1. Introduction

1. Introduction

Netting is used to protect agricultural crops from excessive solar radiation (shade nets), environmental hazards, or pests (Sivakumar and Jifon, 2018). Black nets are used for shading, while clear, transparent nets are used to protect from abiotic environmental hazards or from biotic pests. Both net types do not alter the spectral composition of light reaching the plants.

In the last decade or so, the photoselective shade netting has become an emerging approach, which introduces further additional benefits, in addition to the mere protective functions of the netting (Shahak et al., 2009; Shahak, 2014). Photoselective shade nets are unique in that they both spectrallymodify, as well as scatter, the transmitted light. The photoselective nets include “colored-ColorNets” (e.g., Red, Yellow, Green, Blue net products) as well as “neutral-ColorNets” (e.g., Pearl, White and Grey) absorbing spectral bands that are shorter or longer than the those in the range of visible light.

Spectral manipulation is aimed at specifically promoting photomorphogenetic-physiological responses, while light scattering improves light penetration into the inner canopy (Ilic and Fallik, 2017).

Modern shade nets are manufactured from woven polypropylene or knitted polyethylene materials, with different dimensions of fibers and holes to achieve specific shade levels. The fraction of light that passes through the holes in the shade cloth remains unchanged in its quality, while the light hitting the threads is spectrally modified and scattered on exit. Depending on the thread pigmentation and knitting design with different fibers and variable density to create specific shade indices, photo-selective colored shade nets provide diverse mixtures of natural, unmodified light and scattered, spectrally modified light (Appling, 2012).

Compared with black shade nets with the same shading factor, as determined by the photosynthetically active radiation (PAR), red and yellow nets were found to specifically stimulate the vegetative growth rate and vigor of foliage and cut-flower crops, while the blue shade nets caused dwarfing. The grey shade nets enhanced branching and bushiness, in conjunction with reduction of leaf size and variegation due to their distinct absorption in the IR range (Ilic and Fallik, 2017).

When blue and red light were increased, plants exhibited improved photosynthesis and enhanced productivity due to chlorophylls a and b (Sivakumar et al., 2017). That was achieved by using red and blue shade nets.

However, photoselective nets can influence the plant’s growth and development already in the nursery.

Citrus seedlings grown under red net had improved seedling diameter, internodal length, leaf area and number, shoot dry weight and budding success. Higher nitrogen, phosphorus, potassium and zinc levels were recorded in the leaves of rough lemon and mandarin plants grown under red shade net. The plant height of rough lemon seedlings was positively correlated to internodal length, budding success and leaf area as well as leaf phosphorus and iron levels.

The highest number of buddable seedlings were recorded in open conditions but, budding success was greater in a red shade net house.