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Annals of Applied Biology ISSN 0003-4746

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Climate-ameliorating measures influence photosynthetic gas exchange of apple leaves J. Gindaba & S.J.E. Wand Department of Horticultural Science, University of Stellenbosch, Matieland, South Africa

Keywords Evaporative cooling; leaf temperature; photosynthetic rate; shade net; stomatal conductance; Surround WP. Correspondence J. Gindaba, Department of Horticultural Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. Email: [email protected] Received: 24 August 2006; revised version accepted: 20 November 2006. doi:10.1111/j.1744-7348.2006.00110.x

Abstract Sunburn has become one of the major threats to apple fruit production in South Africa and other countries with Mediterranean climate. Some climate-ameliorating measures have been developed to control sunburn in apples. Effects of the climate-ameliorating measures, viz. evaporative cooling, Surround WP and shade net, on leaf gas exchange of a 5-year-old orchard of ‘Cripps’ Pink’ apple were investigated during hot summer days in Stellenbosch, South Africa. Evaporative cooling increased net photosynthetic rate (A) and stomatal conductance (gs) because of its lowering of leaf temperature and leaf-to-air vapour pressure difference (VPD). Shade net also reduced leaf temperature because of reduction in photosynthetic photon flux density (PPFD). Quantum efficiency of photosynthesis was increased under shade net to compensate for reduced PPFD. Shade net also reduced transpiration rate more than A, resulting in increased midday water-use efficiency. The diurnal trends of A and gs in the Surround WP and control treatments were similar, indicating limited ameliorative impact of Surround WP. Furthermore, Surround WP typically reduced maximum rate of carboxylation and the light-saturated rate of electron transport. In all treatments, A decreased by 70% when leaf temperature increased from 35C to 40C. In conclusion, all treatments affected leaf photosynthetic gas exchange. Evaporative cooling enhanced leaf A and gs because of distinct ameliorative effects on leaf temperature and VPD. Shade net reduced leaf temperature with no consistent effects on leaf gas exchange attributes. Surround WP had limited or no impact on leaf temperature and negatively affected leaf gas exchange attributes.

Introduction Several studies (Wu¨nsche et al., 2001; Glenn et al., 2002; Schupp et al., 2002; Gindaba & Wand, 2005, 2006; Wand & Gindaba, 2005; Wand et al., 2006) have been carried out on sunburn of fruit. Sunburn damage is caused by high summer air temperature and radiation (Glenn et al., 2002; Schrader et al., 2003), the degree of the damage being influenced by cultivar, climate fluctuations, canopy density and orchard management practices (Parchomchuk & Meheriuk, 1996; Schrader et al., 2003). In the Western Cape region of South Africa, the incidence of sunburn in apples (Malus domestica Borkh.) is generally 15–50% (Gindaba & Wand, 2005, 2006; Wand et al., 2006).

Ann Appl Biol 150 (2007) 75–80 ª 2007 The Authors Journal compilation ª 2007 Association of Applied Biologists

Some climate-ameliorating measures, i.e. evaporative cooling, kaolin-based particle film called Surround WP and shade net, have been developed to control sunburn of apple and other fruits. Evaporative cooling involves an over-tree irrigation system to cool fruits when air temperature exceeds threshold for sunburn incidence (Parchomchuk & Meheriuk, 1996). Surround WP is sprayed on the plant canopy to reduce incident radiation on fruits surfaces, thus lowering fruit temperature (Glenn et al., 2002, 2003). Covering the plant canopy with shade cloth reduces solar radiation and therefore lowers fruit surface temperature. It is vital to ensure that any product or technology applied to control sunburn in orchards does not interfere with the gas exchange processes of the leaves that have 75

Climate-ameliorating measures affect leaf gas exchange

direct bearing on productivity or yield. Although kaolin particle film application increased net photosynthetic rate (A) of grapefruit (Jifon & Syvertsen, 2003a) and ‘Empire’ apple leaves (Glenn et al., 2003), studies by Le Grange et al. (2004) and Wu¨nsche et al. (2004) indicate that effects of kaolin on leaf photosynthetic rate lack consistency in apples. Moderate shading increased leaf A and stomatal conductance in grapefruit and navel orange because of reduced radiation and leaf temperature (Jifon & Syvertsen, 2003b; Syvertsen et al., 2003). However, shading of apple orchards generally resulted in reduced A or no effect depending on the degree of shading (Stampar et al., 2001). The main theme of the current study was to compare the effects of the climate-ameliorating measures on physiological processes of leaves under the same orchard conditions. We hypothesised that sunburn control measures improve leaf gas exchange because of their climate-ameliorative effects, but their effectiveness would depend on the extent to which they reduce leaf temperature, leaf-to-air vapour pressure differences (VPD) or light during hot summer days. We also investigated the photosynthetic capacity of leaves that were acclimated to evaporative cooling, Surround WP and shade net by measuring photosynthetic responses of the leaves to increasing CO2 concentration, light and air temperature under orchard conditions.

Materials and methods Study site A 5-year-old orchard of ‘Cripps’ Pink’ apple (M. domestica) at Welgevallen Experimental Farm, Stellenbosch, South Africa (3355#S, 1853#E), was used during the 2003– 2004 growth season. The trees were grafted on M793 rootstock, planted in NE–SW row orientation at a spacing of 3.8 m 1.25 m and trained to a central leader on a three-wire system. Tree canopies were relatively open, and there was minimal self-shading. The trees were irrigated with microjet sprinklers scheduled using neutron moisture probe measurements. Standard orchard management practices were applied as for commercial orchards in the region. Experimental design and treatments A randomised complete block design with six blocks and four treatments was used. One tree per block was assigned to each of the four treatments: evaporative cooling, Surround WP, shade net and control. The evaporative cooling system used over-tree sprinkler jets over each row, spaced every 2.5 m along a suspended pipe to cover a radius of about 1.5 m, and with a discharge rate of 28 Lh21 (4.5 mmh21). The system was activated for 5 min and closed 76

J. Gindaba & S.J.E. Wand

for 15 min at air temperatures of 30C and above between 06:00 and 18:00 h, and above 22C between 18:00 and 21:00 h from 10 December 2003 to 20 April 2004. Black shade net manufactured to intercept 20% solar radiation was installed around individual trees on 8 December 2003. Surround WP (Engelhard Corporation, Iselin, NJ, USA) was applied at rates of 6% and 3% on 10 December 2003 and 16 January 2004, respectively. A surfactant (Agral) was used at 20 mL 100 L21 water. The whole tree was sprayed from both sides and top for uniform coverage using a hand-held spray apparatus. No treatments were applied to the control trees. Gas exchange measurements Net photosynthetic rate (A), stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (E), air and leaf temperatures, leaf-to-air VPD and photosynthetic photon flux density (PPFD) were measured using an infrared gas analyser (LI-6400, Li-Cor, Lincoln, NE, USA) under ambient radiation (1700–2100 lmol m22 s21 PPFD) and temperature (34–39C), and cuvette CO2 concentration of 360 lmol mol21 on 16 January, 30 January and 10 February 2004. All measurements were made three times on two sun-exposed spur leaves per tree at about 1.3 m height on the north-western side of the row during the midday (12:00–14:00 h). For the evaporative cooling treatment, measurements were carried out during the 15-min intervals, as soon as leaves were dry (;5 min after system deactivation). Instantaneous leaf water-use efficiency (WUE) was calculated as the ratio of net leaf photosynthetic rate to transpiration rate and expressed in units of millimoles of CO2 per mole of H2O. Diurnal gas exchange measurements were made under ambient light and temperature conditions, and a cuvette CO2 concentration of 360 lmol mol21 on sun-exposed spur leaves on 16 January 2004. Photosynthetic light and CO2 responses were measured at 25C air temperature and 1.0–1.5 kPa VPD soon after treatments (12 and 17 December) and late during the season (10 and 12 March) on sun-exposed spur leaves during midday. An internal red/blue LED light source provided different light levels and CO2 concentration set at 360 lmol mol21. Light-response data were fitted to a model of nonrectangular hyperbola (Prioul & Chartier, 1977) to estimate light-saturated rate of net photosynthetic rate (Amax) and apparent quantum efficiency. Photosynthetic CO2 response was measured at 1500 lmol m22 s21 PPFD and different cuvette CO2 concentrations obtained using the LI-6400 CO2 injection system. The maximum rate of carboxylation by Rubisco (Vcmax) and the light-saturated rate of electron transport (Jmax) were estimated using the mechanistic model Ann Appl Biol 150 (2007) 75–80 ª 2007 The Authors Journal compilation ª 2007 Association of Applied Biologists



Results Diurnal and midday gas exchange No distinct midday depression in leaf A was observed for evaporative cooling unlike that for the other treatments (Fig. 1A). Lower A was observed for shade net compared to evaporative cooling except during the morning. Stomatal conductance (gs) followed a similar diurnal trend as A in all treatments (Fig. 1B). Evaporative cooling increased leaf gs during midday and early afternoon compared to the other treatments. Measurement of midday gas exchange on three hot summer days indicated that treatments had significant variations with evaporative cooling showing increased midday A and gs (Table 1). Reduced PPFD under shade net resulted in reduced leaf temperature, which in turn reduced transpiration rate (E) and increased WUE. Evaporative cooling also reduced leaf temperature and leaf-to-air VPDs because of heat removal from the leaf surface and increased humidity around the leaf. All the sunburn control measures increased temperature differences between the air and the leaf (T) compared to control. Photosynthetic light and CO2 responses Photosynthetic light and CO2 responses were measured under milder atmospheric conditions (25C and 1.0–1.5 kPa VPD) early and late during the season (Table 2). Treatments did not affect light-saturated photosynthetic rate (Amax) during the early season. However, Surround WP and shade net reduced Amax after acclimation to treatment compared to evaporative cooling and control treatments. Shade net significantly increased quantum efficiency of leaves, possibly to compensate for reduced radiation. Surround WP reduced light-saturated rate of electron transport (Jmax) and maximum rate of carboxylAnn Appl Biol 150 (2007) 75–80 ª 2007 The Authors Journal compilation ª 2007 Association of Applied Biologists

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proposed by Farquhar et al. (1980) and modified by Harley et al. (1992). The response of A and gs to increasing leaf temperature (20C, 25C, 30C, 35C and 40C) was measured on sun-exposed spur leaves at cuvette CO2 concentration of 360 lmol mol21 and radiation of 1500 lmol m22 s21 PPFD on 24 and 25 March. The VPD was maintained at 1.5 kPa for leaf temperatures of 20–30C and at 2.5 kPa for leaf temperatures of 35C and 40C. Different VPDs were used because it was not possible to achieve the higher leaf temperatures without correspondingly increasing the VPD. All statistical tests were made using SAS (version 6.12). Data were subjected to ANOVA to test the significance of differences between means.

0.3

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Time (HR) Figure 1 Mean (±SE, n = 6) diurnal (A) net photosynthetic rate, (B) stomatal conductance of ‘Cripps’ Pink’ apple leaves grown under evaporative cooling (EC), Surround WP (SU), shade net (SN) and control (CO) during 2003–2004 season. Measurements were taken on 16 January 2004 on exposed leaves under ambient conditions of temperature, photosynthetic photon flux density, humidity and a CO2 concentration of 360 lmol m22 s21.

ation (Vcmax) during both periods, while shade net reduced Jmax and Vcmax only during the late season. Photosynthetic temperature response A and gs responses to increasing leaf temperature are shown in Fig. 2. A increased with increasing leaf temperature up to 35C and decreased 70% when leaf temperature increased from 35C to 40C in all treatments. gs increased with increasing leaf temperature up to 30C under evaporative cooling, control and Surround WP, and it decreased above 30C. Except at 20C, lower values of A and gs were observed for shade net leaves compared to all the other treatments.

Discussion Apples in the Western Cape province of South Africa suffer from sunburn during hot summer days. Sunburn damage of ‘Cripps’ Pink’ apple was estimated to 15–27% during 2003–2004 and 2004–2005 seasons (Gindaba & Wand, 2005; Wand et al., 2006). Sunburn loss could be 77



Table 1 Mean (±SE, n = 6) midday gas exchange attributes of ‘Cripps’ Pink’ apple leaves, air and leaf temperatures, differences in air and leaf temperature (T), air-to-leaf VPD and PPFD, as affected by evaporative cooling, Surround WP, shade net or no treatment during the 2003–2004 season. Measurements were made on three hot summer days (16 and 30 January and 10 February) and averaged Attributes

Control 22

21

A (lmol m s ) gs (mol m22 s21) Ci (lmol mol21) E (mmol m22 s21) WUE (mmol mol21) Air T (C) Leaf T (C) T (C) VPD (kPa) PPFD (lmol m22 s21)

17.9 0.26 203 9.0 2.1 36.6 36.7 20.1 3.3 1861

Evaporative Cooling 0.6 0.016 7.9 0.39 0.09 0.4 0.2 0.25 0.13 35

20.9 0.33 208 9.4 2.3 36.7 35.2 1.5 2.9 1906

Surround WP

0.5 0.013 7.9 0.32 0.10 0.4 0.3 0.23 0.08 19

18.1 0.27 206 8.4 2.2 36.8 35.9 0.9 3.1 1882

Shade Net

0.4 0.011 7.8 0.27 0.08 0.3 0.3 0.17 0.10 47

18.3 0.26 195 7.0 2.9 36.9 35.3 1.6 3.4 1231

P-Value

0.6 0.013 8.6 0.41 0.19 0.3 0.3 0.20 0.11 59

0.000 0.001 ns 0.000 0.000 ns 0.002 0.000 0.022 0.000

WUE, water-use efficiency; VPD, vapour pressure difference; PPFD, photosynthetic photon flux density; ns, not significant, P > 0.05.

45–56% in sensitive cultivars such as ‘Granny Smith’ and ‘Fuji’ (Wand et al., 2006). Climate-ameliorating measures have shown considerable promise not only to reduce sunburn but also in terms of modifying orchard microclimate for leaf gas exchange. Evaporative cooling and shade net significantly reduced leaf temperature, resulting in significant reduction of VPD. During evaporative cooling application, the water directly cools the leaves, fruit and the orchard air as it evaporates. The degree of cooling depends on the rate of water application and the local climate conditions (Evans, 2004). Therefore, the application of evaporative cooling to reduce fruit sunburn in apples enables the plant to keep its stomata open and assimilate more CO2 during hot summer days compared to the application of shade net, Surround WP or no application. The result may be improved fruit size or solids content as reported by Unrath & Sneed (1974), Parchomchuk & Meheriuk (1996) and Gindaba & Wand (2005). Therefore, increased A and gs in evaporative cooled leaves was the result of reduced leaf temperature that lowered VPD. Former studies have established an inverse relationship between VPD and gs,

i.e. as VPD declines stomata tend to remain open (Jifon & Syvertsen, 2003b). Shade net increased midday WUE during midseason compared to control plants because shading reduced the rate of transpiration. However, shaded leaves had lower Amax because of a lower Jmax and Vcmax during the late season, in agreement with former studies that reported lower Amax for shade leaves compared to sun leaves (Evans, 1996; Lambers et al., 1998). In our trial, shaded leaves had higher apparent quantum efficiencies during both early and late seasons, suggesting the possibility of increased efficiency or concentration of the light-harvesting chlorophylls in shade leaves. Surround WP increased canopy photosynthetic rate in ‘Empire’ apples (Glenn et al., 2003) and A in ‘Red Chief’ and ‘Red Spur Delicious’ (Glenn et al., 2001) under higher radiation and air temperature but decreased in other studies (Le Grange et al., 2004; Wu¨nsche et al., 2004). However, the current result showed that Surround WP did not significantly affect midday leaf A in ‘Cripps’ Pink’ apple under midseason conditions of high air temperature and radiation compared to control

Table 2 Light-saturated net photosynthetic rate (Amax), apparent quantum efficiency, maximum electron transport rate (Jmax) and maximum rate of carboxylation (Vcmax) of ‘Cripps’ Pink’ leaves treated with evaporative cooling, Surround WP, shade net and control as measured during the early (12 and 17 December) and late (10 and 12 March) season. Measurements were made at air temperature of 25C and vapour pressure difference of 1.0–1.5 kPa Attributes

Season

Control

Amax (lmol m22 s21)

December March December March December March December March

18.5 20.0 0.052 0.064 186 144 65 53

Quantum efficiency (mol mol21) Jmax (lmol m22 s21) Vcmax (lmol m22 s21)

78

Evaporative Cooling 1.10 0.77 0.002 0.002 7.2 8.6 0.9 3.9

20.0 19.0 0.056 0.067 192 144 65 50

0.46 0.95 0.001 0.000 7.0 5.7 1.5 1.2

Surround WP 16.8 15.0 0.048 0.060 152 108 56 39

1.11 1.00 0.0 0.001 11.4 14.6 3.5 6.1

Shade Net 18.0 13.3 0.058 0.074 193 107 65 37

0.39 1.34 0.001 0.001 32.3 8.4 2.5 3.9

P-Value 0.027 0.001 0.006 0.000 0.028 0.018 0.035 0.033




A(µmol m-2 s-1)

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Leaf temperature (°C) Figure 2 Mean (±SE, n = 6) temperature response of (A) net photosynthetic rate (A) and (B) stomatal conductance (gs) of ‘Cripps’ Pink’ apple leaves acclimated to evaporative cooling (EC), Surround WP (SU), shade net (SN) and control (CO) during 2003–2004 season. Measurements were made at CO2 concentration of 360 lmol mol21, photosynthetic photon flux density of 1500 lmol m22 s21 and vapour pressure differences of 1.5 kPa (20–30C) and 2.5 kPa (35–40C).

plants. Abou-Khaled et al. (1970) reported significant reflection of visible light by kaolinite mineral with resulting reduction of A at low light intensities. However, Glenn et al. (1999) reported 90% light transmission in processed kaolin particle film with possible reduction depending on the amount of residue on the leaf. In the current study, Surround WP did not significantly reduce leaf temperature and improve WUE compared to untreated control unlike the reports of Abou-Khaled et al. (1970) using a kaolinite mineral. Glenn et al. (2001) discussed the possibility of lower WUE in Surround WPsprayed plants due to increased gs and hence transpiration. However, we did not observe significant increase in either gs or transpiration rate in Surround WP-treated plants compared to the control. Although Surround WP did not significantly affect midday leaf A and gs during the midseason, compared to control plants, it significantly reduced leaf Amax, Vcmax and Jmax under milder atmospheric conditions during the early and late seasons (at 25C leaf temperature and 1500 lmol m22 s21 PPFD), indicating the suppressive effect of Surround WP on the photosynthetic capacity of


the leaf. The observed leaf Jmax and Vcmax were, however, within the estimated ranges for fruit trees (Wullschleger, 1993). The lower Vcmax and Jmax observed during the late season in all treatments could be due to leaf ageing or possibly a decline in sink strength towards harvest (Palmer, 1986; Pretorius & Wand, 2003). Although the response of A to temperature in apple leaves generally shows optima ranging from 25C to 30C (Lakso, 1994; Lakso et al., 1999; Pretorius & Wand, 2003), we observed a range of 30–35C possibly because of acclimation of leaves to high ambient temperature in this region compared to other apple-growing regions. A decreased when air temperature was above 35C in all treatments, in accordance with the results of Lakso (1994). Decreases in gs with increasing leaf temperature are often attributable to increased VPD (Lakso, 1994; Pretorius & Wand, 2003). According to Lakso (1994), A probably controls gs in apple leaves. Nevertheless, the decrease in gs could not fully account for the observed 70% drop in A at 40C leaf temperature. Thus, the reduction of A at high temperature was the result of increased nonstomatal limitation (Pretorius & Wand, 2003). In conclusion, the use of evaporative cooling to reduce fruit sunburn in ‘Cripps’ Pink’ apples during hot summer days reduced both leaf temperature and VPD, resulting in increased leaf A and gs. Shade net also reduced leaf temperature, maintained high midday WUE and increased quantum efficiency to compensate for low light under the shade cloth. Surround WP had limited or no impact on leaf temperature, VPD and midday gas exchange under high radiation and temperature. However, Surround WP and shade net reduced late-season photosynthetic capacity (Vcmax and Jmax) under relatively cooler atmosphere.

Acknowledgements We are grateful to the South African Deciduous Fruit Producers’ Trust, the National Research Foundation, the University of Stellenbosch and the Lombardi Trust for financial support; Mr M. du Toit for managing the orchard and evaporative cooling system; and Dr W.J. Steyn for his contribution to establishing the evaporative cooling system. Engelhard Corporation provided the Surround WP.

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