Agriculture Reference
In-Depth Information
component contributing to increased yields was increases in the number of fruits. This agrees with
Evans' (1980) description of how yield was increased with smaller but more numerous fruits in other
major crops.
1.2.4.2.1.7 Dry Bean
Vandenberg and Nleya (1999) summarized traits that might optimize
canopy structure in common bean at harvest, which could be modified through breeding. These
were (1) long internodes in the lower stem; (2) consistent internode elongation under a wide range of
environmental conditions; (3) reduction of stem stunting during early season growth; (4) increased
stem length; (5) increased stem strength, particularly in the more basal internodes; (6) reduction of
pod length without decreasing seed size; (7) increase in pod curvature so that pod tips do not extend
below the combine cutterbar; (8) long upright peduncles; (9) commencement of flowering at the
upper nodes; (10) high fertility at the upper nodes; and (11) a sufficient number of main stem nodes
to maximize productivity in the available growing season.
1.2.4.2.2 Decreasing Photorespiration
A large part of the plant photosynthetic product is lost in photorespiration. If this loss is minimized,
there is a possibility of increasing photosynthesis in crop species or genotypes within species. This
may be achieved by converting C
3
into a C
4
or by improving the specificity of Rubisco for CO
2
(Long et al., 2006). Long et al. (2006) discussed this issue in detail and readers may refer this article.
1.2.5 l
eaf
n
ItroGen
C
ontent
versus
r
adIatIon
u
se
e
ffICIenCY
The interception of the radiation of the sun by plant leaves is an important factor in determining dry
matter yield. Radiation use efficiency (RUE = dry matter produced per unit of intercepted light) is
widely used in the analysis of crop growth and the calculation of biomass production in crop simu-
lation models (Sinclair and Muchow, 1999; Milory and Bange, 2003). For simulation purposes, the
intercepted radiation over a time period is multiplied by RUE to estimate the dry matter production
for that period (Milory and Bange, 2003). RUE is related to the N content of the plants, especially in
the leaves (Sinclair and Muchow, 1999). Leaf N content and light intensity affect leaf photosynthesis
(Milroy and Bange, 2003).
The sensitivity of leaf photosynthetic rates to changes in leaf N was explored by Sinclair and
Horie (1989) as a potentially important source of variation in RUE and reviewed by Sinclair and
Muchow (1999). The theoretical study of Hammer and Wright (1994) provided a detailed analysis
of the linkage between RUE and leaf N content. In the model of Sinclair and Horie (1989), the
maximum leaf CO
2
assimilation rate was argued to be a direct function of leaf N content per unit
leaf area. They assumed that the leaf N content was uniform throughout the leaf canopy. From
this model, Sinclair and Horie (1989) calculated substantial differences among crop species in the
relationship between RUE and leaf N content. At high leaf N contents, the calculated RUE response
curves were saturated and there were only small changes in RUE with changes in leaf N. The maxi-
mum values of RUE from this analysis for each species were about 1.8 g MJ
−1
for corn, 1.5 g MJ
−1
for rice, and 1.3 g MJ
−1
for soybean.
Sinclar and Muchow (1999) reported that, for C
3
species, the maximum RUE was generally cal-
culated to be in the range of 1.4-1.5 g MJ
−1
intercepted solar radiation. When N content in the leaves
decreased to a minimum value, the RUE was very sensitive and decreased linearly in corn, rice,
and soybean (Sinclair and Horie, 1989). Similar decrease in RUE was also reported by Hammer and
Wright (1994) and Sands (1996) at lower N content in the leaves of peanuts. In sunflower, Gimenez
et al. (1994) reported that increased soil N fertility resulted in both an increased specific leaf N con-
tent and increased RUE. A curvilinear relationship between RUE and leaf N content of sunflower
was reported by Fisher (1993), Hall et al. (1995), and Bange et al. (1997). Sinclair and Muchow
(1999) concluded that, based on the review of the literature, RUE achieved a saturated value at high
