ICRISAT Archival Report 2006 - The seedlings of success in the ...

Milestone 5B.2.1: Putative relation between stay-green QTL introgression lines and root traits identified (VV/CTH,
2009)
Root traits of stay-green lines: A repeat trial with long (2.40 m) PVC pipes was done to evaluate root growth at
different stages and under different timing of imposed stress, using 2 stay-green genotypes (B 35, E 36-1) and 2
senescent genotypes (R 16 and ISIAP Dorado). The purpose of the experiment was to determine the most suitable
timing of stress imposition to identify differences, if any, in the rooting pattern. Stress was imposed at 21, 35, 49 and
63 days after sowing. For each date, fifteen plants per genotype were grown under well-watered conditions until the
seedset date. At each date, 5 plants were used to assess their root characteristics at that date. Then 5 plants were
exposed to water stress and the remaining 5 plants were kept under well-watered conditions. Water-stressed and
well-watered plants were used at 42, 40, 37, and 26 days after stress imposition, respectively, for sets of plants
treated at 21, 35, 49, and 63 days after sowing.
In the sets of plants that were stressed at 21 and 35 DAS, we did not find any root depth differences between staygreen
and senescent genotypes at harvest (respectively at 63 and 75 DAS). By contrast, we found that when stressed
at 49 DAS, both stay-green genotypes had deeper rooting systems than senescent R 16 and ISIAP Dorado (15−35
cm deeper). Also, the two stay-green genotypes had a larger proportion of roots in the deeper layers of soil than R
16. Overall, this experiment confirmed the earlier finding of a deeper rooting and more profuse rooting in deeper
layers of stay-green genotypes compared to senescent materials, under conditions of water deficit. What appeared
also from this experiment is that compared to control, there was little difference in the root distribution in the layers
below 90 cm in water-stressed and well-watered R 16, whereas B 35 was able to allocate a much larger proportion
of roots in deeper layers under water stress than under well-watered conditions. In this experiment, the shoot dry
weight under water stress, expressed as a percentage of control well-watered plants, was also higher in B 35 and E
36-1 than in R 16.
Root traits of stay-green introgressed lines: RSG 04001 and RSG 04005 are BC 1 F 3 and BC 2 F 3 derivatives from B
35 in the background of recurrent parent R 16, whereas IDSG 04211 is a BC 2 F 3 derivative from B 35 in the
background of recurrent parent ISIAP Dorado. Fifteen plants of these genotypes were grown in PVC pipes as above
and stress was applied from 21 days onwards. The plants were harvested at 60 DAS. In this experiment, similar to
the one reported above, there was no difference in root depth between B 35 and R 16. By contrast, two of the
promising derivatives (RSG 04001 and RSG 04005) had deeper rooting and more profuse rooting in the 120−150
cm layer than R16. In this experiment also, it was evident that, compared to well-watered plants, B 35, RSG 04001
and RSG 04005 were able to increase their deep rooting relatively more than R 16. Also, the shoot dry weight under
water stress, expressed as a percentage of control well-watered plants, was higher in B 35 and RSG 04001 than in R
16.
Several conclusions and future orientations can be drawn from the different sets of experiments that have been
carried out to investigate root traits in sorghum: (i) Stay-green donor parent B 35 and in some cases E 36-1, and
derivatives from B 35 appear to have deeper rooting and more profuse rooting in deeper layers than R 16. However,
these differences seem to be conditioned by the timing of stress imposition and duration of stress. Major differences
between senescent and stay-green materials are found when stress is applied close to the reproductive period and/or
when plants are exposed to water stress for a sufficient time; (ii) Though we found some differences, which tend to
indicate a role of roots in the stay-green material, we have had difficulties to obtain significant results in all
experiments, because of plant to plant variations and because of the fairly small differences in rooting traits; (iii) We
have mostly assessed, so far, the parents of the stay-green trait, and have only recently started working with
derivatives, which are showing promising results. There is a need to move forward and study the advanced
backcross generations of these derivatives that have been produced from RSG 04001 and RSG 04005, the two
promising introgression lines mentioned before; (iv) So far, we have assessed rooting characteristics. However, this
does not give any indication on whether roots do contribute to a better water uptake. We observed that B 35 tended
to senesce and dry later than R 16 under water stress, which would indicate that water uptake occurs for a longer
period of time. Therefore, we are currently designing a lysimetric system, by which we could assess plant water
uptake from the time of stress imposition. We believe that putative differences in the rooting characteristics would
get integrated over time in larger differences for water uptake (easier to measure). If successful, such assay could
eventually replace the time-consuming process of root extraction and its characterization.
Vincent Vadez and CT Hash
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