Salinity-induced alterations in growth morphology, gas exchange,
chlorophyll fluorescence and ion accumulation patterns of P.
pinnata
In order to assess salt tolerance capacity in P. pinnata , 30 days
old seedlings were exposed to 300 and 500 mM NaCl concentrations for 1,
4 and 8 days. After 8 days of salt treatment (DAS), leaves of 300 mM
NaCl treated plants remained healthy and green as that of controls,
while 500 mM NaCl stress exposure caused tip burns at 8DAS (Figure 1A).
Reason behind selection of the specific time points for this study was
that plants were treated with two different salt concentrations (300 mM
NaCl and 500 mM NaCl) and kept under observation each day for induction
of salt-induced morphological symptoms including yellowing of leaves,
necrosis, wilting and shedding of old leaves. However, we couldn’t
observe any morphological changes till 7DAS (Days After Salt-treatment)
in 500 mM NaCl treated plants. Further, at 8DAS, 500 mM NaCl treated
plants started to show wilting symptoms (which was shown in
Supplementary Figure S1) and with increasing treatment time further
(After 10DAS) these plants were continued to shed off its leaves
(Supplementary Figure S1). Interestingly, at this point of time, 300 mM
NaCl treated plants did not display any salt-induced morphological
symptoms. With the above observation, we concluded that the 8DAS plants
could probably give us possible clues regarding salt tolerance
mechanisms. We observed that carbon exchange rate (CER) was changed with
salt concentration and treatment time (Supplementary Figure 2A). Upon
300 mM NaCl stress, the CER was not changed significantly at 1 and 4DAS,
while a significant reduction of ~50% was observed at
8DAS. In 500 mM NaCl treated plants, the CER values were progressively
decreased
with the treatment time. However, the levels of chlorophylls pigments
were not changed significantly in both 300 and 500 mM salt treated
plants at all-time points (Supplementary Figure 2B). Further, 300 mM
NaCl treated plants could maintain a constant Fv/Fm values like control
plants at all-time points (Supplementary Figure 2C). Conversely, 500 mM
NaCl treated plants showed a little decrease in Fv/Fm values at 4 and
8DAS. Similar trend was observed with relative water content (RWC) in
leaves and roots of Pongamia. The LRWC was maintained equal to that of
controls in 300 mM NaCl treated plants, whereas in 500 mM NaCl treated
plants these values tend to decrease progressively with the treatment
time (Supplementary Figure 2D). In contrast, the RRWC values did not
change significantly in both 300 and 500 mM NaCl treated plants.
Differences in photosynthetic and morphological responses to salt stress
showed in Figure 1A and Supplementary Figure 2A-D might result of
difference in their Na+ accumulation patterns across
the plant, we tested the following hypothesis. As shown in the
Supplementary Figure 2E-G, ions such as Na+,
Cl-, and Ca2+ content were increased
dose dependently in both 300 and 500 mM NaCl treated plants with
treatment time. In detail, root showed a significant increase in
Na+ content when compared to leaves. The peak value of
root Na+ ion content was about 65 mg.
g-1 DW in 500 mM NaCl treated plants at 8DAS.
Similarly, root Cl- levels were enhanced by
~2.7, ~2.7, and
~1.4-fold in 300 mM NaCl treated plants, and
~3.8, ~4.0 and ~4.3-fold
up-regulation was observed in roots of 500 mM NaCl treated plants at 1,
4 and 8DAS, respectively. Further, Ca2+ levels were
slightly increased in roots of 300 mM NaCl treated plants at 1, 4 and
8DAS. While, in roots of 500 mM NaCl treated plants,
Ca2+ levels were increased significantly at 1 and
4DAS, while these levels were unchanged at 8DAS.