As in other plant organs, the epidermis (skin) of fruit and vegetables plays an important role in gas exchange between the product and the surrounding environment. The skin allows the fruit to maintain a high water content despite the low relative humidity in the air around the product. This protection against dehydration is particularly important after harvest, when the fruit will not be receiving any more water from the plant. Bell pepper fruit quality and postharvest life are highly determined by fruit water loss or transpiration.1–3 Fruit water loss occurs through the stomata, lenticels, cuticle, and epicuticular wax platelets, as well as through the calyx, pedicel or floral ends.3 Fruit water loss accounts for most of the weight loss in the majority of horticultural produce.4 In tomatoes, transpiration represents 92–97% of fruit weight loss.5
Temperature and humidity are the environmental factors that have the strongest influence on fruit quality.4 The effect of humidity on fruit quality varies among crops. In non-climacteric fruit, such as bell pepper, storage in high humidity conditions which results in reduced fruit transpiration has a stronger effect in delaying senescence than storage at low temperatures.1, 6 Lurie et al.7 found that water stress hastens and triggers the onset of senescence in bell pepper fruit. Excessive fruit water loss results in softening and reduced shelf-life in citrus, bell pepper, and eggplant fruit.1, 2, 8
Using Fick's law of diffusion, fruit transpiration can be described as being determined by the water vapor pressure difference between the fruit and the surrounding air and by the resistance to water evaporation through the fruit epidermis. The rate of fruit water loss differs among species and sometimes even among cultivars of the same species.1, 4, 9 Some of the fruit factors that affect transpiration in fruits are the fruit surface area/volume or surface area/mass ratio,3, 10, 11 the surface structure of the fruit, including the number and size of stomata and lenticels, and the thickness and composition of the cuticle.1, 3, 12 The stem scar and the calyx have also been found to play an important role in fruit transpiration. The surface of pepper fruit lack stomata and thus gas diffusion is primarily through the cuticle of the skin, and the contribution of the calyx to whole-fruit transpiration is not known.13 In tomato, 67% of fruit transpiration is through the stem scar,14 and in eggplants at least 60% of fruit transpiration is through the calyx.10 Cultural factors and maturity stages affect water loss in some commodities such as potato4 and tomato15 but the relation between maturity and water loss is still unclear in most fruits and vegetables.
Permeance of a plant organ to a gas is the rate at which the gas can diffuse in or out of the organ per unit surface area for a given gradient of partial pressure of the gas.16 Permeance is also known as conductance and represents the reciprocal of resistance to gas diffusion.17 The objectives of this study were to determine the changes in transpiration and permeance to water vapor of bell pepper fruit as affected by fruit stage of development, ripeness, and storage.
MATERIALS AND METHODS
The study was conducted at the Coastal Plain Experiment Station, Tifton, GA, USA, during spring 2001 and fall 2002. Bell pepper (Capsicum annum L. ‘Camelot’ (Seminis, Oxnard, CA)) plants were grown according to the recommendations of the Georgia Extension Service. The soil was a Tifton Sandy Loam (a fine loamy-siliceous, thermic Plinthic Kandiudults) with a pH of about 6.5. Fruit with a wide range of sizes (5–250 g) were harvested and taken to the laboratory within 15 min after harvest. Fruit were classified as immature or mature (green, turning, or red). Mature green fruit were firmer and heavier (>100 g) compared to immature fruit (<100 g). Mature fruit with 0–30% of a shade of red were designated as ‘turning’, while fruit with > 30% of shade of red were designated as ‘red’.
Measurement of fruit diameter, length, weight, and surface area
Diameter (D), length (L), and weight (FW) were measured in fruit of a range of sizes. Models based on measurements of D, L, and FW were evaluated as a means to estimate fruit surface area non-destructively. The actual surface areas (SA) of fruit skin and calyx were measured using a photocopy of the skin peelings and calyx portions.10 Whole-fruit SA was calculated as the sum of skin SA plus calyx SA.
Measurement of water loss rate, permeance, and firmness
Fruit were placed on trays (12 fruit per tray) and kept in a controlled-temperature room at 20 °C (vapor pressure difference, VPD = 1.50 kPa) and air velocity of < 0.2 m s−1. Fruit water loss was measured gravimetrically on individual fruit. Fruit were weighed daily for 7–10 days. The rate of water loss (WLR) was determined as a daily percent weight loss of the fruit with respect to the fruit weight the day before each measurement. The WLR and transpiration ratio or permeance to water vapor (PH2O) were calculated as
where ΔFW is the change in fruit FW (g) and t is the time period (day) between two consecutive fruit FW determinations; FWo is fruit FW at the beginning of the weighing period; 6.43 × 10−3 µmol m−2 s−1 is a conversion factor; SA is whole-fruit surface area (m2); and VPD is water vapor pressure deficit (kPa) under storage. Mean values of WLR and PH2O were calculated for individual fruit from measurements made over a 7- to 10-day period. WLR values were normalized by expressing them as a function of VPD.
Values of WLR and PH2O for the skin and calyx were determined in immature fruit and in fruit at the mature green (MG), turning and red stages. To estimate WLR and PH2O of the skin, petrolatum was applied to the calyx and pedicel of the fruit.10 The water loss of fruit whose calyces were covered with petrolatum was attributed to transpiration only through the skin. Calyx water loss was calculated as the difference between the mean whole-fruit water loss minus skin fruit water loss. Permeances to water vapor of skin and calyx were calculated from their respective WLRs and SAs. Fruit firmness was measured hedonically using a 1–5 scale (1 = spongy soft; 2 = soft; 3 = firm soft; 4 = moderately firm; 5 = firm).
For the determination of allometric attributes, water loss rate and permeance to water vapor of fruit of a range of sizes, the experimental design was a completely randomized, where individual fruit were the experimental units. For determination of the effect of stage of ripeness on WLR and PH2O, the experimental design was a randomized complete block with 10 replications, where the block was a carton box containing 12 fruit. For the determination of the effect of storage time on WLR, fruit firmness and PH2O, the experimental design was a randomized complete block with eight replications. Experiments were repeated twice. Data were analyzed using the GLM, NLIN and REG procedures of SAS (SAS Inst., Inc., Version 9, Cary, NC, USA). The means were separated by the Fisher's protected LSD test.
Relationships among fruit diameter, length, weight, and surface area
Fruit diameter (D) and fruit length (L) were both correlated with fruit fresh weight (FW), although D (FW = 2.464D2 − 20.27; R2 = 0.954; P < 0.01; n = 1024) was a better estimator of FW than L (FW = 25.48L − 88.24; R2 = 0.756; P < 0.01; n = 1024). The relationship between L and D was: D = 0.964L + 0.81 (R2 = 0.698; P < 0.01).
Various fruit allometric properties were evaluated as a means to non-destructively estimate fruit SA. Fruit SA fit a quadratic relationship with fruit FW (SA = − 0.0026 FW2 + 1.767 FW + 23.06; R2 = 0.985; P < 0.01; n = 164) and linear relationships with D (SA = 39.56D − 103.1; R2 = 0.952; P < 0.01; n = 164) and L (SA = 31.95L − 71.81; R2 = 0.827; P < 0.01; n = 164). The measurements of D and L were used to calculate SA assuming the fruit was a cylinder (SA = 3.142DL) or a prism (SA = 4DL). Fruit SA calculated on the basis of a cylinder produced values that were within 2% of the actual values of SA, while SA calculated on the basis of a prism overestimated actual SA by 30%. Fruit SA estimated using the Dversus SA relationship had a correlation coefficient (r = 0.976) that was similar to that of fruit SA estimated on the basis of a cylinder (r = 0.981). Thus, fruit SA of bell pepper fruit may be estimated by using either fruit FW or D. Fruit L was not a sensitive estimator of either FW or SA. Fruit SA/FW ratio decreased logarithmically with increases in FW, with smaller fruit showing marked reduction in the ratio than larger fruit (Fig. 1).
Effect of fresh weight, stage of maturity, and calyx on water loss and permeance of the fruit
Fruit FW, whole-fruit SA, and calyx SA increased as fruit developed (Table 1). Calyx SA increased linearly with increasing fruit SA (r2 = 0.51; P < 0.01). The calyx was found to cover about 2.5–4.1% of total fruit SA. The water loss rate (WLR) and fruit SA/FW ratio were both highest in small, immature fruit, and WLR decreased as ripening proceeded The WLR of individual fruit declined asymptotically with increases in fruit size (Fig. 2). In small fruit (<50 g per fruit), WLR decreased with minor increases in fruit size. In large fruit (>50 g per fruit), however, increases in fruit size had little effect on WLR. Application of petrolatum to the calyx to stop water loss through the calyx reduced fruit WLR by 47% (immature stage), 23% (MG stage), 27% (turning stage), and 28% (red stage) (Table 1). Thus, about 26% of water loss occurred via the calyx of mature fruit. The WLR versus FW relationship was linearized by plotting WLR as a function of the inverse of FW. WLR increased quadratically with increases in fruit SA/FW ratio (Fig. 3).
Table 1. Effect of stage of ripeness on fruit fresh weight, surface area, and water loss rate and permeance to water vapor through the skin and calyx in bell pepper
Permeances to water vapor of the whole fruit and fruit skin were highest at immature stage and decreased with increasing fruit weight and maturity (Table 1 and Fig. 4). Calyx PH2O was also highest at immature stage but showed no change once the fruit reached the MG stage (Fig. 4). In mature fruit, reductions in fruit PH2O as fruit ripened were mostly explained (R2 = 0.998; P < 0.01) by reductions in skin PH2O. Calyx PH2O was about 14 times higher than skin PH2O. The average PH2O of mature fruit were 37 µmol m−2 s−1 kPa−1 (whole fruit), 29 µmol m−2 s−1 kPa−1 (skin), and 398 µmol m−2 s−1 kPa−1 (calyx).
Fruit water loss and firmness during storage
Fruit WLR and PH2O were about constant for the first 6 days in storage and then declined linearly after day 6 with increasing fruit water loss during storage (Fig. 5). Changes in PH2O explained 92% (R2 = 0.92; P < 0.01) of the changes in fruit WLR during storage.
Fruit firmness showed little change during the first 2 days in storage and then declined linearly. In order to describe the changes in firmness during storage, a model was built using the means of fruit FW (163 g; n = 480 fruit), WLR (0.56% d−1 kPa−1), and the mean daily change in fruit firmness (−0.38 d−1) (Fig. 6). The model predicted that the lowest permissible firmness (i.e., firmness = 3) was reached on day 8, after the fruit had lost about 4.5% of their initial weight.
The mean WLR of bell pepper fruit with marketable size was 0.56% d−1 kPa−1. This value is similar to the 0.63% d−1 kPa−1,3 although smaller than the 2.2% d−1 kPa−1.11 In a study with various pepper types, WLR at 20 °C varied from 4.3% d−1 kPa−1 (‘TAM Jalapeño’) to 10.9% d−1 kPa−1 (‘NuMex Conquistador’), with two bell pepper cultivars having WLRs of 6.1% d−1 kPa−1 (‘Mexibell’) and 6.6% d−1 kPa−1 (‘Keystone’).1 The high variability in WLR for bell pepper in the literature suggests that either bell pepper genotypes differ significantly in their fruit transpirational properties, or that there may be errors in the measurement of some of the reported WLR values.18 An example of the large variability in WLR among types or cultivars of the same species is a study that shows that ‘Classic’ eggplants (Solanum melongena) had much lower WLR values (0.6% d−1 kPa−1) compared to ‘Japanese’ eggplants (5.7% d−1 kPa−1).8
Fruit water loss is a function of fruit surface area rather than fruit fresh weight.4 Compared to other fruit of similar shape and size, peppers have a high SA/FW ratio because the fruit are hollow.19 This high SA/FW ratio makes peppers particularly sensitive to fruit water loss. In many postharvest studies, fruit water loss is often expressed as a percentage change in weight (weight loss) over time. However, fruit water loss expressed as a function of fruit FW may be misleading when comparing fruit of different sizes or shapes since fruit with similar FWs may have different surface area/weight ratios.11, 12 In this study, there was a reduction in the SA/FW ratio as bell pepper fruit increased in size (Fig. 1), consistent with previous reports.3, 10, 18, 19 The reduction of WLR with increases in fruit size was probably due, at least partly, to the decreases in the SA/FW ratio, as suggested by the relationship of WLR with the SA/FW ratio. Reductions in WLR with increasing in fruit size have also been found in other vegetable crops.10, 18
In addition to the SA/FW ratio, fruit WLR may also be affected by changes in PH2O during fruit development and ripening. Possibly, changes in the cuticle thickness or composition as fruit ripen affect fruit PH2O. There seems to be a direct relationship between the amount of epicuticular wax on the surface of bell pepper fruit and the PH2O.9, 12, 19 The literature, however, is not clear about the relationship of the amount or thickness of the cuticle with water loss from epidermal surfaces.19, 20 Both WLR and PH2O of bell pepper fruit decreased during storage. Our results are consistent with reports that show a decrease in PH2O during storage of tomato,21 orange,3 and other crops.18, 21 Changes in PH2O explained 92% of the changes in fruit WLR (R2 = 0.92; P < 0.01). The decrease in PH2O over time is probably a result of modifications of the water transport properties of the cuticular matrix due to fruit surface dehydration during storage. Possibly, the water lost from the outermost cells of the fruit and from the cuticle is not replenished from the cells in the interior of the fruit at the same rate as the water is lost from the fruit surface. As fruit surface dehydrates, there is an increased resistance to water vapor diffusion through the cuticle. The effect of fruit surface dehydration was found in a study that showed that water vapor diffuses 50 times faster at high humidity (50% RH, 20 °C) than at low humidity (0% RH, 20 °C) through the surface of tomato fruit.22
Water loss through the calyx or stem scar may be an important route for water loss and can significantly affect fruit quality and postharvest life, as in eggplants, where calyx transpiration accounts for about 65% of whole-fruit transpiration,10 and tomatoes, where transpiration through the stem scar accounts for about 67% of whole-fruit water loss.14 In a recent study on various types of peppers, transpiration through the calyx represented only a small portion of fruit transpiration.19 However, consistent with other studies on solanaceous crops,10, 14, 16, 23 permeance of the calyx or stem scar was higher than skin permeance in bell pepper. Calyx PH2O (398 µmol m−2 s−1 kPa−1) of bell pepper was about 14 higher than skin PH2O, compared to eggplants, where calyx PH2O (310 µmol m−2 s−1 kPa−1) is 18 higher than skin PH2O,10 and tomato, where stem scar PH2O is 1000 higher than skin PH2O.24 The high rate of water loss through the calyx of bell pepper indicates that the calyx must be protected from dehydration in order to prolong fruit postharvest life. Utilization of waxing and natural and synthetic polymeric films to cover the fruit help in reducing fruit water loss and maintaining fruit quality.25 Calyx or stem scar may also be an important route for oxygen, ethylene and carbon dioxide diffusion in fruit. In bell pepper, 80–90% of the diffusion of O2 and CO2 is through the pedicel and stem scar,16 while it is 97% in tomatoes.24
Firmness is an important quality attribute in bell pepper.6 Among marketable fruit, smaller fruit lost firmness faster than larger fruit. This was because of the higher WLR due to the higher PH2O values and higher SA/FW ratios in smaller compared to larger fruit. In apples, the changes in firmness in storage are also related to fruit water loss.26 The output of our model showed that, for a medium-size marketable fruit, the lowest permissible fruit firmness (firmness = 3) was reached on day 8, after the fruit lost about 4.5% of their initial weight. This maximum permissible weight loss of 4.5% in bell pepper is similar to the 5% reported by Wills et al.27 or the 7% reported by Ben-Yehoshua.3
In conclusion, there was a reduction in fruit firmness, rate of water loss and permeance to water vapor of bell pepper with increasing water loss during storage. Permeance to water vapor of fruit skin and the rate of water loss in bell pepper decreased with increasing fruit size and ripeness. Permeance to water vapor of the fruit skin was about 14 times smaller than that of the fruit calyx, and about 26% of the water loss in mature bell pepper fruit occurred through the calyx.
We are graciously thankful to B. Mullinix for statistical assistance and to Drs John Ruter and John Silvoy for reviewing the manuscript. Thanks also to Denne Betrand and David Giddings for technical support and to Gabriel Acuña and Rashard Dawson for their help with field and laboratory work.