1. INTRODUCTION
In the process of growth and development, plants often suffer from the stress imposed by biotic and abiotic factors (Salvucci et al., 2004). Among the latter, high temperature and drought are the main environmental factors that limit the growth and productivity of various plants (Sita et al., 2017). The fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) points out that global warming is an indisputable fact and the global temperature is predicted to rise by 1.8–4℃ by the end of the 21st century (Pachauri et al., 2014). Recent climate change models predict that the frequency and duration of periods of high temperatures and moisture deficits are on the rise, especially for extreme climatic events (Chen et al., 2010); for instance, high temperatures are accompanied by exceptional drought in some regions (Field et al., 2012). These harsh climates cause changes in the physiological and ecological processes of plants in the region, hinder plant growth and development (Allen et al., 2010), and seriously affect plant distribution and productivity. It follows that such climates have become important environmental factors affecting plant growth and development (Song et al., 2018). Recent studies have revealed that the response of plants to a combination of different abiotic stresses is unique and cannot be directly extrapolated from the response of plants to each of the different stresses applied individually (Ahmed et al., 2013). This emphasizes the necessity to study the vegetation response of terrestrial ecosystems to drought, high temperature, and their combined stress, which have become the focus of research in recent years (Mahalingam, 2015; Zandalinas et al., 2017; Lawas et al., 2018).
The co-occurrence of drought and high temperature is more harmful to plant growth than the single influence of one of these factors (Nankishore & Farrell, 2016; Sehgal et al., 2019). The response of the photosynthetic parameters of plant leaves and plant productivity to high temperatures and drought reflect the light energy utilization efficiency and growth rate of the plant photosynthetic system. Research has shown that when heat and drought stress co-occur, they initiate various processes, such as the accumulation of large amounts of harmful substances in plants, a decreased photosynthetic rate by changing the activity of various enzymes involved in plant photosynthesis and metabolism coupled with abnormal respiration, closed stomata, high leaf temperature, and reduced water use efficiency in plants (Hao et al., 2019; Qaseem et al., 2019). Furthermore, various studies have demonstrated that the impact of heat and drought stress on plant growth, yield, and physiology varies among different crops (Bakhshandeh et al., 2019), which is also related to the different characteristics of each species. The effects on yield or any other trait may be synergistic, antagonistic, or hypo-additive (Pradhan et al., 2012; Prasad et al., 2011).
The effects of the global climate change are most visible on the physiological and ecological processes of plants in high latitudes and altitudes, owing to the faster temperature rise in these regions (Peñuelas et al., 2013; Li et al., 2018). Kobresia humilis, Poa annua, Oxytropis ochrocephala, and Saussurea pulchra are typical representative functional group Sedge, Graminoid, Legume, and Forb species, respectively, in a K. humilis meadow, and are also key plant species related to the stability of alpine grassland ecosystems (Ren et al., 2013; Zhao et al., 2015). These plants, which adapted to living in cold, humid plateau climates long ago, are more sensitive to the combined effects of drought and heat caused by global climate change. The existing research on the combined effects of high temperature and drought mainly focuses on herbaceous ground covers (Song et al., 2018), arbor forest species (Huang et al., 2011; Read et al., 2014), and crops such as rice (Lawas et al., 2019), wheat (Pradhan et al., 2012), and lentils and their seeds (Sita et al., 2017; Sehgal et al., 2019). However, there are relatively few studies on alpine grassland plant species distributed in the Qinghai-Tibet Plateau. Moreover, in some studies, it proved difficult to control the environmental factors accurately in field experiments, while their applicability was poor (Zhang et al., 2010; Yu et al., 2015), owing to a lack of research data on the physiological ecology of individual alpine meadow species under the influence of drought and high temperature or their combined effects.
As there are relatively few indoor studies on the physiological and ecological response and adaptation strategies of alpine meadow plants, our research objects in this study were typical alpine meadow plants (K. humilis, P. annua, O. ochrocephala, and S. pulchra ). These were subjected to drought, heat, and the combined stress of the two in an artificial climate chamber, with the aim of revealing the physiological response of alpine meadow species to continuous environmental stress. More specifically, the present study aimed:
(1) To evaluate the effects of drought, heat, and their combined stress on the productivity and photosynthetic characteristics of four different alpine meadow species under a controlled environment.
(2) To detect differences in the performance of different alpine meadow species under heat, drought, and their interaction.
(3) To explore the dominant environmental factors that affect the response of different alpine meadow plants under drought, heat, and their interaction.
Our results can provide a theoretical basis for the rational utilization of grassland resources in alpine regions under the threat of climate change.