Effects of PEG stress on seed germination process
The germination process of all three species was significantly affected by PEG stress (P<0.05; Fig 1). In the control, seeds of all three species began to germinate on day 1 and maximum GP was reach on day 3 for G. uralensis and G. glabra and on day 4 for G. inflata. Under mild PEG stress (5%-15%), all three species also began to germinate on day 1; however, the time to maximum GP for G. glabra and G. uralensis seeds was delayed by 1 d. Further delays were observed under severe PEG stress (25%-30%). At 25% PEG, the germination onset of G. uralensis, G. glabra G. inflata seeds was delayed to day 2 and the time to maximum GP was delayed by 2, 3 and 3 days, respectively, compared with the control. At 30% PEG, the seeds of G. uralensis and G. inflata did not germinate until day 7. These results show that showed that PEG solution could delay the germination initiation time and the time to reach the final germination percentage of the three liquorice seeds, but low concentration (5%) PEG solution could accelerate the seed germination process of G. glabra and G. inflata improve the final seed germination percentage of G. inflata seeds. This may be related to the fact that low concentration PEG solution can be used as a osmotic regulator to regulate the degree and state of water absorption of cells, which can stabilize the water absorption of seeds thus improve the germination rate and tidiness rate of seeds (Jia et al., 2020).
Effects of PEG stress on GP
GP is an important indicator of the germination ability and quality of seeds. PEG stress had a significant effect on the GPs of all three species (
P<0.01). As PEG concentration increased, the GPs of all three species showed a downward trend; however, there were interspecific differences in their changing patterns (Fig 2). In the control, the GP of
G. uralensis seeds was highest. Under mild PEG stress, the GP of
G. uralensis and
G. inflata seeds were significantly higher than those of
G. glabra seeds. Increasing the concentration of PEG from 15% to 25% decreased the GP of
G. inflata seeds by 71.11%, while increasing the concentration from 25% to 30% decreased the GP of
G. uralensis seeds by 56.67%. At 30% PEG, the GPs of
G. uralensis and
G. inflata seeds were 0, while that of
G. glabra seeds was 44.60% of that in the control. These results show that under mild PEG stress,
G. uralensis and
G. inflata seeds had higher GPs than
G. glabra seeds; however, under severe PEG stress, the germination of
G. uralensis and
G. inflata seeds were severely inhibited, whereas the seeds of
G. glabra maintained a high GP. The results of this study also showed that high concentration of PEG solution inhibited the germination of three liquorice seeds and the inhibition degree increased with the increase of solution concentration, which might be due to low water potential or high osmotic pressure seriously hindered the absorption of water in the initial stage of seed germination and inhibited the normal germination of seeds
(Lamia et al., 2012) .
Effects of PEG stress on GI and VI
GI and VI can be used to measure the germination rate and vigour status of certain plant seeds. PEG stress had a significant effect on the GIs and VIs of all three species (
P<0.01). As PEG concentration increased, the GI and VI of
G. inflata seeds first increased then decreased, while the GIs of
G. uralensis and
G. glabra seeds gradually decreased (Fig 3, Fig 4). Under the control and mild PEG treatments, the GI and VI of
G. uralensis seeds were significantly higher than those of
G. glabra and
G. inflata seeds. Increasing the concentration of PEG from 15% to 25% decreased the GI and VI of
G. inflata seeds by 91.53% and 92.94%, respectively. At 30% PEG, the GIs and VIs of
G. uralensis and
G. inflata seeds decreased to 0, whereas those of
G. glabra seeds were 24.66% and 16.17% of those in the control, respectively. These results show that the seeds of
G. uralensis had the highest germination speed and seed vigour under mild PEG stress; under severe PEG stress, the germination speeds of
G. uralensis and
G. inflata seeds were severely inhibited, while
G. glabra seeds maintained a high GI and VI. Imbibition is a necessary process of seed germination and the reason for seed imbibition is the hydrophilic substances in the embryo or endosperm, such as protein and starch. And compared to starch, protein is more hydrophilic (
Zamora-Briseno and De Jimenez, 2016). The results of our study showed that the seeds of
G. glabra have a stronger ability to obtain water from a dry or water-deficient environment to make themselves swell and germinate subsequently, while compared with
G. inflata and
G. uralensis. However, whether the differences among the three species in their seed germination percentage or germination rate is related to the inter-specific differences in composition of hydrophilic substances in the three seeds is still unknown.
Effects of PEG stress on GSI
GSI can be used to assess the tolerance of plant seeds to drought stress more intuitively. PEG stress had a significant effect on the GSI of all three species (
P<0.01). As PEG concentration increased, the GSI of
G. inflata seeds first increased and then decreased, whereas those of
G. uralensis and
G. glabra seeds continuously decreased (Fig 5). Under mild PEG stress, the seeds of
G. inflata had the highest GSI. Increasing the PEG concentration from 15% to 25% decreased the GSI of
G. inflata seeds by 74.63%. Increasing the concentration from 25% to 30% decreased the GSI of
G. uralensis seeds by 38.15%. At 30% PEG, the GSIs of
G. uralensis and
G. inflata seeds were reduced to 0, whereas that of
G. glabra seeds remained at 27.26%. These results show that
G. inflata seeds had the strongest drought tolerance under mild PEG stress and
G. glabra seeds had the strongest drought tolerance under severe PEG stress.
Han et al., (2022) pointed out that drought-resistance of
G. inflata seeds was stronger than those of the other two liquorice, the result was different from ours, which may be due to the genetic differences among the seeds in the two researches, Han’s seeds was obtained from Ningxia provience and ours was collected in Xinjiang province. Populations with different genotypes may have different phenitypes while being exposed to drought stress conditions.
Effects of PEG stress on RG
RG reflects the ability of plant seeds to recover to normal germination after being stressed and is an important indicator of plant stress tolerance. PEG stress had a significant effect on the RGs of all three species(
P<0.05). As PEG concentration increased, the RGs of
G. uralensis and
G. inflata seeds showed a gradual upward trend, whereas that of
G. glabra seeds first increased and then decreased (Fig 6). Under mild PEG stress, the RG of
G. glabra seeds was significantly higher than those of
G. uralensis and
G. inflata seeds. Under severe PEG stress, the RG of
G. inflata seeds was the highest. These results show that
G. glabra seeds exhibited the strongest germination recovery ability under mild PEG stress and
G. inflata seeds exhibited the strongest germination recovery ability under severe PEG stress.
Comprehensive evaluation of drought tolerance
The membership function method can eliminate the one-sidedness of individual indicators and reflect the stress resistance of plants more comprehensively and objectively and it has been adopted by many researchers. And MFV ranges between 0 and 1, which simplifies the comparison of resistance performance between species.
Under mild PEG stress, the average MFV of
G. uralensis,
G. glabra and
G. inflata seeds were 0.61, 0.40 and 0.42, respectively (Table 1). The order of drought resistance from strong to weak was:
G. uralensis >
G. inflata >
G. glabra. Under severe PEG stress, the average MFV of
G. uralensis,
G. glabra and
G. inflata seeds were 0.47, 0.76 0.2, respectively (Table 2). The order from strong to weak was:
G. glabra >
G. uralensis >
G. inflata.
Under mild PEG stress, drought tolerance was highest in
G. uralensis seeds and lowest in
G. glabra seeds. However, under severe PEG stress, drought tolerance was highest in
G. glabra seeds and lowest in
G. inflata seeds.This may explain why
G. uralensis and
G. inflata seeds are more sensitive to elevated PEG concentrations than
G. glabra seeds. Therefore,
G. uralensis should be preferentially planted in areas with better water conditions or less severe water shortages and
G. glabra should be preferentially planted in areas with severe drought and water shortage or uncertain water supply.