Agriculture Reference
In-Depth Information
between leaf N content and carbon dioxide exchange (CER) for corn, rice, and soybean. The value
of CER for corn was much higher (2.4 mg CO
2
m
−2
s
−1
) at about 1.2 g m
−2
leaf N compared to
soybean and rice (about 1.2 mg CO
2
m
−2
s
−1
). The substantial advantage in biomass accumulation
under N-limited conditions for corn, compared with rice and soybean, is noteworthy (Harper, 1994).
The higher photosynthesis rate in C
4
plants such as corn as compared to C
3
plants is well known
(Fageria, 1992).
1.2.4.2 Improving Photosynthesis
Photosynthesis in crop plants is influenced genetically as well as determined by environmental
conditions. Crop plants provided with adequate environmental conditions such as mineral nutrition,
water availability, and temperature produce higher photosynthates as compared to those grown
under adverse environmental conditions. In addition, photosynthesis can be improved with the
use of genotypes having ideal architecture. In addition, photosynthesis can also be increased by
decreasing photorespiration. Hence, here, two options (ideal architecture and biotic and abiotic
stresses and decreasing photorespiration) will be discussed.
1.2.4.2.1 Ideal Architecture and Biotic and Abiotic Stresses in Important Food Crops
Ideal plant architecture for a higher yield can be achieved through plant breeding. It has been rec-
ognized that the spectacular yield increase of crops during the second part of the twentieth century
has been attributed in almost equal measure to breeding and to the use of inputs or better manage-
ment practices (Fageria et al., 2006). The most important morphological characters that have been
bred into high-yielding cereal cultivars such as rice, wheat, and corn are short, stiff culm for lodging
resistance, and erect leaves for increased interception of solar radiation. In addition to this, most of
the crop cultivars released had resistance to biotic and abiotic resistance. The use of an adequate
rate of nutrients and plant spacing or stands was also part of the improved management practices
adopted.
Genetic improvement in grain yield has been intensively studied in wheat, barley, oat, corn,
and rice (Austin et al., 1980; Wych and Rasmusson, 1983; Wych and Stuthman, 1983; Tollenaar,
1989; Feil, 1992; Peng et al., 2000). Most of these studies reported a positive historical cultivar
trend in grain yield. Studies of historical cultivars often show that genetic improvement in yield
potential has resulted from increases in GHI (Lawes, 1977; Austin et al., 1980; Riggs et al.,
1981), which is associated with ideotype characters, for example, short stature in wheat and
uniculm habit in corn and sunflower (
Helianthus annuus
L.) (Sedgley, 1991). Others reported
that the improvement in yield potential has been associated with increases in the biomass yield
in wheat (Waddington et al., 1986), corn (Tollenaar, 1989), oat (Payne et al., 1986), and soybean
(Cregan and Yaklich, 1986). McEwan and Cross (1979), Wych and Rasmusson (1983), and Wych
and Stuthman (1983) stated that the improvement in grain yield was related to both dry matter
accumulation and GHI in wheat, barley, and oat. In the author's opinion, both the improvement
(dry matter and GHI) contributed to yield increase in cereals and legumes and breeding and
improved management practices have contributed to this increase. Several examples of crop
improvement through breeding for ideal plant architecture and biotic and abiotic stresses are
provided in this section.
1.2.4.2.1.1 R ice
Rice is a staple food for more than 50% of the world population (Li et al.,
2013). China and India are the world's largest rice producer as well as consumer. In South America,
rice is eaten everyday with dry bean by all sections of the population. Rice is mainly grown under
two ecosystems, known as upland and lowland. Upland rice is defined as the rice grown on undu-
lated well-drained soils, without water accumulation in the plots or fields, and totally depends on
rainfall for water requirements (Fageria, 2011a, 2014). It is also known as aerobic rice because it
is grown on well-drained soils. Upland rice is mainly grown in South America, Africa, and Asia.
Brazil is the largest upland rice-producing country in the world. Lowland rice is mainly grown on
