Agriculture Reference
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from polymer-coated urea decreased the wheat grain yield as compared with the uncoated urea but
increased protein concentration. Previous studies have reported that the maximum grain protein
concentration is generally attained at N levels much higher than those required to reach maximum
yield (Campbell et al., 1997; Fowler, 2003).
1.2.4 e ssentIals for p hotosYnthesIs
Photosynthesis is the basic process underlying plant growth and production of the food, fuel, and
fiber required to sustain human life (Tolbert, 1997). An understanding of photosynthesis is therefore
necessary to appreciate processes that determine yield in agriculture, forestry, ecology, and many
other fields. This extremely important biochemical process in green plants, which literally means
building by light, probably evolved 3500 million years ago (Shopf, 1993). In general, photosynthesis
is the process by which plants synthesize organic compounds from inorganic substances using light.
During photosynthesis, carbon (C) from atmospheric carbon dioxide (CO 2 ) is fixed to become part
of many organic molecules that constitute plant tissues (Fageria et al., 2006). Because of this, the
total dry matter production of crop plants is correlated with photosynthetic rates integrated over
plant growth cycles.
Photosynthesis is defined as a photochemical process involving the absorption of light energy by
plant pigment and its conversion into stable chemical potential (e.g., adenosine triphosphate (ATP)
and nicotinamide adenine dinucleotide phosphate, reduced (NADPH)) (Crop Science Society of
America, 1992). Conversion of the light energy into chemical energy involves an electron flow
between photosystem I (PS I) and photosystem II (PS II). This electron transport system is associ-
ated with the regeneration of NADPH and the establishment of an electrochemical gradient across
the thylakoid membranes to produce ATP. The NADPH and ATP are used as electron and energy
sources at various steps of CO 2 reduction and carbohydrate synthesis (Cakmak and Engels, 1999).
The photosynthetic capacity of plants is determined by several factors, such as water availability,
light conditions, and mineral nutrients. Mineral nutrients play several roles in the formation, par-
titioning, and the utilization of photosynthates. Therefore, mineral nutrient deficiencies, including
N, substantially impair the production of dry matter and its partitioning between the plant organs
(Marschner et al., 1996).
In green plants, in the photosynthetic process involving the photochemical reaction, carbohy-
drates are produced and O 2 and water are released according to the following equation:
6CO
+
12H O(light energy)
CH O O HO
+
+
2
2
6 26
2
2
Based on the pathways of carbon metabolism and their behavior in CO 2 uptake, three types of
higher plants are distinguished. These are known as C 3 plants, C 4 plants, and CAM (crassulacean
acid metabolism) plants. In C 3 plants, the three-carbon phosphoglyceric acid is the first prod-
uct (following the Benson-Calvin pathway). In C 4 plants, the four-carbon malic or aspartic acid
(Kortschak-Hartt, Hatch-Slack pathway) is present. The CAM plants, fix CO 2 into four-carbon
acids in the dark and reduce them during a subsequent light period (Crop Science Society of
America, 1992). The C 4 plants have higher photosynthetic efficiency and produce more leaf area
at lower leaf N contents as compared to C 3 plants (Sinclair and Horie, 1989). As the irradiance of
the leaf increases, the photosynthetic rate of C 3 species reaches a maximum at a lower irradiance
and at a lower value of photosynthesis than the C 4 species, and therefore has a lower RUE. Mitchell
et al. (1998) found that the average RUE values during vegetative growth under optimum condi-
tions were wheat 2.7 g MJ −1 , rice 2.2 g MJ −1 , corn 3.3 g MJ −1 , and soybean 1.9 g MJ −1 . Lindquist
et al. (2005) reported in modern corn hybrids an RUE value of 3.8 MJ −1 , indicating an improve-
ment in RUE with selection. The use of an adequate rate of N can improve leaf canopy and struc-
ture and can improve RUE.
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