Coloured hailnets alter light transmission, spectra and phytochrome, as well as vegetative growth, leaf chlorophyll and photosynthesis and reduce flower induction of apple

In view of alleged positive effects of coloured (red) hailnets on phytochrome, photosynthesis, yield and fruit quality, the objective of the present work was to investigate a range of red and green hailnets using apple as a model crop with cvs. ‘Pinova’ and ‘Fuji Kiku 8’. Light transmission of green or red hailnets peaked between 500 and 570 nm (green) or above 570 nm (red–orange) and was reduced by 12% (white) or 14% (red–white), 18% (red–black) and 23% (green–black) hailnets; there were no effects on phytochrome. Leaf chlorophyll concentration increased under coloured hailnets by up to 46% under the green–black hailnet, while air temperature was reduced by 0.2°C. Under sunny conditions, photosynthesis of ca. 18 μmol CO2 m−2 s−1 was not reduced under coloured hailnets, in contrast with a 21% reduction under cloudy conditions with a concomitant reduction in transpiration by 13%. Vegetative growth was affected in different ways: shaded trees showed smaller trunk diameter, but increased the number and length of their 1-year shoots under coloured hailnets, particularly with cv. ‘Fuji’ when grown under green–black hailnet. Hailnets reduced flower induction in June and return bloom in the next spring to the same extent as they reduced the light transmission. Overall, tree growth under coloured hailnets was genetically influenced, with cv. ‘Fuji’ being more prone and sensitive to adverse effects of coloured hailnets than cv. ‘Pinova’, but is also influenced by the environment.