**2. Interception of radiation**

In the interception of light (LI) by a canopy, difference between the solar incident radiation and reflected radiation by the soil surface (Villalobos et al., 2002), is a determining factor in crop development and provides the energy needed for fundamental physiological processes such as photosynthesis and transpiration.

Plants intercept direct and diffuse sunlight. The upper leaves receive both types of radiation, while the lower leaves intercept a small portion of direct radiation. Diffuse radiation therefore, becomes more significant in the lower leaves due to radiation transmitted and reflected from the leaves and the soil surface. Solar radiation transmitted by the leaves is predominantly infrared. From a practical point of view, the solar radiation spectrum is divided into regions, each with its own characteristic properties. Appropriate procedures and sensors must be chosen according to the specific objectives of the radiation measurements. Visible radiation, between the wavelengths of 400 and 700 nm, is the most important type from an ecophysiological viewpoint, as it relates to photosynthetically active radiation (PAR). Only 50% of the incident radiation is employed by the plant to perform photosynthesis (Varlet-Gancher et al, 1993). The quantity of radiation intercepted by plant cover is influenced by a series of factors such as leaf angle, the properties of the leaf surface affecting light reflection, the thickness and chlorophyll concentration, which affect the light transmission, the size and shape of the leaf phyllotaxis and vertical stratification, and the elevation of the sun and distribution of direct and diffuse solar radiation. Of the 100% total energy received by the leaf only 5% is converted into carbohydrates for biomass production later. Losses of energy are: by non-absorbed wavelengths: 60%. Reflection and transmission: 8%.Heat dissipation: 8%. Metabolism: 19%.

Of the global radiation incident on the plant canopy only a proportion is used to carry out photosynthesis: PAR (photosynthetic active radiation). The plant's response differs with different wavelengths. Chlorophyll is the main pigment that absorbs the light, other accessory pigments are the b-carotene, red isoprenoid compound which is the precursor of vitamin A in animals and the xanthophyll, a yellow carotenoid.

Essentially the entire visible light is capable of promoting photosynthesis, but the regions from 400 to 500 and 600 to 700 nm are the most effective (figure 4). In addition, pure chlorophyll has a very weak absorption, between 500 and 600 nm. The accessory pigments complement the absorption of light in this region, supplementing the chlorophylls.


One difference lies in the fact that photorespiration is very active in C3 plants. The photorespiration makes plants increase the oxygen consumption when they are illuminated by the sun, and this is very important for agriculture in temperate zones. In a hot day with no wind, the CO2 concentration in the plant decreases considerably for photosynthesis consumption, therefore, the relationship between carbon and oxygen decreases, and the CO2

In the interception of light (LI) by a canopy, difference between the solar incident radiation and reflected radiation by the soil surface (Villalobos et al., 2002), is a determining factor in crop development and provides the energy needed for fundamental physiological processes

Plants intercept direct and diffuse sunlight. The upper leaves receive both types of radiation, while the lower leaves intercept a small portion of direct radiation. Diffuse radiation therefore, becomes more significant in the lower leaves due to radiation transmitted and reflected from the leaves and the soil surface. Solar radiation transmitted by the leaves is predominantly infrared. From a practical point of view, the solar radiation spectrum is divided into regions, each with its own characteristic properties. Appropriate procedures and sensors must be chosen according to the specific objectives of the radiation measurements. Visible radiation, between the wavelengths of 400 and 700 nm, is the most important type from an ecophysiological viewpoint, as it relates to photosynthetically active radiation (PAR). Only 50% of the incident radiation is employed by the plant to perform photosynthesis (Varlet-Gancher et al, 1993). The quantity of radiation intercepted by plant cover is influenced by a series of factors such as leaf angle, the properties of the leaf surface affecting light reflection, the thickness and chlorophyll concentration, which affect the light transmission, the size and shape of the leaf phyllotaxis and vertical stratification, and the elevation of the sun and distribution of direct and diffuse solar radiation. Of the 100% total energy received by the leaf only 5% is converted into carbohydrates for biomass production later. Losses of energy are: by non-absorbed wavelengths: 60%. Reflection and transmission:

Of the global radiation incident on the plant canopy only a proportion is used to carry out photosynthesis: PAR (photosynthetic active radiation). The plant's response differs with different wavelengths. Chlorophyll is the main pigment that absorbs the light, other accessory pigments are the b-carotene, red isoprenoid compound which is the precursor of

Essentially the entire visible light is capable of promoting photosynthesis, but the regions from 400 to 500 and 600 to 700 nm are the most effective (figure 4). In addition, pure chlorophyll has a very weak absorption, between 500 and 600 nm. The accessory pigments


complement the absorption of light in this region, supplementing the chlorophylls.

fixation increases the photorespiration.

such as photosynthesis and transpiration.

8%.Heat dissipation: 8%. Metabolism: 19%.

chlorophyll.

vitamin A in animals and the xanthophyll, a yellow carotenoid.



**2. Interception of radiation** 

Fig. 4. Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids. From Whitmarsh and Govindjee, 1999.
