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.