AGRONOMIC CHARACTERISTICS OF CASSAVA
Agboola (1968) gave an outline of the
agronomic characteristics of cassava, a summary of which was given by Wheatley et
al (1995) as shown in Table 1 below.
Table 1:
Agronomic characteristics of cassava
Characteristic Period
Growth period
(month) 9-24
Optimal
temperature (0C) 25-29
Optimal rainfall
(cm) 100-150
Optimal pH 5-6
Fertilizer
requirement Low
Planting
material Stem
Source: Wheatley
et al (1995)
PHYSIOLOGICAL CHARACTERISTICS OF CASSAVA
Physiological characteristics of
cassava tolerance to prolonged drought in the tropics: implications for
breeding cultivars adapted to seasonally dry and semiarid
environments
The paper summarizes research conducted at
International Center for Tropical Agriculture (CIAT) on responses of
cassava to extended water shortages in the field aided by modern
gas-exchange and water-relation techniques as well as biochemical
assays. The aim of the research was to coordinate basic and applied
aspects of crop physiology into a breeding strategy with a
multidisciplinary approach. Several physiological
characteristics/traits and mechanisms underpinning tolerance of
cassava to drought were elucidated using a large number of genotypes
from the CIAT core germplasm collection grown in various locations
representing ecozones where cassava is cultivated. Most notable among
these characteristics are the high photosynthetic capacity of
cassava leaves in favorable environments and the maintenance of
reasonable rates throughout prolonged water deficits, a crucial
characteristic for high and sustainable productivity. Cassava possess
a tight stomatal control over leaf gas exchange that reduces water
losses when plants are subjected to soil water deficits as well as to
high atmospheric evaporative demands, thus protecting leaves from
severe dehydration. During prolonged water deficits, cassava reduces
its canopy by shedding older leaves and forming smaller new leaves
leading to less light interception, another adaptive trait to drought.
Though root yield is reduced (but much less than the reduction in top
growth) under water stress, the crop can recover when water becomes
available by rapidly forming new canopy leaves with much higher
photosynthetic rates compared to unstressed crops, thus compensating
for yield losses with final yields approaching those in well-watered
crops. Cassava can extract slowly water from deep soils, a
characteristic of paramount importance in seasonally dry and semiarid
environments where deeply stored water needs to be tapped. Screening
large accessions under seasonally dry and semiarid environments
showed that yield is significantly correlated with upper canopy leaf
photosynthetic rates, and the association was attributed mainly to
nonstomatal (anatomical/biochemical) factors. Parental materials with
both high yields and photosynthetic rates were identified for
incorporation into breeding and selection programs for cultivars adapted
to prolonged drought coupled with high temperatures and dry air,
conditions that might be further aggravated by global climate changes
in tropical regions.
Key words: agriculture, breeding, canopy, carboxylation enzymes, C3-C4 intermediate, ecophysiology, growth, leaf photosynthesis, Manihot sp., PEP carboxylase, productivity, stomata, water stress, yield