Light spectrum modifications under photo-selective hail-nets

The hail-nets in fruit orchards can influence several environmental factors with specific modification of metabolism and production of plants. The aim of the present work was to study sunlight spectrum modification under six photo-selective plastic hail-nets (yellow, fluo, pearl, red, blue and grey), progressively wide spread in fruit orchards after their introduction in early 2000, in comparison with the most largely used black and neutral nets. Light spectra under the nets were measured 5 times in a fixed structure, with the nets almost perpendicular to solar light, using an array spectrometer. The spectra were measured between 320 and 800 nm, cumulating the irradiance in 40 nm intervals, on clear sunny days of September. Black nets reduced irradiance more than all other nets, while neutral nets the least. Neutral, grey and black nets induced a spectrum like solar light, but with an increasing shadow effect of 7, 20 and 24%, respectively. The photo-selective nets induced an intermediate level of shade (21% blue, 21% red, 18% yellow, 15% fluo and 13% pearl) but with specific differences in light spectra. Blue nets reduced irradiance in the interval between 600 and 720 nm more than all other photo-selective nets. Yellow and fluo nets reduced the irradiance more in the interval between 400-520 nm, while red nets between 400 and 600 nm. The irradiance in the 320-400 nm interval (Ultra-Violet A) was reduced between 4 and 6 W m^‑2 by all the nets (photo-selective and black), except for the neutral net, which reduced light less than 2 W m^‑2 in the same interval. Black nets reduced the irradiance 14 W m^‑2 in the 720 and 800 nm interval, more than all other colored nets, which ranged between 6 and 10 W m^-2. Pearl and neutral nets reduced the least (4 W m^‑2). The red:far red ratio was not significantly modified by any of the photo-selective nets in comparison to that of sun light (1.23).