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.