Chlorophyll levels decreased as a result of sunburn in all apple cultivars studied ( Felicetti and Schrader, 2009). Photo-oxidative browning can occur at much lower peel temperatures (less than 30 ☌) when fruit peel is suddenly exposed to high irradiance resulting in photobleaching followed by necrosis ( Felicetti and Schrader, 2008). Sunburn necrosis occurs as a result of thermal death of epidermal and subepidermal cells that leads to formation of a necrotic spot and is caused by extreme fruit surface temperature (52 ± 1 ☌), whereas sunburn browning is a result of high fruit surface temperatures (46 to 49 ☌) in the presence of light ( Schrader et al., 2008).
These blemishes occur in high light environments ( Tustin et al., 1988 Warrington et al., 1996), where high light in combination with low or high temperatures initiates the synthesis of red anthocyanins or yellow carotenoids, respectively. In South Africa, sunburn and red blush may decrease Class 1 fruit by 35% and 20%, respectively (Griessel, personal communication). Hence, ways have to be found to increase the yield of Class 1 fruit. Although older GS orchards are being replaced with more profitable cultivars, GS will remain an important component of the South African industry for the time being. Fruit with sunburn or red blush are downgraded to Class 3 and will not pass for export. Apart from realizing lower returns, the proportion of Class 1 fruit from GS trees is lower compared with other cultivars as a result of zero tolerance for red blush, sunburn blemishes, and whitening of the peel. However, the total area of GS planted in South Africa has decreased by almost 15% during the last 15 years as a result of competition from more lucrative blushed cultivars.
‘Granny Smith’ (GS) is the most widely planted apple cultivar in South Africa, accounting for 25% of apple plantings in 2008 ( Deciduous Fruit Producers’ Trust, 2009). Keywords: sunburn red blush chlorophyll fruit quality peel temperature GS may benefit significantly from the installation of shade netting if combined with rigorous pruning and vigor control. In conclusion, maximum chlorophyll synthesis and dark green color require an open canopy during the first half of fruit development, whereas shading is necessary during the latter half of fruit development to avoid the occurrence of sunburn, red blush, and photothermal destruction of chlorophyll.
#GRANNY SMITH APPLE COLOR FULL#
The 10% darkest green fruit received moderately high irradiance (25% to 45% of full sun PPF) during early fruit development (until ≈80 DAFB) but became progressively shaded (3% of full sun PPF) during the latter half of the season. Deeply shaded inner canopy fruit received ≈2% of full sunlight, had low chlorophyll concentrations, and were lighter green in color. Partially shaded fruit from the southern side of east–west rows received ≈5% of full sunlight and had the highest chlorophyll concentrations and darkest green color at harvest. Only 22% to 39% of exposed fruit from the outer canopy did not develop sunburn or red blush. A high proportion of exposed fruit from either side of the row developed red blush. Exposed fruit from the northern side of east–west rows received the highest irradiance throughout the season and had the highest fruit surface temperature (on average 5 ☌ above ambient). Good green color at harvest relied on exposure of fruit to high irradiance at this stage because 50% shading between 14 and 56 DAFB significantly decreased dark green color at harvest.
During early fruit development, chlorophyll concentrations were the highest in fruit from higher light environments. This study was conducted to investigate the relationship between canopy position and external fruit quality with the ultimate aim to devise pruning and training strategies to maximize export yield. However, production of GS is becoming unprofitable as a result of a high incidence of sunburn, red blush, and pale green fruit that decreases the proportion of Class 1 fruit that is suitable for export to more lucrative markets. The dark green apple cultivar, Granny Smith (GS), makes up 25% of the South African apple industry.
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