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

RD34) also had TE superior to JL 24. Event RD33 had higher TE than JL 24 across water regimes, and even higher
TE than best lines identified so far RD2. We also recorded transpiration response as a function of FTSW (index for
soil moisture content) and found that all DREB1A plants showed a decline in transpiration in dryer soil than JL 24.
This pattern is fully consistent with previous experiments using DREB1A transformants. A repeat of that experiment
is planned to confirm the differences.
The lysimetric system described above has been used to assess 5 DREB1A transgenics (RD2, RD11, RD12, RD19,
and RD20) and their non-transformed parent. The experimental design was similar to that described above, except
we had 6 replications for each set (pre-treatment harvest at 30 DAS, WW, and WS treatment). Shoot dry weight data
are still pending. Normalized transpiration dropped to about 50 - 60% of control at 12 days after stress imposition.
Thereafter, NTR of RD 11 was above that of control JL 24. The total transpiration was also greater in several
transgenics than in JL 24. For instance, transpiration of RD 19 during the 12 - 42 days after stress imposition was
about 600 times higher than that in JL 24 (1788 g vs 2375 g water) under WS conditions. Under WW conditions,
RD 19 used about 600 g more water (5933 g vs 5333 g). This showed that under water stress, RD 19 was able to
take up relatively more water than JL 24. This trend was true in all transgenic plants. In fact, a remarkable finding of
that work was that root dry weight of all 6 genotypes was within a very narrow margin under WW conditions (1.48
– 1.63 g, with JL 24 having 1.61 g). By contrast, under WS conditions, root dry weight of JL 24 remained
unchanged (1.73 g) whereas it increased in all transgenics dramatically and reached a range of 2.27 – 2.65 g (30%
increase). Under WS conditions, all transgenics had more profuse rooting in deep layers compared to JL 24, whereas
under WW, there were no differences. There was a good correlation between the root dry weight within the 40 - 120
cm depth and the total transpiration (r 2 = 0.41). It was rather unexpected that DREB1A transgenic could have such a
root-related response. This trial has shown the importance of understanding the kinetics of water uptake, more than
the importance of knowing about the roots.
Evaluation of transgenic plants in culture chambers: There is a convergence of evidence from the literature and
from our own work, in particular in groundnut, that there may be genotypic differences in the sensitivity of stomata
to the vapor pressure deficit (VPD). That fact is getting documented in a “slow-wilting” genotype of soybean (PI
416937) that shows no further increase in transpiration at VPD level above 2 kPa, whereas other genotypes of
soybean maintain a linear increase in transpiration with VPD level increasing above 2 kPa. Practically, such
behavior would shutdown stomata during the hours of the day reaching the highest VPD, trading-off some loss of
carbon fixation for water saving, and therefore, contributing to higher transpiration efficiency. Here the purpose of
the experiment was to test whether a transgenic line having consistent higher TE than wild type JL 24 differed in
stomatal sensitivity to VPD.
The plants were grown in the P2 facilities until about 30 days, and then transferred to growth chamber having
different combinations of temperature and humidity, resulting in VPD of 0.63, 1.01, and 1.66 kPa. Plants were left
for acclimatizing for 2 days, after which they were submitted to a dry-down experiment. Under WS conditions, there
were no differences in TE between JL 24 and RD 2. By contrast, under WW conditions, TE was higher in RD 2 than
in JL 24, across the 3 VPD environments.
Several tests to measure response to stepwise increase in VPD were conducted. A ladder of VPD conditions was set
up throughout the day, with 60 min for each VPD value. Prior to that, the transpiration was measured
gravimetrically from 9.00 am to 16.00 pm (by weighing pots every hour) to check any possibility of a diurnal effect.
Data showed that transpiration was virtually constant throughout the day. Therefore, the response of transpiration to
stepwise increase in VPD could be assessed without any data adjustment. The first ramp of VPD increases (0.95,
1.13, 1.35, 1.6, 1.88, 2.21, and 2.58 kPa) gave a strict linear increase in transpiration at each VPD interval, with no
significant difference between the slopes of the two genotypes. Since plants in the natural environment face higher
VPD, a ramp exploring higher values of VPD was also tested (0.95, 1.51, 2.02, 2.31, 2.64, 3.01, and 3.43 kPa). This
ramp also gave a linear increase in transpiration in both genotypes, with no significant differences in the slope
between the two genotypes. The same ramp carried out 5 days later also gave a linear increase in transpiration, until
VPD was 2.31 kPa. At VPD values above 2.31 kPa, there seemed to be no increase in transpiration.
Vincent Vadez and KK Sharma
Progress reported towards the achievement of milestone for 2007 will contribute towards achievement of the
milestones listed below.
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