Because most selection among accessions of newly introduced species is carried out on the basis of
high dry matter yields, other things being equal, the outcome of the process is grasses with high potential
growth rates due to C 4 photosynthesis [221]. The legumes not only have the less efficient C 3 photosyn-
thesis but also have the added burden of having to provide energy to the symbiont organisms in the nod-
ules. The outcome of these differences is that, unless the legume has some other advantage, it will in-
evitably be dominated by the grass component [223].
It has been a major challenge to find tropical forage legumes that could persist in association with
aggressive grasses in tropical pastures. Among the several tropical forage legumes tested over the past 20
years.Arachis pintoihas been the only tropical legume that has persisted with such aggressive grasses as
Brachiariaspecies over longer periods [224]. Because of its growth habit, environmental adaptation, and
grazing tolerance, A. pintoihas been considered a pasture legume ideotype for grass-legume associations
in the tropics [221].
- Adaptive Responses to Low Nutrient Supply
An essential part of germplasm selection and improvement is to identify morphological, physiological,
and biochemical mechanisms by which forage plants adapt to acid-soil conditions. Although most trop-
ical grass and legume cultivars show wide adaptation to a range of edaphic and climatic conditions,
their environmental adaptation has not been well studied. Only recently have attempts been made to un-
derstand the physiological and biochemical bases of their adaptation to abiotic constraints [210,219,
225–229].
CIAT researchers conducted a series of investigations to identify plant attributes of tropical forage
ecotypes that help plants acquire and efficiently use nutrients from low-fertility acid soils
[205,228,230–237]. The outcome of this research has been an improved understanding of the physio-
logical basis of forage grass and legume adaptation to acid soils. This research is essential for improv-
ing selection and breeding; identifying plant-soil, plant-plant, and soil-plant-animal nutrient interactions
in forage-based production systems; and assisting the identification of ecological niches for forage
germplasm.
Low supply of nutrients, particularly P, N, and Ca, greatly limits forage adaptation and production
in acid soils. Widespread adoption of forage cultivars depends on their efficiently acquiring nutrients
from the soil and using them for growth. Identifying plant attributes that confer adaptation to low-fertil-
ity acid soils is needed to develop tropical forages rapidly through agronomic evaluation and genetic im-
provement. Plant attributes appear to be linked to different strategies to acquire and use nutrients
[7,18,127]. Understanding these linkages is fundamental in integrating plant attributes into a selection in-
dex. Plant attributes (indices) conferring adaptation to low-fertility acid soils must be identified to de-
velop rapid and reliable screening procedures.
Adaptation of forage plants to acid soils involves changes in the partitioning of biomass between
shoots and roots in response to growth conditions [205,231]. Greenhouse studies were conducted to de-
termine the effects of acid-soil stress and nutrient supply on biomass production, dry matter partitioning
between shoots and roots, and nutrient uptake, transport, and use efficiency in several forage grasses and
legumes adapted to acid soils [231]. Soil texture and fertility (nutrient supply) affected biomass produc-
tion and dry matter partitioning between plant parts. Forage grasses had higher biomass production in a
clay loam soil, especially at lower fertility levels. This higher production was attributed to the higher or-
ganic matter content and N availability in the clay loam soil.
In contrast, legumes, because of their nitrogen-fixing capacity, showed similar biomass production
in both types of soil at low fertility levels. The effect of soil fertility on the allocation of fixed carbon by
grasses and legumes is manifested in their root production. At the higher fertility level, root production in
grasses was higher in the sandy loam soil. In contrast, legumes showed little increase in root production
in either soil type. The change in their allocation of fixed carbon toward shoot growth probably helped
improve the nitrogen-fixing ability of legume roots.
In addition to changes in allocation of dry matter, grasses and legumes showed marked differences
in uptake and use efficiency of nutrients [231]. Efficiency of P uptake in legume roots was twice that of
grasses. However, the N and Ca use efficiency of grasses was about four times that of legumes. The su-
perior efficiency of legumes for P uptake was probably a result, in part, of the higher activity of the en-
zyme acid phosphatase in their roots, thus favoring mobilization of P from organic sources in acid soils
[234].
ADAPTATION OF BEANS AND FORAGES TO ABIOTIC STRESSES 599