Protected Crops - Horticulture: Plants for People and Places - page 322

growers, traders, processors to the consumer, have their own expectations on the
quality of horticultural products. Furthermore, the aspect of multidimensionality
adds to the complexity and specificity of product quality. For instance, the quality
parameters could be either intrinsic or extrinsic, the quality either external or inter-
nal and the criteria for its evaluation either objective or subjective (Gruda 2005 ).
The chapter outlines the objective, well measurable aspects of quality related to the
reaction of plants under the influence of intrinsic factors expressed in both external
and internal qualities. Thus, it records a quality evaluation based on market, utiliza-
tion, sensory, nutritional and health value of horticultural crop protected products.
The influence of extrinsic factors and the use of subjective criteria are excluded
here.
Finally, the optimization of management in protected cultivations will be
highlighted, in order to yield high quality products, on time, applying energy
savings methods and at minimal expense.
The Radiation Balance of Protected Environments
General
Radiation is essential for crop photosynthesis and hence plant production (Hem-
ming 2011 ). In comparison with open field plant production, light is especially im-
portant for greenhouse crops because the amount of daylight that they receive is
reduced e.g. by 30 % or more by the glasshouse structure or plastic cover (Wilson
et al. 1992 ).
In regions short in radiation protected cultivation is always a compromise be-
tween the required protection and the need to maintain maximum penetration of ra-
diation. Despite new developments such as using new cover materials, changing the
size and the height of greenhouses, using reflective covers on the ground, adapting
the canopy structure and cultivation technologies to promote growth, the light loss
in greenhouses will remain an important issue for the near future. On the other hand,
in climates with supra-optimal radiation, the attenuation induced by the cover may
sometimes be advantageous in avoiding excess heat and sun damage. Nevertheless,
in all cases, the radiative properties of the cover are a significant design feature in
protected cultivation.
Covers have three major effects on the radiation balance of crops: (i) attenu-
ating the amount of light or other electromagnetic radiation (Teitel et al. 2012 );
(ii) increasing the fraction of diffuse radiation that reaches more shaded regions of
the canopy (Hemming et al. 2008 ) and (iii) modifying the light spectrum (see e.g.
Shahak 2008 , for colored screen materials). These three effects depend on several
attributes of the system. The first is the radiative properties of the cover material,
including its reflectance, transmittance and absorbance at different wavelengths
and solar elevation angles (Möller et al. 2010 ). Another property is the structure