Mild reductions in guard cell NtSUS2 expression
substantially reduce whole plant transpiration (WPT) under well-watered
and fluctuating environmental conditions
We next used a gravimetric methodology to determine WPT in plants grown
under well-watered (WW) and non-controlled greenhouse conditions, which
varies substantially in terms of light, temperature and humidity
throughout the days (Figure 4A). Under these conditions, both L3 and L13
showed significantly lower WPT (up to 44%) compared to WT throughout
the eleven days of the experiment (Figures 4B,C). At day 5, plants
reached a peak of WPT (Figure 4B), which was associated to a sequence of
sunny days (Figure 4A). Following the interruption of irrigation at day
5, a strong reduction in WPT in WT plants was observed at day 6, whilst
no changes in WPT of the transgenic lines were observed (Figure 4B). On
this day, the percentage of WPT was invariant across the genotypes
(Figure 4C). When the cumulative WPT across the experiment were
compared, L3 and L13 were found to transpire approximately 32% less
than the WT (Figure 4D), meaning that they respectively conserved 416
and 433 g H2O plant-1 during the
experiment.
The leaf area was initially smaller in L3 and L13 plants compared to WT.
However, from day 5 to 11 of the experiment, only the leaf area of line
L13 remained smaller than WT (Figure S3). WPT per leaf area unit (g
H2O m-2 d-1) was
also lower in L3 (28%) and L13 (22%) than WT (Figures 4E,F), which
lead to a lower cumulative water loss per leaf area in these lines
(Figure 4G). No changes in leaf stomatal density (SD) and water loss
from detached leaves among the genotypes was observed (Figures S4A,B).
At the end of the experiment, plants were harvested and growth
parameters determined. Although relative growth rate (RGR), total leaf
number, specific leaf area and stem length were not significantly
altered in the transgenic lines, both L3 and L13 exhibited a reduced
total biomass (Table 1). This was associated to a reduced carbon
allocation toward the roots, given that reduced roots DW and % of DW
roots were observed in these lines (Table 1). By contrast, line L3
showed higher % DW leaf, % DW shoot and shoot-to-root and leaf-to-root
ratios than WT (Table 1). It is noteworthy that the leaves are the
harvestable part of tobacco plants and the harvest index of L3 and L13
reached 0.58 and 0.54 g g-1, but only L3 was
significantly higher than WT (0.52 g g-1). Line L3
also have higher yWUE than WT, whilst no difference in
season-long WUE (slWUE) under WW condition was observed
(Table 1). Taken all WUE parameters together, reduced expression of the
guard cell NtSUS2 did not alter both steady-stateiWUE and slWUE, while L3 showed higheryWUE and increased iWUE in both
transgenic lines during dark-to-light transition was observed.