Final version of Tropical Legumes II Project Report for Phase 1 - icrisat

Technology generation
Mechanistic studies on drought resistance
Work at CIAT headquarters in Colombia involved both breeding in support of the program in Africa, and
physiology work as a complement to breeding. At the outset of the TL II project, substantial progress
had been made in improving the drought resistance of the small seeded Mesoamerican beans. This was
carried out in part under a BMZ-Germany, funded project in which Nicaragua, Rwanda and Malawi
participated. In 2009 and 2010, small red-seeded varieties with drought resistance were formally
released from that effort in Rwanda and Nicaragua. The research in TL II built on that experience,
and sought to extend it to the medium-to-large seeded beans of the Andean gene pool. Mechanisms
being investigated involve both roots to access soil moisture, particularly from deeper soil layers, and
remobilization of photosynthate to grain (partitioning of biomass).
Studies on mechanisms of drought resistance: We continued to evaluate a set of 36 elite lines in
our drought season from June to September in CIAT-Colombia. Year to year rainfall variability created
different patterns of drought stress, and over the past three years we have experienced light intermittent
drought (2008), terminal drought (2009), and low rainfall that extended over much of the vegetative
period (2010) (Figure 6-8). Each pattern of drought affected the crop response differently, and ranking
of genotypes changed somewhat under different patterns (Table 6-20).
Light intermittent drought in 2008 resulted in very acceptable yields of drought-selected lines, although
there were still wide differences in relation to the commercial check. Under terminal drought (2009),
early materials ranked relatively better, as would be expected. These included SER 125 (released in
Nicaragua), SER 16 (released in Rwanda), and G40001 (Phaseolus acutifolius). SER 78 was the highest
yielder in 2009 and was mediocre in other years. In 2010 occasional but light rainfall during the
vegetative phase of the crop resulted in more modest shoot development, and the ranking of some lines
changed substantially. SXB 412 and SXB 405, both types developed for Brazil, and with adaptation
to poor soil, ranked relatively better, whereas early materials were mid-to-low in ranking. This pattern
of drought stress due to low rainfall during vegetative growth might have stimulated more early root
growth in some genotypes, while the early materials did not benefit from this capacity. On the other
hand, SXB 412 and SXB 405 were both mediocre under intermittent and terminal drought in 2008 and
2009, respectively.
A few entries, particularly black-seeded lines, were excellent under all patterns of drought, as well
as with adequate soil moisture provided through irrigation. SEN 56 stood out among these. In spite
of being relatively early to mature, it was the best line in 2010 when other early materials slipped in
ranking. NCB 226 was especially noteworthy, since it was also one of the best lines under combined
stress of low P and drought (see Table 6-20 below). Both lines have excellent remobilization of
photosynthates to grain, a trait that is shared with SER 118 (small red) which was also very good in
2010, and has performed well under low soil fertility conditions. These observations bode well for the
potential of developing varieties of common bean with multiple stress resistance. However, these results
also indicate how complex the interactions of stress are on the crop. Plant responses to different types
of stress may well be independent traits, and if this is the case, it appears that they can be recombined
in lines such as NCB 226.
Correlations between yield and plant attributes under irrigated and rainfed conditions of the 36 lines
varied somewhat from year to year, probably reflecting the different rainfall patterns in each year (Table
6-21). For example, days to flowering varied from -0.28 to -0.59, and leaf area index varied from
-.09 to 0.44 in rainfed trials. However, pod harvest index (PHI) or the percent of pod biomass that is
represented by seed weight, was the most consistently positive, and is a trait that should be considered
as a selection criterion for breeding.
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