Agriculture Reference
In-Depth Information
In the second group are genotypes that produced reasonably well at lower as well as at higher
rates of N. Genotypes fall in this group are Perola, CNFM 6911, CNFR 7847, CNFR 7865, and
CNFP 7777. This group is most desirable because they produce high yield at low as well as at high
levels of N. The third group includes genotypes that produced low yield at low as well as at higher
N rates. The genotypes fall in this group are CNFP 7792, CNFR 7866, and BRS Valente. Crop spe-
cies respond differently to soil and fertilizer N (Sinclair and Horie, 1989; Fageria and Baligar, 1993),
including bean genotypes (Park and Buttery, 1989). Morphological, physiological, or biochemical
mechanisms might be responsible for different response of crop genotypes to applied N (Fageria
and Baligar, 1993; Marschner, 1995; Baligar et al., 2001).
1.2.1.7 Leaf Area Index
LAI is defined as the leaf area per unit soil area (cm
2
m
−2
). This growth index can be calculated as
follows (Fageria et al., 2006):
LAI AN)1
=×/0 000
where A is the leaf area (cm
2
) and N is the number of tillers (cereals), branches (legumes), or plants
per square meter. CGR is related to LAI and NAR (net assimilation rate) as follows:
CGR
=
AI
×
NAR
NAR is defined as the dry matter accumulation per unit of leaf area and expressed as grams per
square meter of leaf per day (g m
−2
leaf area per day), and can be calculated using the following
equation (Brown, 1984):
NAR
=(
A1 dt
/
)(
/
dW
)
where A is the leaf area and dW/dt is the change in plant dry matter per unit time. The objective of
measuring NAR is to determine the efficiency of plant leaves for dry matter production. The NAR
values decrease with the advancement of plant growth due to the mutual shading of leaves and the
reduced photosynthetic efficiency of older leaves (Fageria, 1992).
The source capacity of plants is primarily determined by the LAI, the leaf area duration, and the
rates of photosynthesis, respiration, and amino acid synthesis (Novoa and Loomis, 1981). LAI is the
main factor in biomass formation, and it varies in amount with plant population and nutrient supply.
LAI is significantly increased with the addition of N in cereals and legumes (Watson et al., 1958;
Langer and Liew, 1973; Pearman et al., 1977; Spiertz and Ellen, 1978; Fageria, 2007; Fageria and
Santos, 2008). The greater LAI can be due to an effect on the leaf number or leaf size. Langer and
Liew (1973) did not find any effect of N on the leaf number of the wheat main shoot. But the leaf
number per plant will increase with N supply due to an increase in tiller number in cereals (Halse
et al., 1969; Pearman et al., 1977). The area of each lamina is also increased by N due to the effect
on both the cell number and the size (Novoa and Loomis, 1981). Leaf area duration is also extended
by N fertilization (Langer and Liew, 1973; Pearman et al., 1977; Thomas et al., 1978).
LAI is one of the principal crop parameters affecting photosynthesis. LAI varies with envi-
ronmental conditions, cultural practices, and stage of crop growth. In corn, N levels significantly
increased LAI compared to control treatment (Uhart and Andrade 1995). For determinate crops, the
best time to measure LAI is when it reaches its maximum at the beginning of reproductive growth;
for indeterminate crops, the maximum LAI may occur well after flowering begins. Optimal leaf
area varied with the crop species and genotypes within species. However, optimum values of LAI
reported for different crops, such as for soybean about 3.2, for corn about 5, and for wheat 6-8.8
(Yoshida, 1972). Critical LAI values for the large leaved tropical legumes generally fall in the range
of 3-4 (Muchow, 1985), but they can exceed 5 for small leaved pigeonpea (Rowden et al., 1981).
