Agriculture Reference
In-Depth Information
been consistently found that the relationship is nonlinear, the slope declining as the N content
increases (Nevins and Loomis, 1970; Takano and Tsunoda, 1971; Wong, 1979; Lugg and Sinclair,
1981; Evans, 1983).
A strong positive correlation has been observed between the light saturated rate of photosynthe-
sis of a leaf and its N content (Field and Mooney, 1986; Evans, 1989; Reich et al., 1994). That is,
generally, higher N contents are associated with higher rates of maximum photosynthesis (Poorter
and Evans, 1998). The reason for this strong relationship is the large amount of leaf organic N
(up to 75%) present in the chloroplasts, most of it in the photosynthetic machinery (Evans and
Seemann, 1989).
The rate of photosynthesis is also influenced by N nutrition (Novoa and Loomis, 1981).
Burstrom (1943) observed an enhancement in apparent CO
2
assimilation with a rise in the nitrate
content of wheat leaf laminae. Osman and Milthorpe (1971) found that the increase in the gross
photosynthesis in that species was linear with N content. Murata (1969) reported a positive cor-
relation between total protein N content and the photosynthetic activity of rice, and Watanabe and
Yoshida (1970) observed a similar relationship in that species between chlorophyll content and
photosynthesis rate.
N deficiency has been reported to reduce photosynthesis in higher plants (Barker, 1979). N and
chlorophyll contents of leaves are closely correlated, and N deficiency brings about a sharp drop
in the chlorophyll content of leaves. Increased resistance to CO
2
transfer into or within leaves has
been demonstrated in N-deficient leaves (Ryle and Hesketh, 1969; Nevins and Loomis, 1970). These
increased resistance may be due to metabolic changes in the mesophyll or to changes in stomatal
aperture (Ryle and Hesketh, 1969). The inhibitory effect of O
2
on net photosynthesis is greater in
N-deficient leaves than in those adequately fertilized with N (Natr, 1972). Possibly, N availability
regulates the amount of substrate (glycolate) available for photorespiration through the formation of
glycine and serine (Barker, 1979).
Generally, fast-growing plant species or genotypes within species have a lower N content
and equal rate of photosynthesis, both expressed per unit leaf area, than slow-growing species
and hence have a higher efficiency of photosynthesis per unit leaf N (Poorter et al., 1990; Boot
et al., 1992). The higher photosynthesis efficiency of fast-growing species at optimum N supply
is explained by two characteristics: (i) they invest less N in nonphotosynthetic components; and
(ii) they exhibit a higher activation state of Rubisco and relatively higher activity of thylakoid
reactions as compared to their capacity (Pons et al., 1994; Werf, 1996). Werf (1996) reported that
there is a strong body of evidence that the total photosynthetic production of a crop during a sea-
son is strongly correlated with the integrated amount of intercepted light and not with the amount
of dry matter production per unit of intercepted radiation energy, at least when determined over
a whole growing season.
Roth et al. (2013) reported that N application in corn improved photosynthesis, especially at the
later reproductive growth stage. Xia (2012) also reported similar effects of N on corn photosynthe-
sis in Indiana, USA. In two studies comparing varied N rates on older and newer maize hybrids,
photosynthesis increased with higher N rates, with a more significant difference later in the season
(Ding et al., 2005; Echarte et al., 2008). In dryland agroecosystems, photosynthetic efficiency is
mainly influenced by internal genetic factors and the external water and fertilizer environmental
status (Shangguan et al., 2000; Jiang et al., 2004; Hou et al., 2013).
1.2.4.1 Leaf Nitrogen Content versus Photosynthesis
Several studies reported in the literature relating leaf N content and photosynthesis in crop plants.
These studies involve wheat (Evans, 1983), rice (Uchida et al., 1982; Cook and Evans, 1983; Sinclair
and Horie, 1989), soybean (Boote et al., 1978; Hesketh et al., 1981; Lugg and Sinclair, 1981; Sinclair
and Horie, 1989), and corn (Wong et al., 1985; Sinclair and Horie, 1989). A strong positive cor-
relation between leaf N content and carbon dioxide exchange rate or photosynthesis was reported
(Harper, 1994). Sinclair and Horie (1989) reported that there was a significant quadratic association
