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Effect of leaf area index on the spectral transmittance of solar radiation in greenhouse cultivation of sweet pepper plants

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
**Agricultural University
**29 Listopada 54, 31-425 Kraków, Poland

The spectral quantum irradiance in the midday hours on a sunny day in summer measured above the greenhouse sweet pepper culture reached a maximum value of 5.3 mmol(quantum) m-2 s-1 nm-1 at a wavelength of 752 nm. The irradiance in the range 300-1100 nm amounted to 2860 mmol(quantum) m-2 s-1, being about 28% lower than that outside the greenhouse. The photosynthetically active radiation (400-700 nm, PAR) at that time reached a value of 1210 mmol(quantum) m-2 s-1.

The spectral transmittance of solar radiation through plants cultivated in a greenhouse depends on the weather conditions and the value of the leaf area index (LAI) of the plants. The lowest PAR transmittance through the canopy was observed in August, being from 0.7% (at LAI 5.0) to 3% (at LAI 3.8) of the PAR value above the culture. This demonstrated the high PAR absorptance by the pepper plants. The transmittance in the near infrared (over 700 nm) was from 6 (at LAI 3.8) to 19 (at LAI 5.0) times higher than that in the PAR range. Regression analysis showed that the correlation between the PAR transmittance and the LAI during the vegetative season was statistically highly significant (76%). A considerably weaker correlation (30%) was found between the near infrared transmittance radiation and the value of the leaf area index.

A single leaf of the pepper plant revealed high absorptance (about 90%), low reflectance (about 7%), and very low transmittance (about 3%) in the PAR range. In the near infrared (over 700 nm) the leaves revealed low absorptance (about 7%), and considerable reflectance (about 53%) and transmittance (about 40%). This protects the leaves from overheating and also from inhibition of the process of photosynthesis.

Czarnowski M., Cebula S., 1996. Effect of leaf area index in the spectral transmittance of solar radiation in greenhouse cultivation of sweet pepper plants. Folia Horticulturae 8/1: 53-72.