Folia Horticulturae

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Solar spectral irradiance in cultivated plants under cover in a submontane region. I. Cucumber plants in the greenhouse

Marian Czarnowski*, Stanisław Cebula**

**The Franciszek Górski Department of Plant Physiology
**Polish Academy of Sciences
**Sławkowska 17, 31-016 Kraków, Poland
**Department of Vegetable Crops
**Faculty of Horticulture, Agricultural University
**29 Listopada 54, 31-425 Kraków, Poland

In winter solar spectral irradiance is characterized by a relatively smaller part of radiation in the blue range and a greater one in the near infrared. During the midday hours on a sunny day the spectral quantum irradiance, measured above the cucumber culture in a submontane region, reached a maximum value of 1.6 μmol m-2 s-1 nm-1 at a wavelength of 752 nm. The irradiance in the range 300-110 nm amounted to 866 μmol m-2 s-1, while the photosynthetically active radiation (400-700 nm, PAR) at that time reached a value of 350 μmol m-2 s-1. The spectral quantum irradiance in the midday hours on a sunny day in June above the greenhouse cucumber culture had a maximum value of 5.2 μmol m-2 s-1 nm-1 at a wavelength of 668 nm. The quantum irradiance (300-1100) amounted to 2910 μmol m-2 s-1, being about 22% lower than that outside the greenhouse. The PAR at that time reached a value of 1300 μmol m-2 s-1.

The transmittance of solar irradiation through the canopy of the cucumber plants depends on the value of the leaf area index (LAI). The lowest PAR transmittance through the canopy was 3-5% (at LAI=2.6-2.9) of the PAR value above the culture. This demonstrated the high PAR absorptance by the cucumber plants. A single leaf revealed high absorptance (75-80%), low reflectance (6-10%), and slightly higher transmittance (8-15%) in the PAR range, while in the near infrared (over 700 nm) the leaves revealed very low absorptance (6-8%), considerable reflectance (about 45%), and transmittance (about 45%). This protects the leaves from overheating and also from inhibition of the process of photosynthesis.

Czarnowski M., Cebula S., 1994. Solar spectral irradiance in cultivated plants under cover in a submontane region. I. Cucumber plants in the greenhouse. Folia Horticulturae VI/2: 3-14.