Comparison of climate ameliorating measures to control sunburn on 'Fuji' apples

A 12-year-old 'Fuji' apple orchard in the Western Cape Province, South Africa, was treated with over-tree evaporative cooling (EC), RAYNOX® (RY), EC + RAYNOX® (RE), 20% white shade net (SN), or Surround® WP (SU) to investigate their effects on sunburn and fruit quality compared to the untreated control (CO) during the 2004-2005 season. When midday air temperature was 30-32°C, fruit surfaces were on average 5.6 (SN), 4.6 (EC), 3.8 (RE) and 2.4°C (SU) cooler than CO fruit. RY did not significantly reduce fruit skin temperature compared to the CO on all measurement days. Total sunburn incidence was 34.3% for the CO. All treatments significantly reduced sunburn browning (Class 2), necrosis (Class 3) and total sunburn compared to the CO. Shade net was most effective, followed by RE, EC, SU and RY, in order of decreasing efficacy. Sunburn browning contributed about 78% to total sunburn in the CO but over 90% in the sunburn control treatments. SN reduced blush color significantly. EC and RE increased fruit mass and diameter while under SU they were reduced compared to the CO and other treatments. However, EC and RE fruit had lower firmness than CO fruit on the shaded side. Starch degradation pattern was reduced by SN and RE, while soluble solids concentrations and titratable acid concentrations were not affected. In conclusion, all control measures can potentially reduce sunburn although none would eliminate it. Availability of water for EC, and financial feasibility are factors which will influence the choice of technology for each farm.Conference Pape

Comparative effects of evaporative cooling, kaolin particle film, and shade net on sunburn and fruit quality in apples

We investigated the effects of evaporative cooling (EC), kaolin particle film (KP) and 20% shade net (SN) on the control of sunburn, fruit temperature amelioration and fruit quality of 'Cripps' Pink' and 'Royal Gala' apples [Malus domestica Borkh.] under orchard conditions during the 2003-04 season in Stellenbosch, South Africa. On days with maximum air temperatures of 34 to 37°C, SN fruit were 5.4 to 9.7°C cooler, EC fruit were 3.1 to 5.8 °C cooler and KP fruit were 1.5 to 6.4°C cooler compared to the control (nontreated, CO) fruit. SN was effective in reducing fruit temperature from mid-morning until midafternoon; KP was most effective during late morning and early afternoon but not at midday; EC was effective from late morning on days when EC was activated. SN, followed by KP, was the most effective technique for controlling sunburn in fruit of both cultivars, with EC being less effective. The different technologies reduced fruit blush color compared to the CO treatment, with SN showing the most reduction and EC the least. EC increased fruit mass compared to all other treatments in 'Royal Gala', and also increased fruit diameter and mass compared to CO in 'Cripps' Pink'. We conclude that under the high radiation levels experienced in South African apple production areas, technologies which reduce irradiance as well as fruit temperature (KP, SN) are more effective in reducing sunburn than those which only reduce fruit temperature (EC). However, radiation-reducing technologies are potentially detrimental to color development on blushed apples.Articl

Late-season stomatal sensitivity to microclimate is influenced by sink strength and soil moisture stress in 'Braestar' apple trees in South Africa

In the Western Cape region of South Africa, mild weather during the late season is conducive to continued high rates of photosynthesis in apple trees. During the post-harvest period, both stressful environmental conditions and altered tree carbohydrate demand (sink strength) may influence stomatal function and thus gas exchange and tree water requirements. In this study we investigated the interactive effects of the main atmospheric determinants of stomatal conductance (light, leaf temperature, leaf-to-air vapour pressure deficit (VPD)) in response to fruit removal at harvest and soil moisture stress after harvest (cessation of irrigation). An 8-year-old 'Braestar' orchard was used. Preand post-harvest (pre-senescent) photosynthetic light response curves were generated at leaf temperatures of 20-40°C. Stomatal conductance showed increased sensitivity to increasing leaf temperature after harvest, resulting in a reduction in the optimum temperature for photosynthesis from 25-30 to 20-25°C. This ensured the maintenance of high instantaneous leaf water use efficiency (WUE) post-harvest at all temperatures, compared to reductions in WUE but higher photosynthetic rates at high leaf temperature pre-harvest. We suggest that this reflects a change from a carbohydrate-demanding strategy (high sink demand) pre-harvest, to a water-conserving strategy post-harvest when sink demand is reduced. Following harvest, daily irrigation was either shut off or continued for 11 days, and light-saturated gas exchange measured regularly at varying levels of VPD in droughted trees. Photosynthetic rates, stomatal conductances and intercellular CO2 concentration (ci) remained relatively constant for the first 6 days, followed by reductions linked to a period of high ambient atmospheric evaporative demand and subsequent partial recovery. Droughted and non-droughted trees showed similar gas exchange values at the end of the droughting period when measured at 25°C, but gas exchange of droughted trees remained repressed when measured at 30°C due to increased non-stomatal limitation. Throughout the study, only cuvette VPD levels above 3.5 kPa (measured at 30°C) significantly reduced gas exchange, with no interaction with duration of droughting. This study showed that post-harvest stomatal control was more attuned to ambient atmospheric evaporative demand in the orchard than to slowly developing soil moisture stress. We conclude that both atmospheric factors and changing sink demand influence stomatal control of apple leaves post-harvest, and that this should be taken into account when determining irrigation strategies. © 2003 Elsevier Science B.V. All rights reserved.Articl

Comparative effects of evaporative cooling, kaolin particle film, and shade net on sunburn and fruit quality in apples

We investigated the effects of evaporative cooling (EC), kaolin particle film (KP) and 20% shade net (SN) on the control of sunburn, fruit temperature amelioration and fruit quality of 'Cripps' Pink' and 'Royal Gala' apples [Malus domestica Borkh.] under orchard conditions during the 2003-04 season in Stellenbosch, South Africa. On days with maximum air temperatures of 34 to 37°C, SN fruit were 5.4 to 9.7°C cooler, EC fruit were 3.1 to 5.8 °C cooler and KP fruit were 1.5 to 6.4°C cooler compared to the control (nontreated, CO) fruit. SN was effective in reducing fruit temperature from mid-morning until midafternoon; KP was most effective during late morning and early afternoon but not at midday; EC was effective from late morning on days when EC was activated. SN, followed by KP, was the most effective technique for controlling sunburn in fruit of both cultivars, with EC being less effective. The different technologies reduced fruit blush color compared to the CO treatment, with SN showing the most reduction and EC the least. EC increased fruit mass compared to all other treatments in 'Royal Gala', and also increased fruit diameter and mass compared to CO in 'Cripps' Pink'. We conclude that under the high radiation levels experienced in South African apple production areas, technologies which reduce irradiance as well as fruit temperature (KP, SN) are more effective in reducing sunburn than those which only reduce fruit temperature (EC). However, radiation-reducing technologies are potentially detrimental to color development on blushed apples.Articl

Apple leaf gas exchange measurements in 'brookfield gala' trees under three water and nutrient management strategies

In this study, the use of water and nutrient management strategies through single (fertigation) and more frequent (pulsating) daily drip systems was compared to the conventional micro jet irrigation system on two rootstocks ('M793' and 'M7') together with hand application of nutrients in the Western Cape Province, South Africa. During the 2004/2005 and 2005/2006 seasons, seasonal gas exchange measurements were made on 2-3 year old 'Brookfield Gala' apple trees under each system, to determine photosynthetic rate, stomatal conductance, transpiration rate, and instantaneous leaf water use efficiency. Pre-dawn and midday leaf water potential measurements were made throughout the season using a pressure chamber. Results indicated higher photosynthetic rates, and higher leaf water use efficiency during the warm summer months in the conventional drip and pulsating drip compared to micro jet irrigation system.Conference Pape

Seasonal gas exchange responses under three different temperature treatments in a leaf-succulent and a drought-deciduous shrub from the Succulent Karoo

Two Succulent Karoo shrubs, an evergreen leaf succulent, Zygophyllum prismatocarpum E. Mey. ex. Sond. and a drought-deciduous species, Tripteris sinuata (DC.) were grown under well watered conditions over 12 months in three greenhouses with distinct temperature characteristics. Temperature differences between greenhouses varied between 3-8°C depending on season. It was predicted that both species would respond to increases in growth (treatment) and seasonal temperature, but that T. sinuata would be more active under favourable conditions and more sensitive to temperature increases than Z. prismatocarpum. Net CO2 assimilation rates (A), stomatal conductances (g(s)), transpiration rates (E) and instantaneous leaf water use efficiency (WUE) were determined in winter, spring and summer. Both species responded to increasing growth (treatment) and seasonal temperature regimes, but their gas exchange characteristics differed significantly. T. sinuata had higher A, g(s), E and WUE. In Z. prismatocarpum, A showed low variability across treatments and seasons, but was slightly higher in the high temperature treatment. Spring was the most favourable season for A and WUE. Conductances increased with increasing treatment temperature in winter, but g(s) was significantly reduced in the high temperature treatment in spring and summer. T. sinuata generally had the highest A in the intermediate and high temperature treatments. Winter was the most favourable season for A and g(s). Conductances decreased with increasing seasonal temperature under all temperature treatments. Both species had the highest transpiration rates in winter. Optimum temperature for photosynthesis was 25-29°C in both species. Although A was not strongly depressed by high temperatures (35-37°C), Z. prismatocarpum had better control of transpiration. Our results are interpreted with respect to climatic differences in the distribution ranges of the two species, and their differing life-history strategies. Implications of predicted climate warming (+1.0-3.5°C in the next 50 years) for Succulent Karoo shrubs are discussed.Articl

Vulnerability and impact of climate change on pear production in South Africa

The Western Cape region of South Africa, with its Mediterranean-type climate and predominantly winter rainfall, has been identified as highly vulnerable to projected climate change within both global and national contexts. Rising temperatures are already detectable and are predicted to increase by a further 1-2°C within the next 30 years, together with decreasing rainfall, especially in winter. Agricultural production will experience a primary impact with resulting socio-economic implications. With plantings of 11,800 ha, pear production contributes 16% to total deciduous fruit production in the region. An analysis of possible impacts of regional projections of climate change on pear production was conducted. Pears are sensitive to the risks posed by extremes in rainfall and temperature and to gradual warming in an already warm production area. Impacts are expected to be felt in both yields and fruit quality, including reduced chilling units and disrupted reproductive processes, increasing incidence of sunburn, poor colour development in some blushed cultivars and higher risk of drought stress in years when water availability is insufficient. Adaptation options, which could include breeding, microclimate amelioration, water conservation methods and climate monitoring systems, were assessed. Opportunities for further research were identified.Conference Pape

Effects of shade netting on gas exchange of blushed apple cultivars

Shade netting over apple orchards can provide protection against sunburn, as well as hail, wind and bird protection. It reduces total incoming radiation, particularly ultraviolet (UV), and increases the proportion of scattered light. However, light reductions could reduce leaf photosynthesis. Trials were conducted over two seasons in the Western Cape Province, South Africa, on the blushed apple 'Braeburn', 'Fuji', 'Royal Gala' and 'Cripps' Pink' using 20% black shade netting which reduced photo-synthetic photon flux density by 22-31%. Fruit surface temperatures were reduced under nets by 0.7-2.9°C on milder days (Tmax 19.0-23.8°C), and by 4.0-5.3°C on a warm day (Tmax 27.4°C). Leaf temperatures were reduced under nets by 1.7-3.9°C on a milder day (Tmax 23.9°C), and by 4.3-6.2°C on a warm day (Tmax 27.4°C). Gas exchange measurements showed that leaf photosynthetic rate was either not affected or was increased, leaf-to-air vapour pressure deficit was reduced, and stomatal conductance was increased under nets. Pre-harvest leaf water potential was generally improved under nets. Maximum light- and CO2-saturated photosynthetic rates and leaf nitrogen concentration were not affected, indicating no long-term acclimation in photosynthetic capacity under shade netting.Conference Pape

Response to elevated CO2 from a natural spring in a C4-dominated grassland depends on seasonal phenology

A South African C4-dominated grassland was exposed to twice-ambient atmospheric CO2 concentration using gas emitted by a natural CO2 spring and distributed over a 7m × 7m plot. A similar control plot was established 20m away at near-ambient CO2 concentrations. Photosynthetic CO2 response curves were performed on three C4 and one C3 grass species under both treatments, in spring (post-fire), mid-summer and autumn. Photosynthetic efficiency of the post-fire dominants, Alloteropsis semialata subsp. eckloniana (C3) and Andropogon appendiculatus (C4) was significantly enhanced in high CO2 only during the early season, when photosynthetic capacity was high. Thereafter, photosynthetic capacity decreased with advancing season in both species, and positive responses to high CO2 were lost or reduced. In the mid- to late-season dominant Themeda triandra (C4), photosynthetic capacity was maintained in elevated CO2 during the mid- to late-season, while decreasing in ambient CO2 relative to high CO2. Eragrostis racemosa (C4), a slow grower, showed no significant responses to high CO2 during the late season. CO2-responsiveness in the field generally followed the seasonal phenologies of each species, being greatest during the most active growth period.Articl