and transport conditions in a supply chain. However, the phenomenon is less studied and understood than leaf transpiration. Moreover, recent research findings show that fruit transpiration can also be crucial in the preharvest stage during fruit development. Find out what is known so far about fruit transpiration.

What is Fruit Transpiration?

Transpiration is the loss of water content from fruits. It is limited to leaves and happens through stems, roots, flowers, and fruits.

Most fruits have few or no stomata, so transpiration occurs through the cuticle or fruit skin. The skin can be a waxy cuticle or periderm resistant to water loss, but some gas exchange does occur. Apples and pears have lenticels through which they lose water but have no control over lenticular opening and closing. Transpiration can also happen through fruit calyx (as in eggplants), stem scar where the fruit is attached to the plant (tomato), broken skin (as in cherries), and fruit hairs (as in kiwis and peaches).

Transpiration occurs while the fruit is on the plant and also postharvest. Once fruits are harvested, the water content lost cannot be replaced. Therefore, in postharvest stages, fruit transpiration is the most critical factor influencing fruit quality.

Preharvest Transpiration Effects

Figure 1: “Diurnal patterns of transpiration flow rates (a) and phloem flow rates (b). Each color line represents a different fruit species at a different time of the season, expressed as days after full bloom (DAFB),” Adapted from Rossi et al., 2022. (Image credits: https://doi.org/10.1093/hr/uhac036)

Fruit transpiration is not constant, and there are diurnal variations, with the rate being the lowest in the morning and rising till mid-day, after which it begins to drop again. As transpiration increases, fruits have less water or negative water potential. Fruit water potential is the difference in water content between stem or roots and fruits. As a result, the water and assimilates move to the fruits, though the rate and timings will differ based on species and the stage of fruit development, see Figure 1.

Fruit water potentials and fruit transpiration will drive diurnal and seasonal variations in fruit development. A reduction can lead to reduced accumulation of assimilates and dry matter content in fruits, see Figure 2. Higher dry matter is associated with improved fruit quality in most fruits and is correlated with consumer satisfaction. Moreover, water uptake replenishes water loss, so the fruit doesn’t suffer.

Figure 2: “Dry matter accumulation in the fruit of apricot (left) and peach (right) measured in

transpiring fruit (control) and under reduced transpiration,” Montanaro et al. 2012. (Image credits: doi: 10.5772/21411)

Since transpiration is the least in the morning, and fruits and vegetables have the maximum water at this time, harvesting in the morning has become standard practice. For example, melons harvested in the morning are heavier than melons harvested in mid-day.

Postharvest Transpiration Effects on Fruits

Water content is highest in any fruit or vegetable at harvest, and any loss is undesirable as the water cannot be replenished. Fruit transpiration can change the appearance, size, shape, weight, and internal quality of fruits, see Figure 3.

Figure 3: “Bell pepper fruit immediately after harvest (left), three days after harvest (center), and seven days after harvest (right). Fruit kept at 20C and 70% RH,” Díaz-Pérez 2019. (Image credits: https://irrec.ifas.ufl.edu/postharvest/HOS_5085C/Reading%20Assignments/2019-PH_Phys_&_Biochem_of_Fruits_&_Vegs-Yahia/2019-Ch.08-Transpiration-Yahia.pdf)

Fruit transpiration first causes dehydration that reduces cell turgor, making the fruits soft (as in bell pepper),  shriveling (as in mango, cucumber, citrus, etc.), and loss of skin glossiness (as in mango, eggplant, cucumber, etc.), reducing the marketability of the products.

Transpiration also results in water stress in fruits, accelerating fruit senescence caused by ethylene, cellular membrane disintegration, and solute leakage.

The amount of water loss depends on the fresh produce, and each species has a different tolerance. Fruits and vegetables can have a high water content of 80-95%, which is responsible for their freshness and crispness, see Table 1. The postharvest water loss beyond which fruits cannot be marketed ranges from 3% to 10% of the fruit weight at harvest. Beyond a loss of an average of 5% water content, the fruit quality is affected.

Even if the water loss is not significant enough to begin affecting fruit quality, any loss in weight can amount to a drop in profits for bulk deliveries.

Table 1: “Water content (%) by weight of some common fruits and vegetables, Holcroft 2015. (Image credits: http://www.postharvest.org/Water%20relations%20PEF%20white%20paper%20FINAL%20MAY%202015.pdf)

The loss of water also reduces the weight of the fruits. Transpiration causes more weight loss than fruit respiration, depending on the fruit. In some cases, like tomatoes, transpiration is responsible for up to 97% of the fruit weight loss.

Factors Affecting Fruit Transpiration

It is necessary to know the factors that affect transpiration to control and reduce its negative impact on food production. These can be genetic, biological, and environmental factors.

Environmental Factors

Air temperature and relative humidity are the two environmental factors that influence fruit transpiration. The relative humidity is more critical because transpiration is driven mainly by the difference in water vapor difference in the air and the fruit. When the air is dry, the atmosphere’s water vapor deficit (VPD) increases, causing transpiration to increase. Conversely, as the relative humidity of the air increases, transpiration decreases. For example, increasing air relative humidity from 40 to 60% can reduce fruit water loss in peaches by 30 to 50%.

However, at very high VPD, there is less transpiration because the fruit skin is dry or its permeability has changed.

Water loss during transpiration is also expressed as the percentage of fresh produce weight loss per unit of vapor pressure difference (kPa) between the surrounding air and the produce.

As air temperature rises, transpiration will increase as a relative humidity reduction accompanies it. For example, a temperature rise from 4 to 20°C increases transpiration by more than five times in strawberries.

Following diurnal patterns of temperature rises and drops in relative humidity through the day, fruit transpiration also peaks at mid-day, see Figure 4.

Figure 4: “Daily oscillation of fruit water loss measured 1-week after fruit-set in attached

kiwifruit (cv Hayward) and detached peach fruit (cv Dixired),” Montanaro et al. 2012. (Image credits: https://doi.org/10.5772/21411) 

As the duration of postharvest storage increases, fruit transpiration reduces for a range of reasons. It could be due to resistance to water movement as the commodity becomes dehydrated. Or due to changes in the cuticle.

Precision instruments like the F-901 AccuRipe & AccuStore, produced by Felix Instruments Applied Food Science, simultaneously measure and control temperature, relative humidity, carbon dioxide, oxygen, and ethylene with sensors. These are excellent for storage and ripening facilities to reduce water and quality loss.

Biological and Genetical Factors

Many biological factors affect the rate of fruit transpiration, like fruit size, surface area/weight ratio, maturity stage, and skin properties.

Fruit Size

As fruit size increases, fruit transpiration decreases. Size influence has been recorded in eggplant, tomatoes, and bell peppers. For example, an eggplant with a commercial size 32 (weighing 180 g) has a transpiration rate of 1.12%/day/kPa, and an eggplant size 16 (or 550 g) transpires at a rate of 0.62%/day/kPa. So smaller-sized eggplants with more transpiration have a shorter shelf-life.

Surface Area/ Weight Ratio

Transpiration is proportional to fruit surface area. As a fruit’s size increases, its surface-to-weight ratio decreases. In the earlier example, smaller eggplants had more surface area than their weight, so they transpired more. Spherical-shaped fruits, for example, tomato and orange, have a lower surface area-to-weight ratio and transpire less than other shaped fresh produce. However, transpiration in tomato cultivars with a similar shape but different size was found to depend on the surface area to weight ratio.

Maturity Stage & Type

The fruit maturity stage can influence the transpiration rate. This is partly due to an increase in size and a decrease in surface area-to-weight ratio. In some cases, like the eggplant, whose transpiration occurs mainly through the calyx, young fruits can be partially covered by the calyx and record high transpiration. As the eggplant grows, the ratio of its surface area covered by calyx diminishes, reducing the transpiration rate.

In most species, transpiration is highest at the fruit set and gradually reduces. For example, transpiration reduces by 80% in apricot from the initial to last stages of fruit development.

Though the transpiration can differ among fruits, a similar trend was seen in peaches, grapevines, kiwis, etc. However, transpiration increases after the tomato ripens from the green to red stages.

Type of maturation- climacteric or non-climacteric – influences transpiration. In non-climacteric fruits is more susceptible to relative humidity than temperature. Therefore, maintaining high relative humidity in postharvest stages is essential to prolong non-climacteric fruits’ shelf life.

Skin Properties

Skin properties crucial to fruit transpiration are thickness, waxiness, or cuticle composition. Skin characteristics can be species and cultivar specific. For example, thick and waxy skin prevents or limits water loss in fruits like apples and tomatoes. However, differences among tomato cultivars can be explained only by cuticle composition.

Any injury or cracks to the fruit skin will increase transpiration, and risks of disease infection increase quality drop. Some fruits like cherries or thin-skinned tomato lines are prone to skin cracking and high transpiration losses.

Plant parts like stems, calyx, and leaves still attached to fresh produce will increase transpiration. For example, bunch tomatoes wilt faster than separated from the stem and calyx.

The stem scar is a known route of water loss. In blueberries, the stem scar is responsible for 40% of the fruit transpiration and is 170% higher than cuticular fruit transpiration.

Genetical Factors

Different species of fruits and vegetables have varying transpiration rates. Moreover, fruits from cultivars of the same species will transpire at different rates due to varying shapes, sizes, or skin properties. Transpiration in blueberries predominantly through stem scar can vary 75% between cultivars.

Controlling Fruit Transpiration

Since most water loss happens due to transpiration, the fruit supply chain relies on high relative humidity (90-95%), low temperatures, and reduced air movement to minimize water loss. Moreover, surface coatings, such as wax, reduce transpiration by around 30%. MAP packaging is another way to control fruit transpiration and extend fruit shelf-life. Though these measures are standard, a thorough understanding of preharvest and postharvest fruit transpiration and its response to environmental changes in different species and cultivars can help stakeholders customize conditions to optimize fruit quality and prevent food waste.

Sources

Bovi, G. G., & Herppich, W. B. (2021). Keeping fruits and vegetables fresh by limiting respiration and transpiration. Frontiers for Young Minds, 9. https://doi.org/10.3389/frym.2021.576906

Dı́az-Pérez, J.C. (1998). Transpiration rates in eggplant fruit as affected by fruit and calyx size. Postharvest Biology and Technology, 13(1), 45–49. https://doi.org/10.1016/s0925-5214(97)00078-1

Díaz-Pérez, J.C. (2019). Transpiration Chapter 8. Retrieved from https://irrec.ifas.ufl.edu/postharvest/HOS_5085C/Reading%20Assignments/2019-PH_Phys_&_Biochem_of_Fruits_&_Vegs-Yahia/2019-Ch.08-Transpiration-Yahia.pdf

Holcroft, D. (2015 May). Water Relations in Harvested Fresh Produce. PEF White Paper No. 15-01. The Postharvest Education Foundation (PEF). Retrieved from http://www.postharvest.org/Water%20relations%20PEF%20white%20paper%20FINAL%20MAY%202015.pdf

Lara, I., Heredia, A., & Domínguez, E. (2019). Shelf Life Potential and the Fruit Cuticle: The Unexpected Player. Frontiers in Plant Science, 10. https://doi.org/10.3389/fpls.2019.00770

Montanaro, G., Dichio, B., & Xiloyannis, C. (2012). Fruit Transpiration: Mechanisms and Significance for Fruit Nutrition and Growth. InTech. doi: 10.5772/21411

Rossi, F., Manfrini, L., Venturi, M., Corelli Grappadelli, L., & Morandi, B. (2022). Fruit transpiration drives interspecific variability in Fruit Growth Strategies. Horticulture Research, 9. https://doi.org/10.1093/hr/uhac036