THE STRUCTURE OF COLOURED HAIL NETS AFFECTS LIGHT TRANSMISSION, LIGHT SPECTRUM, PHYTOCHROME AND APPLE FRUIT COLOURATION

The greater incidence of hailstorms due to climate change results in fruit crops being increasingly grown under white or black hailnets, which adversely affect vegetative and reproductive growth primarily due to light loss.
Recently, coloured, “photo-selective” hailnets came on to the European market, which are supposed to improve photosynthesis, yield, fruit quality and fruit colouration.
The objective of this project was to examine these coloured hailnets and study their plant physiology effects under temperate zone conditions.
White, black, red and green hailnets comprised double twisted longitudinal (parallel to the tree rows) and single transverse high density polyethylene (HDP) fibres of 0.28 mm to 0.33 μm diameter, irrespective of hailnet colour.
Black and green-black hail nets contained double black longitudinal fibres, while white translucent, grey (or red) hail nets contained double longitudinal white translucent (or red) fibres.
White translucent hail nets reduced visible or photosynthetically active radiation (PAR; 400-700 nm) by 8.4%, red-white by 10.7%, grey by 11.6%, green by 13.0%, red-black and black by 16.1%. With green hail nets, light transmission increased by 3% between 500-570 nm (green) and with red hail nets by 5% above 570 nm (orange-red), without affecting the red (666 nm) : far red (730 nm) (R:FR) ratio or the phytochrome system.
The mesh size, i.e., the distance between the fibres, varied from white translucent nets with the largest mesh size of 3 x 9 mm, followed by 3.9 x 6.9 with green-white, 3.3 x 7.7 mm with red-white, 3.5 x 6.5 with green-black, 3 x 8.5 mm with grey as well as red-black and black hail nets both with the smallest mesh of 2.5-3 x 6.5 mm.
These large variations in mesh size between hail nets predominantly influenced their light transmission, which was also affected by the proportion of translucent or black fibres in a hail net.
Hence, calculation of a “geometric light transmission” is proposed.
Overall, the visual colour of a hail net was not indicative of its magnitude of light transmission.
Fruit colouration of the poorly coloured apple cv. ‘Pinova’ followed the proposed geometric light transmission, while that of the late-ripening, well-coloured cv. ‘Fuji Kiku 8’ was sufficient in 2006 and unaffected by hail net colour; fruit yields of the young apple trees were unaffected by net colour.
Black hail nets appear suitable for single-coloured green, or bi-coloured apple cultivars with good colouration, or those otherwise susceptible to sunburn in Southern Europe.
Crystal hail nets (with their translucent fibres, widest mesh size and largest light transmission), or grey hail nets (with twin translucent longitudinal, single black 0.31 mm transverse fibres and 3 x 8.5 mm mesh) appear suitable for apple crops in NW Europe or the US with sunlight deficiency and without risk of sunburn.
Red-white nets appear unsuitable, due to their greater shading effect, than the white hail nets with their translucent fibres and for landscape reasons and for countryside conservation; their proposed photo-selective effects, as reported from Southern countries, are interpreted to be due to a reversal of photo-inhibition under high light intensities and heat in these regions.
Labelling of hail nets with tear and Langley values for UV durability is hence suggested.

I International Symposium on Horticulture in Europe

M.M. Blanke

Malus domestica Borkh., apple, climate change, fruit colouration, fruit quality, hail, hail net, mechanically-induced stress (MIS); phytochrome, photosynthesis, stomata, stress physiology

https://doi.org/10.17660/ActaHortic.2009.817.17