4. DISCUSSION
Several organisms gradually demonstrate adaptive changes in their
morphological and physiological characteristics and the greatest plastic
response (heat resistance and drought resistance) under continuous
drought and high temperature stress (Seki et al., 2007; An & Liang,
2012). However, the amount of drought and high temperature stress that
species can withstand differ (Li & Li, 2016; Pradhan et al., 2012);
more specifically, alpine meadow plants grown in low-temperature
restricted environments are extremely sensitive to temperature and
drought
Leaves, the main function of which is photosynthesis, are the most
flexible and sensitive plant organs to environmental stress (Xue et al.,
2012). Drought or high temperature stress will generally reduce the LWC
(Zhang et al., 2020). The results of the present study showed that
drought stress and high temperature stress decreased and increased the
LWC, respectively. The LWC was more affected by drought stress than by
heat; these results corroborate those of Keyvan (2010). The LWC ofK. humilis and P. annua increased under high temperature
stress, decreased significantly under excessive drought, and increased
significantly under the compound heat and drought stress, thereby
indicating that temperature played a key role in the effect of the
compound stress on LWC. The LWC change of S. pulchra demonstrate
that it is sensitive to both excessive heat and the excessive combined
stress, which is consistent with the research results of Zhang et al.
(2020). Excessively high temperature and drought destroy the
photosynthetic mechanism of mesophyll cells in plant leaves, cause
irreversible damage, reduce the LWC of plants, and result in yellowing
(Zandalinas et al., 2017). The different responses of the four plant
species to drought and high temperature stress in terms of their LWC
changes, indicate that different plant characteristics play an important
role in a species’ response mode.
High temperatures can increase the consumption of organic matter by
plant respiration in order to reduce the biomass or promote organic
matter accumulation by increasing photosynthesis or the absorption of
mineral nutrients (Ma et al., 2017). Drought and high temperature stress
lead to a biomass decline through negative effects on plant growth,
physiology, and reproduction (Barnabás et al., 2007). The results of our
study showed that the AGB of the four plant species examined was
inhibited by high temperature stress; this could be related to a variety
of factors such as reduced Pn (Flexas et al., 2004) and interfered
assimilate distribution (Farooq et al., 2009), resulting in a decreased
biomass trend under high temperature and compound heat and drought
stress, which is consistent with the results of many studies (Fahad et
al., 2017; Daryanto et al., 2016). The combined drought and high
temperature stress had an inhibitive effect on O. ochrocephalaand S. pulchra , while drought had a cushioning effect on the
effects of the combined drought and heat stress on K. humilis andP. annua, with the temperature being dominant in the combined
treatment. The BGB of O. ochrocephala and S.pulchra was the highest under the high temperature stress. The
BGB of K. humilis and P. annua had a decreasing trend
under the high temperature stress. Our results showed that high
temperature stress was beneficial for root growth, which made plant
productivity shift to an underground distribution and inhibited the BGB
of plants with shallow roots. The rising temperature would lead to a
decrease in the surface soil water content, making the SM condition a
key factor limiting the root growth of plants with shallow roots (Yu et
al., 2015).
Photosynthesis, the most important physiological and biochemical plant
activity, affects material conversion and energy metabolism in plants,
and it has a strong response to drought and high temperatures (Chakhchar
et al., 2016; Jumrani et al., 2017). The net Pn of plants decreases
under drought and high temperature stress, with the extent of the
decrease varying among plants (Jing et al., 2013; Jumrani et al., 2017).
Stomatal factors are the main reasons for the Pn decrease under mild
water stress, while under severe water stress the chlorophyll structure
damage results in a Pn decrease (Bray, 1997). The results of this study
showed that the increasing drought resulted in the gradual water loss by
the leaves of the four plant species, while the net Pn and Tr of the
leaves also decreased. These results indicate that photosynthetic
restriction factors consist mainly of stomatal restriction under drought
stress and can prevent excessive water loss and ensure effective water
utilization (Zhao et al., 2002). The net Pn of K. humilis andP. annua increased with the high temperature stress increase,
while that of O. ochrocephala and S. pulchrainitially increased and then decreased significantly with the high
temperature stress increase, which was also true for the LWC. Compared
with K. humilis, P. annua , O. ochrocephala, and S.pulchra are less able to withstand high temperatures and regulate
water. Under severe high temperature stress, O. ochrocephala andS. pulchra may have non-stomatal restriction and inhibit
photosynthesis; this reflects the difference in the plant species’
ability and manner of regulating water (Xu et al., 2013). Lawlor and
Cornic (2002) found that stomatal responses are highly variable under
drought conditions across plant species; the results of our study showed
that the Gs differed significantly among the four plant species and did
not change based on obvious rules, thereby confirming that there was not
necessarily a linear relationship between Gs and drought stress.
The combined effect of drought and heat stress on plants is greater than
that of a single stressor (Dreesen et al., 2012). When drought and heat
stress occur simultaneously, the Gs and transpiration decrease, and heat
persecution is caused by the increase of the leaf temperature (Lamaoui,
et al., 2018). The results of the present study demonstrated that the
promotion effect of high temperature stress on Pn did not alleviate the
influence of drought stress on the photosynthetic characteristics of the
four plant species, but produced a superimposed effect together with
drought stress, thereby leading to a Pn decrease under combined stress.
This indicates that the combined stress had a greater influence on Pn
than a single stress, and the water condition played a dominant role.
Rollins (2013) found that photosynthesis is not affected under drought
stress, but decreases significantly under a combined drought and high
temperature treatment, with temperature playing a dominant role in the
combined stress; these results are significantly different from those
obtained in the present study. The most plausible explanation for this
outcome may be related to the characteristics of alpine meadow plants,
which are more sensitive to water changes than temperature changes owing
to their long-term development and survival in cold and wet environments
(Xu & Xue, 2013).
High temperature stress inhibits the Tr of O. ochrocephala andS. pulchra , and higher stomatal resistance reduces the
transpiration rate after a temperature rise, thus preserving the plant
moisture status (Liu et al., 2005). This was also demonstrated by the
WUE analysis (Figure 5). The WUE of O. ochrocephala increased
under the H2 treatment and that of S. pulchra increased
significantly under the H1 treatment (P <0.05). The
effects of drought were dominant in the compound stress and high
temperature stress treatments as these promoted the WUE of alpine meadow
plants. However, this would reduce the cooling effect of transpiration,
leading to an increase in leaf temperature, especially under the
combined stress of two factors, which would affect the photosynthetic
capacity of the plants. The WUE of Kobresia humilis and P.
annua increased significantly under the H1 treatment and decreased
under the H2 treatment, showing a certain range of adaptation to high
temperature stress, with the influence of high temperature being
dominant in the compound stress (Figure 5). The mitigation of heat
stress is achieved by a reduction in Tr, an increase in WUE, an
enhancement of photoenzyme activity, and an increase in the defense
molecules of the four plant species (Blum, 2009).
Due to the lack of continuous observation of physiological indexes of
alpine meadow plants, this study could not obtain the characteristics
and rules of alpine meadow species changes in small-scale time series,
which had certain limitations. Therefore, in the face of the
environmental effects of climate warming and frequent bouts of extreme
weather, it is necessary to research the heat and drought tolerance
mechanisms of alpine meadow plants with stress time and identify the
genetic differences that determine their stress responses, in order to
improve plant stress performance.