Source; Adapted from Corlett et al., 1992.
Source; Adapted from Corlett et al., 1992.
predicts that redistributing direct solar radiation over twice the leaf area at half the intensity would give an increase of 22 percent of annual productivity. The model gives reasonable values for the reduction in productivity reported for shade regimes. The results of this study suggest that in protected cultivation, screens of partially reflective material could be used to redistribute solar radiation from leaves exposed to high intensities onto shaded leaves and so raise the photosynthetic efficiency. Assuming absorption of direct light by the screens of 0.1, the increase in productivity is estimated to be 17 percent. Li, Kurata, and Takakura (1998) also demonstrated that solar radiation enhancement through reflected radiation on the cultivated area could be achieved to raise the photosynthetic productivity throughout the winter.
When water or nutrient supplies do not limit growth, the quantity of biomass produced by monocrops is limited primarily by the quantity of radiation captured, and seasonal biomass accumulation for a given species may be expressed as the time integral of the product (Monteith, 1990,1994). Numerous studies of annual crops, and some with perennial species, have demonstrated the existence of close correlations between dry matter production and cumulative intercepted radiation. For example, Stirling and colleagues (1990) examined the impact of artificial shade imposed on groundnut between the onset of peg initiation and pod filling, and final harvest using bamboo screens. A close linear correlation between aboveground biomass and cumulative intercepted radiation was found in all treatments, although the quantity of biomass produced per unit of intercepted radiation was sub stantially greater when shading was imposed from peg initiation onward. In the absence of stress, e is often conservative, typically ranging between 1.0 and 1.5 g MJ1 for C3 species in temperate environments, 1.5 to 1.7 g MJ1 for tropical C3 species, and up to 2.5 g MJ-1for tropical C4 cereals under favorable conditions (Squire, 1990). However, the work of Stirling and colleagues (1990) showed that e may vary substantially within a single season between 0.98 g MJ1 in the unshaded control and 2.36 g MJ1 in crops shaded from peg initiation onward. Thus, plants in the latter treatment intercepted approximately one-quarter of the radiation received by the unshaded control but converted this to dry matter 2.4 times more efficiently (Monteith and Elston, 1983; Russell, Jarvis, and Monteith, 1988). Choudhury (2000) also observed a strong linear relationship between RUE and diffuse fraction of the incident solar radiation.
The observed variability in experimentally determined e values contrasts with earlier views that e is highly conservative except during severe water stress but complies with more recent suggestions that the assumption of a constant value within species or cultivars may be incorrect (Demetriades Shah et al., 1994; Sumit and Kler, 2000; Bonhomme, 2000).
This leads to criticism of the concept that biomass accumulation may be linked directly with cumulative intercepted radiation, and those meaningful e values may be derived from such correlations. It is argued that the concept of radiation use efficiency is oversimplistic, cannot improve our understanding of crop growth, and is of limited value in predicting yield. This argument concludes little evidence exists that incident radiation is a critical limiting factor determining crop growth under normal field conditions. Demetriades Shah and colleagues (1992) advocated that analysis of crop growth in terms of cumulative intercepted radiation and the conversion efficiency of solar energy during dry matter production should be approached with caution. A major plank in this argument was that photosynthesis, and hence crop growth rate, depends on numerous soil, atmospheric, and biological factors, of which radiation is only one component. They suggested that good correlations would always be found between radiation interception and any growing object, even when radiation is not the limiting variable. So a close correlation between crop growth and radiation interception may be expected even when light is not a major limiting factor. Therefore, although solar energy may be the most fundamental natural resource for crop growth from a physical viewpoint, from a biological viewpoint it is no more important than water, nutrients, CO2, or any other essential commodity. As such, analysis of crop growth in terms of its radiation conversion coefficient may be inappropriate when variables other than radiation are the primary limiting factor. Further experimental support for this view was provided by Vijaya Kumar and colleagues (1996), when they showed that the conversion coefficient for rainfed castor beans (Ricinus communis) was less stable than previously suggested. The values obtained varied from year to year and were influenced by sowing date, decreasing with lateness of planting within the range 0.79 to 1.10 g MJ1. Values recorded prior to flowering were more stable than those obtained after flowering began. Campbell and colleagues (2001) also demonstrated that RUE steadily declined during growth of the rice crop and suggested that when RUE is used as a model parameter, it must be changed for differing LAI and for pre- and postanthesis periods.
Monteith (1994), however, defends the validity, generality, and robustness of correlations between intercepted radiation and growth and the con-servativeness of e. Monteith concludes that few of the arguments advanced against conversion coefficient e are not convincing, and errors involved in measuring intercepted radiation can be minimized. In contrast to the view of Demetriades Shah and colleagues (1992), he saw no reason to abandon the concept, but instead highlighted the need to test and improve methodology as new information becomes available.
Monteith's arguments are supported by Kiniry (1994) and Arkebauer and colleagues (1994), who suggested that Demetriades Shah and colleagues (1992) had overlooked the fact that many environmental stresses that limit growth act through physiological pathways directly involving the photo-synthetic process and its products. Arkebauer and colleagues (1994) argued that e cannot be expected to be constant, even within a single species or genotype, in the face of changes in other environmental variables. They argued that the definition of e involves three separate factors. First, the type and energy content of the carbon involved, i.e., net CO2 uptake by the canopy, total aboveground dry matter production, or total plant dry matter including roots and storage organs. Second, the way in which radiation is characterized, i.e., total incident solar radiation, intercepted shortwave radiation, intercepted PAR, or absorbed PAR. Third, the time scale over which e is calculated is extremely important and may range from instantaneous to hourly, daily, weekly, or seasonal estimates. Because widely differing definitions of e have been adopted, the values obtained may be expected to show substantial variation.
Weighing arguments for and against the concept of solar radiation use efficiency, it can be concluded that RUE is likely to remain as a tool in understanding and predicting crop growth and yield.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.