to N and Ca were very much site specific. The lack of clear response to micronutrients may be attributed
to their efficiency in acquisition from low amounts of soil reserves and/or their efficiency in using ac-
quired amounts.
Considerable progress has been made in selecting germplasm accessions of tropical forage grasses
and legumes adapted to low-fertility acid soils [205,216,225,226,246,247,249–252]. Such accessions will
make the most efficient use of scarce fertilizer inputs, permitting a decrease in the required rates of fer-
tilizer application until minimal but adequate quality and yield are attained. The relative importance of
different soil nutrients in influencing growth and productivity of adapted plants may depend on the plants’
physiological adaptation to low-fertility acid soils.
At CIAT, efforts to screen forage germplasm for tolerance of toxic levels of Al and Mn in nutrient
solution [225,253] led to the identification of several promising grass and legume accessions. Brazilian
workers successfully developed solution culture techniques to identify acid-soil tolerance in Leucaena
leucocephala, a multipurpose forage tree species [254]. They also found that cotyledon retention on
seedlings was the most reliable, nondestructive, and easily assessed indicator of Al tolerance [255,256].
Improving acid-soil tolerance was also a breeding objective for two forage legumes, Centrosema
pubescensandL. leucocephala[257,258], although commercial cultivars were not developed.
An ongoing Brachiariabreeding program at CIAT aims to combine superior acid-soil adaptation,
found in B. decumbenscv. Basilisk, with resistance to spittlebugs, found in B. brizanthacv. Marandú.
Both species are natural tetraploid apomicts but produce fertile pollen, which can be used to pollinate
the closely related sexual species B. ruziziensisonce its normal diploid chromosome number is doubled
[259]. Among the resulting hybrids, both apomictic and sexual plants are found. Hence, combining
genes of the two apomictic species becomes possible. Field evaluation was conducted for 43 genetic
recombinants from a breeding population, four parents, and eight germplasm accessions for their toler-
ance of low-fertility acid soils. The study identified two genetic recombinants that combined several de-
sirable attributes, such as superior leaf area and leaf biomass, greater N content in leaves, and greater
partitioning of N and P to leaves, that would contribute to adaptation and persistence in low-fertility
acid soils [235].
Continued progress in the selection and improvement of Brachiariagenotypes will depend on iden-
tifying plant attributes that contribute to tolerance of low-fertility acid soils and on developing rapid and
reliable screening methods [231,235]. Adapted and persistent genotypes are able to acquire key nutrients
(e.g., N, P, and Ca) in a soil environment characterized by low pH and high Al [231–237]. Adaptation
may be through several root and shoot attributes [228], including (1) maintenance of root growth at the
expense of shoot growth; (2) acquisition and use of N (both forms), nitrate, and ammonium (e.g., B. hu-
midicola); (3) ability to acquire N through associative biological fixation (B. decumbens); (4) ability to
acquire P through an extensive root system and association with vesicular-arbuscular mycorrhizae; and
(5) development of an extensively branched root system (more root tips), which facilitates greater acqui-
sition of Ca (B. ruziziensis).
CIAT [260] conducted research to elucidate the physiological basis of acid-soil adaptation in
Brachiariaand to develop a high-throughput screening procedure to evaluate genetic recombinants of
Brachiariafor this trait. Results from these studies indicated that the high level of adaptation to acid soils
ofB. decumbenscv. Basilisk is due to its superior resistance to toxic levels of Al, combined with excel-
lent adaptation to P and N deficiencies. A particularly important finding was that B. decumbenscv.
Basilisk exhibits a level of Al resistance markedly superior to that of Al-resistant varieties of crops such
as maize, upland rice, or wheat. Unexpectedly, secretion of organic acids—an apparently widespread
mechanism for Al resistance [261]—is unlikely to be the principal mechanism conferring such outstand-
ing resistance [260]. Patterns of accumulation of Al and callose in root apices suggested that mechanisms
excluding Al from apices contribute to the superior resistance of B. decumbens. These physiological stud-
ies led to the development of a rapid and reliable screening procedure to evaluate Al resistance of those
genetic recombinants, making it possible to improve the efficiency of genetic improvement of Brachiaria
germplasm [260]. This procedure is currently being incorporated into the breeding program.
- Contribution of Adapted Grasses and Legumes to Sustainable Production Systems
Tropical forage species contribute to the sustainability of land management by helping regenerate de-
graded soils and replenish the N supply of the production system. Forage plants as cover crops can con-
trol weeds and reduce soil erosion. Grass and legume cultivars adapted to low-fertility acid soils support
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