Apple tree and fruit responses to shade netting
Thesis (MScAgric (Horticulture))--University of Stellenbosch, 2007.
The production of high quality fruit that meet international standards is of vital importance to the South African fruit industry. Detrimental climatic conditions are forcing producers to examine alternate production methods. In this study the effect of protective netting on apple tree (Malus domestica) physiology, microclimate and fruit quality was investigated to determine the potential of apple production under netting in the Western Cape Province of South Africa. The influence of 20% black shade netting on the photosynthetically photon flux density, leaf and fruit surface as well as air temperature was measured on four blush apple cultivars over a two year period in the Koue Bokkeveld area, in the Western Cape, South Africa. Measurements were also taken on leaf gas exchange, including net CO2 assimilation rate under ambient and saturating CO2 conditions, stomatal conductance, and transpiration rate. A decrease in light conditions of 22 - 31% was measured under black netting thus reducing the amount of light available to the plant significantly. Leaves under the netting were cooler by 1.7 – 3.9°C on a milder day (11 January 2005) and by 4.3 – 6.2°C on a hot day (28 February 2005) than control leaves. Fruit surface temperature under the netting was cooler by 0.7 - 2.9°C on milder days, and by 4.0 - 5.3°C on a hot day (28 February 2005) than control fruit. Significant increases in CO2 assimilation, stomatal conductance and transpiration were found under the netting. Netted treatments also showed a lower leaf vapour pressure deficit and tended to be less water stressed having higher leaf water potential than the control treatments. Differences in leaf nitrogen were not significant between treatments except for ‘Cripps’ Pink’ where non netted treatments had higher leaf nitrogen levels than the netted treatments during the 2004 season. Heat tolerance and the ability to recover after exposure to different temperatures for different lengths of time, was evaluated for the same four blush apple cultivars grown under protective netting. The results showed that fruit taken from the netted treatments had a higher incidence of injury based on chlorophyll fluorescence measurements, and did not recover well at temperatures higher than 45°C. The higher tolerance of the non-netted fruit at 45°C could be due to better acclimation to high light and heat levels than the fruit from the netted treatment. Fruit quality of ‘Royal Gala’, ‘Fuji’, Cripps’ Pink’ and ‘Braeburn’ was evaluated for netted and non-netted treatments as well as between five different crop loads as sun-treatments. It was found that cultivars reacted differently to the altered microclimate under the netting. Significant interaction between net and crop load treatments was found for fruit mass of ‘Royal Gala’ and ‘Fuji’. Fruit mass declined more rapidly with increased crop load under nets than in the open. The net treatment reduced sunburn significantly in all the cultivars except ‘Cripps’ Pink’. Ground colour was found to be positively influenced by netting on ‘Braeburn’ and during 2005 on ‘Royal Gala’. Blush colour was reduced under the netting for ‘Braeburn’ and ‘Fuji’ and unaffected for ‘Royal Gala’ and ‘Cripps’ Pink’. Fruit firmness, total soluble solids and titratable acidity, were reduced under nets for most of the cultivars. Netting thus seems to have a positive effect in reducing sunburn damage but a varying effect on other fruit quality parameters, which seem to be cultivar specific. The influence of black protective netting on vegetative growth was determined by measuring total prunings and trunk circumference. Reproductive bud development analysis was also done. An increase was found in summer prunings for ‘Fuji’ and ‘Cripps’ Pink’ which was most likely due to the topping of the trees to prevent them from growing into the netting. Seasonal trunk growth was affected significantly for ‘Braeburn’ with a higher percentage recorded under the netting. Reproductive bud development was higher for the netted treatments and on trees with lower crop loads. Photosynthetic photon flux density was reduced significantly in a ‘Granny Smith’ orchard at harvest by black, blue and grey type netting. Sunburn was significantly reduced under the black and blue netting. Fruit mass was higher under the netted treatments during the second season of measurements. Firmness and total soluble solids were lower under the netting. Blush colour was found to be significantly lower under the black netting. Seed viability, ground colour, titratable acidity, starch breakdown, stem end russet and the occurrence of Fusicladium pyrorum damage was unaffected by the netting.