**4. Effect of intercepted radiation and leaf area index on growth and crop production**

The productivity of a crop depends on the ability of plant cover to intercept the incident radiation, which is a function of leaf area available, the architecture of vegetation cover and conversion efficiency of the energy captured by the plant into biomass. Most production strategies are directed towards maximizing the interception of solar radiation. In the case of crops, this implies adapting agricultural practices in such a way as to obtain complete canopy cover as soon as possible. Deficiencies in water and nutrient inputs may reduce the rate of leaf growth, reducing yield below optimum levels due to insufficient energy capture (Gardner et al., 1985).

Fig. 5. Typical presentation of the variation in the active (green) Leaf Area Index over the

**4. Effect of intercepted radiation and leaf area index on growth and crop** 

The productivity of a crop depends on the ability of plant cover to intercept the incident radiation, which is a function of leaf area available, the architecture of vegetation cover and conversion efficiency of the energy captured by the plant into biomass. Most production strategies are directed towards maximizing the interception of solar radiation. In the case of crops, this implies adapting agricultural practices in such a way as to obtain complete canopy cover as soon as possible. Deficiencies in water and nutrient inputs may reduce the rate of leaf growth, reducing yield below optimum levels due to insufficient energy capture

The leaf area index (LAI) is other concept for estimate the crop's ability to capture the light energy. LAI is often treated as a core element of ecological field and modeling studies. LAI is broadly defined as the amount of leaf area (m2) in a canopy per unit ground area (m2) Watson (1947). Because it is a dimensionless quantity, LAI can be measured, analyzed and modeled across a range of spatial scales, from individual tree crowns or clusters to whole regions or continents. As a result, LAI has become a central and basic descriptor of vegetation condition in a wide variety of physiological, climatological, and biogeochemical studies. LAI is a key vegetation characteristic needed by the global change research community. For example, LAI is required for scaling between leaf and canopy measurements of water vapour and CO2 conductance and flux, and for estimates of these variables across the global biosphere–atmosphere interface. Because solar radiation covers the entire surface of the ground, the LAI is a robust measure of leaf area per unit of solar

growing season for a maize crop. From Allen et al., 1998

radiation available.

(Gardner et al., 1985).

**production** 

The efficiency of interception of PAR depends on the leaf area of the plant population (Varlet-Grancher et al., 1989) as well as on the leaf shape and inclination to the canopy. Gallo & Daughtry (1986) observed that the difference between the intercepted and absorbed PAR, along the maize crop cycle, was lower than 3.5%. According to this, Müller (2001) showed that maize leaves absorb 92% of the intercepted radiation by the canopy. The efficiency of interception of a canopy corresponds to the capacity of the plant population in intercepting the incident solar radiation, which is the main factor influencing the photosynthesis and the transpiration processes (Thorpe, 1978). The efficient crops tend to spend their early growth to expand their leaf area; they make a better use of solar radiation. Agronomic practices, such as fertilization boot, high stocking densities and better spatial arrangement of plants (eg narrow rows) are used to accelerate ground cover and increase light interception.

Solar radiation also has an important role in the processes of evaporation and transpiration. Evaporation takes place mainly from the soil surface and transpiration is the evaporation that occurs across different plant organs, mainly leaves. Because both processes are closely linked, they are often considered together (evapotranspiration); water consumption account, linked to the crop itself, is considered "crop water needs" and is a fundamental aspect in the planning and designing irrigation strategies. Apart from the availability of water in the surface horizons, the cultivated soil evaporation is determined mainly by the fraction of solar radiation reaching the soil surface. This fraction decreases over the growing season, and at the same time the crop canopy cover grows (figure 6). The development of a crop can be divided into four stages (Allen et al., 1998):

Initial Stage: The early growth of individual plants, with little plant-plant competition is very fast. As the LAI develops, there is a shade of lower leaves, so that descriptions of crop growth are based on leaf area depending on the soil surface (Gardner et al., 1985). The water lost during this phase is mainly due to direct soil evaporation.

Fig. 6. The partitioning of evapotranspiration into evaporation and transpiration over the growing period for an annual field crop. From Allen et al., 1998

Crop Development Stage: LAI grows exponentially, changing the dominant component of evapotranspiration, predominating evaporation in the initial period and the plant transpiration at the end of the stage. As the leaf area grows, the radiation intercepted by leaves increases. At flowering time, leaf area development ends, with the goal of cultural practices to maximize crop photosynthesis intercepting virtually all of the incoming solar radiation.

Mid-season stage: The late season stage runs from the start of maturity to harvest or full senescence. In the vegetative period radiation interception does not increase, starting from fruit ripening to leaf senescence. (Late season stage).

From the point of view of optimizing the use of irrigation water, it is important to have an accurate estimate of the needs of the plant at any time. All of this will be determined in the development stage, which affect the distribution of solar energy in the process that occurs in the water consumption. Crop conditions (cultural practices, climate, soil, etc) that modify the development of vegetation cover along the life cycle change the water needs of the plant, which would imply a change in the watering schedule when the goal is meet in those needs. There are different procedures to determine the needs of the crop (ETc): the most popular is that proposed by (Doorenbos & Pruitt, 1977) ETc = ETo x Kc[1]

Where ETo is the evapotranspiration reference, (Kc) is the crop coefficient, which varies with the state of crop development and is adapted as the reference evapotranspiration (ETo) for each crop. It is related directly to the LI or the PGC, since it determines the distribution of energy available from plant surfaces and bare soil.

Because the leaf surface is the main photosynthetic organ of the plant, it is sometimes convenient to express the growth per unit leaf area. The rate of accumulation of dry matter per unit leaf area and per unit time is called net assimilation rate (NAR) and is usually expressed in g/m2 (leaf area) day. The NAR is a measure of average photosynthetic efficiency of leaves in a population. This is high when the plants are small and most of the leaves are exposed to direct sunlight. As the plant grows and the leaf area index increases, the leaves begin to shade, causing a decrease in NAR. For covers with a high LAI, the young leaves at the top take the highest proportion of absorbed radiation, thus having a high rate of CO2 assimilation and also assimilate many other parts translocated. In contrast, the older leaves at the bottom of the cover, which are shaded, have a low rate of assimilation of CO2 and provide a small assimilation to other parts of the plant.

Under no-stressed environmental conditions, the amount of dry matter produced by a crop is linearly related to the amount of solar radiation, specifically photo synthetically active radiation (PAR), intercepted by the crop. The slope of the regression between biomass and cumulative radiation intercepted by a crop has been used to determine the radiation use efficiency (RUE), which is calculated as the ratio of the biological yield (Kg/ha) to the intercepted PAR (MJ) by the crop plants. Monteith (1977), demonstrated that cumulative seasonal light interception for several crops grown with adequate soil water supply was closely related to biomass production. He formalized and fully established the experimental and theoretical grounds for the relationship (RUE) between accumulated crop dry-matter and solar radiation, arguing that this approach is robust and theoretically appropriate to

Crop Development Stage: LAI grows exponentially, changing the dominant component of evapotranspiration, predominating evaporation in the initial period and the plant transpiration at the end of the stage. As the leaf area grows, the radiation intercepted by leaves increases. At flowering time, leaf area development ends, with the goal of cultural practices to maximize crop photosynthesis intercepting virtually all of the incoming solar

Mid-season stage: The late season stage runs from the start of maturity to harvest or full senescence. In the vegetative period radiation interception does not increase, starting from

From the point of view of optimizing the use of irrigation water, it is important to have an accurate estimate of the needs of the plant at any time. All of this will be determined in the development stage, which affect the distribution of solar energy in the process that occurs in the water consumption. Crop conditions (cultural practices, climate, soil, etc) that modify the development of vegetation cover along the life cycle change the water needs of the plant, which would imply a change in the watering schedule when the goal is meet in those needs. There are different procedures to determine the needs of the crop (ETc): the most popular is

Where ETo is the evapotranspiration reference, (Kc) is the crop coefficient, which varies with the state of crop development and is adapted as the reference evapotranspiration (ETo) for each crop. It is related directly to the LI or the PGC, since it determines the distribution

Because the leaf surface is the main photosynthetic organ of the plant, it is sometimes convenient to express the growth per unit leaf area. The rate of accumulation of dry matter per unit leaf area and per unit time is called net assimilation rate (NAR) and is usually expressed in g/m2 (leaf area) day. The NAR is a measure of average photosynthetic efficiency of leaves in a population. This is high when the plants are small and most of the leaves are exposed to direct sunlight. As the plant grows and the leaf area index increases, the leaves begin to shade, causing a decrease in NAR. For covers with a high LAI, the young leaves at the top take the highest proportion of absorbed radiation, thus having a high rate of CO2 assimilation and also assimilate many other parts translocated. In contrast, the older leaves at the bottom of the cover, which are shaded, have a low rate of assimilation of CO2 and provide a small assimilation to other parts of

Under no-stressed environmental conditions, the amount of dry matter produced by a crop is linearly related to the amount of solar radiation, specifically photo synthetically active radiation (PAR), intercepted by the crop. The slope of the regression between biomass and cumulative radiation intercepted by a crop has been used to determine the radiation use efficiency (RUE), which is calculated as the ratio of the biological yield (Kg/ha) to the intercepted PAR (MJ) by the crop plants. Monteith (1977), demonstrated that cumulative seasonal light interception for several crops grown with adequate soil water supply was closely related to biomass production. He formalized and fully established the experimental and theoretical grounds for the relationship (RUE) between accumulated crop dry-matter and solar radiation, arguing that this approach is robust and theoretically appropriate to

fruit ripening to leaf senescence. (Late season stage).

that proposed by (Doorenbos & Pruitt, 1977) ETc = ETo x Kc[1]

of energy available from plant surfaces and bare soil.

radiation.

the plant.

canopies and can be influenced by several factors, namely, extremes temperature, water, and nutrient status. This is indicated by the variation reported in RUE among and within crop species and across locations and growing environments (Subbarao et al 2005). The literature reported quite a large number of RUE values for different crops and locations (Gallagher & Biscoe, 1978; Gosse et al., 1986; Kiniry et al., 1989). Stockle & Kemanian (2009) at intervals showed the value of RUE in g / MJ for large groups of plants: C3 Annuals (1.2- 1.7), C4 Annuals (1.7-2.0), C3 Oil crops (1.3-1.6), Legumes (1.0-1.2) and Tuber and root (1.6- 1.9). Moreover, the radiation use efficiency (RUE) approach that relates dry mass accumulation to the amount of intercepted PAR (Monteith, 1994; Kiniry, 1999) is widely used to estimate biomass accumulation in horticultural crops, fruit trees and forest (Landsberg & Hingston, 1996; Kiniry et al., 1998; Mariscal et al., 2000).

The efficiency of radiation interception is also influenced by the levels of nutrients in plants, mainly by nitrogen (Dewar, 1996; Scott Green et al., 2003). High crop RUE is directly dependent on obtaining the maximum leaf photosynthetic rate (Sinclair and Horie, 1989; Hammer and Wright, 1993). Nearly 70% of the soluble protein in leaf is concentrated in the carboxylation enzymes (i.e., Rubisco). A positive relationship between leaf nitrogen content per unit area (specific leaf nitrogen) and photosynthetic rates has been reported for a number of crops including wheat, maize, sorghum, rice, soybean, potato, sunflower, peanut, and sugarcane (Muchow & Sinclair, 1994; Sinclair & Shiraiwa, 1993; Sinclair & Horie, 1989; Hammer and Wright, 1993; Evans, 1983; Marshall and Vos, 1991; Giminez, et al 1994; Anten, et al, 1995; Peng, et al, 1994 and Vos & Van Der Putten, 1998 as cited in Subbarao et al 2005). The quantum yield of CO2 assimilation, which is one of the major determinants of the photosynthetic efficiency of crop canopies, reportedly decreases under N deficiency Meinzer and Zhu, 1998). Levels of photoinhibition also increase under N deficiency (Henley et al., 1991). Thus, a favorable crop nitrogen status appears to be necessary for the realization/expression of maximum RUE in a given crop species. Several studies have reported a positive response of RUE to N fertilization in a number of crops (Muchow & Sinclair, 1994; Hall et al., 1995; Green, 1987). Nitrogen deficiency should decrease the range where there is a linear response between PAR and increased light and thus the range of maximum RUE (Sinclair, 1990; Muchow, 1988). A substantial decrease in RUE under nitrogen stress has been reported for maize (Muchow & Davis, 1988; Muchow, 1994), sorghum (Muchow, 1988), kenaf (Muchow, 1992), wheat (Green, 1987), sunflower (Hall et al., 1995 and Bange et al., 1997), and peanut (Wright et al., 1993). Uhart & Andrade (1995) showed the differences in RUE produced in a crop of corn with nitrogen and without nitrogen, the latter being 40% lower (Figure 7).

The water deficit reduces the interception of solar radiation due to rolling up the leaves (Müller, 2001). If the water deficit is prolonged, the number and size of leaves may be reduced or the total leaf area may decrease, reducing as a result, the interception of radiation (Collinson et al., 1999). Soil water and the resulting plant water status play a key role in determining stomata conductance and canopy photosynthesis. Soil water deficit results in plant water deficits that lead to stomata closure and reduced photosynthesis, and results in loss of photosynthetic efficiency of the canopy and thus to a decrease in RUE (Monteith, 1977). Plants have developed a number of adaptive mechanisms to cope with water deficits to minimize the impact on their productivity (Subbarao et al 1995 and Tunner, 1997).

Fig. 7. Effect of water stress and nutrition in two trials in corn, adapted from Otegui (1992) and Uhart & Andrade (1995)

Nearly a 70% decline in RUE due to drought stress was observed in a number of grain legumes (Subbarao et al 2005). Though RUE of C4 crop species is generally higher than that of C3 crop species, the photosynthetic advantage disappears as the water stress increases (Subbarao et al., 2005). When drought stress is imposed from flowering until physiological maturity, a 25% decline in RUE occurred in pigeonpea (Nam et al., 1998). The growth of many field crops can be slowed down or even stopped by a relatively moderate water stress (Boyer, 1970). Stress of this magnitude develops following only a few days without rain, resulting in stomata closure, thus limiting photosynthesis (Sheehy et al., 1975). For rice, wheat, maize, sorghum, and pearl millet, drought stress has been reported to decrease RUE (Gallagher and Biscoe, 1978; Lecoeur & Ney, 1991; Inthapan & Fukai, 1988; Muchow, 1989; Whitfield & Smith, 1989; Robertson and Giunta, 1994; Jamieson et al, 1995 as cited in Subbarao et al 2005). A variety of mechanisms that include leaf movements (that can reduce the radiation load on the canopy when exposed to water deficits) and osmotic adjustment, and root attributes (that can maintain water supply during drought spells) play a major role in maintaining high levels of RUE during water stress (Subbarao et al., 2005). Otegui (1992) compared two maize crops under irrigation and no irrigation during a particular time of cycle, LAI experienced a decrease in cultivation without irrigation (figure 7).

Figure 8 shows a test conducted by the author (unpublished data), processing tomato crop irrigated with two doses. According to crop requirements (T100) and a deficit treatment of 75% of crop needs throughout all crop cycle (T75), LAI measurement and the evolution of dry biomass (aerial biomass) deficit treatment has a lower accumulation of biomass and LAI throughout the crop cycle. This aspect affected the final crop production. Reductions in RUE due to water deficits have been reported by Hughes and Keatinge (1983) and Singh and Sri Rama (1989) in grain legumes. Tesfaye et al., (2006) indicated that dry matter production in grain legumes is highly associated with the fraction of PAR intercepted, which in turn is highly associated with LAI. Li et al. (2008) showed that furrow planting pattern should be used in combination with deficit irrigation to increase the RUE and grain yield of winter

Fig. 7. Effect of water stress and nutrition in two trials in corn, adapted from Otegui (1992)

Nearly a 70% decline in RUE due to drought stress was observed in a number of grain legumes (Subbarao et al 2005). Though RUE of C4 crop species is generally higher than that of C3 crop species, the photosynthetic advantage disappears as the water stress increases (Subbarao et al., 2005). When drought stress is imposed from flowering until physiological maturity, a 25% decline in RUE occurred in pigeonpea (Nam et al., 1998). The growth of many field crops can be slowed down or even stopped by a relatively moderate water stress (Boyer, 1970). Stress of this magnitude develops following only a few days without rain, resulting in stomata closure, thus limiting photosynthesis (Sheehy et al., 1975). For rice, wheat, maize, sorghum, and pearl millet, drought stress has been reported to decrease RUE (Gallagher and Biscoe, 1978; Lecoeur & Ney, 1991; Inthapan & Fukai, 1988; Muchow, 1989; Whitfield & Smith, 1989; Robertson and Giunta, 1994; Jamieson et al, 1995 as cited in Subbarao et al 2005). A variety of mechanisms that include leaf movements (that can reduce the radiation load on the canopy when exposed to water deficits) and osmotic adjustment, and root attributes (that can maintain water supply during drought spells) play a major role in maintaining high levels of RUE during water stress (Subbarao et al., 2005). Otegui (1992) compared two maize crops under irrigation and no irrigation during a particular time of

cycle, LAI experienced a decrease in cultivation without irrigation (figure 7).

Figure 8 shows a test conducted by the author (unpublished data), processing tomato crop irrigated with two doses. According to crop requirements (T100) and a deficit treatment of 75% of crop needs throughout all crop cycle (T75), LAI measurement and the evolution of dry biomass (aerial biomass) deficit treatment has a lower accumulation of biomass and LAI throughout the crop cycle. This aspect affected the final crop production. Reductions in RUE due to water deficits have been reported by Hughes and Keatinge (1983) and Singh and Sri Rama (1989) in grain legumes. Tesfaye et al., (2006) indicated that dry matter production in grain legumes is highly associated with the fraction of PAR intercepted, which in turn is highly associated with LAI. Li et al. (2008) showed that furrow planting pattern should be used in combination with deficit irrigation to increase the RUE and grain yield of winter

and Uhart & Andrade (1995)

wheat in North China. Miralles and Slafer (1997) indicated that post-anthesis RUE appeared to be closely and positively associated and with the number of grains set per unit biomass at anthesis in winter wheat, and Uhart & Andrade (1995) found that stresses reduced the leaf photosynthetic rate and could result in lowering RUE. Whitfield and Smith (1989), Chen et al. (2003), and Li et al. (2008) showed that crop yield was positively related to RUE in winter wheat.

Fig. 8. Effect of water stress in processing tomato crop irrigated with two doses. According to crop requirements (T100) and a deficit treatment 75% of crop needs (T75)

In the modern agricultural research one of the methods in analyzing the crop production along the growth season is simulation by means of the crop production model (Aquacrop, Cropsys, CERES,…); mathematical crop simulation models can quantify the different processes that lead to the yield formation. Once calibrated and validated for a zone, a theoretical harvest with different types of soil management and certain climatic conditions is obtained. The predictive ability of these models can be significantly improved by adjusting the model input data on biomass generated at certain stages of crop development. (Baret et al., 1989; Chistensen & Goudriaan, 1993). Water stress and nutrition reduces LAI for a smaller size and greater leaf senescence. The smaller size of LAI agrees with light capture and thus crop growth, decreasing the efficiency of radiation.

The measurement of the radiation intercepted by a crop for the formation of leaf area is an important factor in monitoring crops, water relations studies, nutrition and crop simulation models. A good measurement of both parameters will be important in studying the effects of solar radiation on crops.
