Effects of Drought Stress During Critical Periods on Photosynthesis Characteristics and Production Performance of Naked Oat (Avena Nuda. L)

Revealing the effects of drought stress during the critical period on the photosynthetic characteristics and production performance of naked oats could provide theoretical basis for optimizing the water managements and selecting the drought resistance cultivars. In this study, a potted experiment consisting of four water levels was conducted, to investigate the response of photosynthesis, chlorophyll uorescence, biomass, yield and yield components of to drought stress during 12-15 days before heading at a typical site in Zhangjiakou. Results showed that the initial chlorophyll uorescence rate (F o ) increased by 9.03-50.92% under drought stress, while the maximum uorescence rate (F m ) and photochemical eciency (Fv/Fm) decreased by 8.49-19.73% and 10.37-24.12%. Moreover drought stress decreased the photosynthetic rate (P n ), transpiration rate (T r ), stomatal conductance (G s ). The CO 2 concentration (C i ) decreased under light drought stress, while increased under moderate and severe drought stress. Drought stress during critical periods also had signicant impacts on oat yield and yield components. Oat yields decreased by 9.5-12.7%, 16.8-27.0% and 44.1-47.7% under light, moderate and sever drought stress, respectively. The yield components such as grains per spike, 1000-grain weight were decreased by 1.7-12.5%, 8.3-24.3%, 32.7-34.2% and 5.7-8.6%, 12.7-14.5%, 16.8-19.1% under light, moderate and severe drought stress, respectively. But the Spike numbers were not signigantly different between different treatments. Our study explored the impacts of drought stress on the photosynthetic characteristics and production performance of naked oats, which had signicance for enhancing the production eciency for oat under drought stress. the mechanism of different use eciency (WUE) in area, results showed that with the extension of drought stress time and the aggravation of drought stress degree, the main limiting factors for the decrease of photosynthetic rate were change from stomatal factors to non stomatal factors, and different germplasm had different responses to water stress.


Introduction
Naked oat (A. nuda L) is a traditional food, feed and forage crop in North China (Ren and Hu, 2013). Due to its rich nutrients content, it is signi cance for keeping the human health (Ren and Hu, 2013). However, oat is a water-costing crop, and the water requirement of which during growing period is more than wheat and barley (Xiao and Yang, 1992). Drought stress has signi cant impacts on oat production, especially during critical growing periods, i.g. 12-15 days before heading (Ren and Hu, 2013). Research shows that water stress at different stages (tillering, jointing and owering stages) can reduce the grain weight per plant, number of effective ears, number of spikelets and 1000 grains weight, which result in grain shrinkage, and then affect the yield of oats. So drought stress at each stage can reduce the photosynthetic products transported from oats to grains (Wang, 2019). Water shortage decreases the photosynthetic rate, stomatal conductance and intercellular CO 2 content .
Photosynthesis is the basis for dry matter accumulation and yield formation (Zhongwu Li et al.,). However, drought stress mainly damages the physiological metabolism and photosynthesis of plants (Xu, 2002;, which reduce crop production (Zheng et al., 2006;Nielsen et al., 2009). Leaf is the main photosynthetic organ, and its photosynthetic capacity was mainly in uenced by water condition (Boyer, 1982;Tezara et al., 1999). When the leaves are damaged, photosystem PS is the most sensitive (Tezara et al., 1999). Under water stress, the light use e ciency and photosynthetic capacity of oat leaves decreased . And drought stress results in the decrease of the whole performance of photosystem , which leads to the decrease of dry matter accumulation (Gao et al., 2015).
Bashang area of Hebei Province is one of the staple naked oats production areas in China. However, water shortage, characterized by low and highly variable amounts of precipitation, is the major limiting factor for oat production in this region (Chavers et al., 2009;Li et al., 2021). Some studies were conducted to evaluate the impacts of drought stress on oat production in this region. Ge (2010) studied the photosynthetic mechanism of different water use e ciency (WUE) in Bashang area, results showed that with the extension of drought stress time and the aggravation of drought stress degree, the main limiting factors for the decrease of photosynthetic rate were change from stomatal factors to non stomatal factors, and different germplasm had different responses to water stress.
However, the evaluation based on the structure and function of light system was rarely in this region.
Therefore, the objectives of this study were to: 1) reveal the response of structure and function of light system of naked oats to different drought stress levels; 2) understand the energy balance of oat leaves under drought stress; 3) explore the change rule of photosynthetic electron transfer and gas exchange parameters under different drought stress.

Study site, climate and soil data
The experiment was conducted at Xishungou station (41°3′54″N, 114°4′18″E) of Zhangjiakou Academy of Agricultural Sciences from 2018 to 2019. The station is characterized by a typical continental climate with abundant solar radiation, warm summers and cold winters. The annual total solar radiation, average temperature and precipitation were 5400-6120 MJ/m 2 , 3.7℃and 400mm respectively. The soil type is chestnut soil and the detailed soil information is shown in Table 1   Table 1 Vertical distribution of the physical and chemical properties of soil in the study site.

Experimental design
To explore the impacts of drought stress on the on photosynthesis characteristics and production performance of Naked oat, potted experiments were conducted for two years from 2018 to 2019, with four water levels each year. The cultivar of Huazao2 was planted, which bred by Zhangjiakou Academy of Agricultural Sciences. The plastic buckets with same speci cations (inner diameter 28.5cm, depth 33.5cm) were used to plant the materials and every bucket load 23.15kg of air dried chestnut soil (the basin weight is 0.85kg). The maximum soil water holding capacity was tested by the cutting ring method and soil nutrient parameters are (to be tested). A plastic lter screen is arranged at the bottom of the barrel to separate the bottom and the upper part of the barrel from a certain space, and a hard plastic pipe beside the bucket leading to the bottom of the barrel is used for watering. Four treatments were set in the experiment: normal water supply (CK), light drought stress (LS), moderate drought stress (MS) and severe drought stress (SS), the soil relative water content were 70-80%, 60-70%, 50-60% and 35-45% of the maximum soil water capacity respectively. There were 6 repetitions in total, 20 seedlings per basin were xed in three leaf stage, water control was started at about 20 days before booting stage, and natural drought to the standard range of soil moisture content. At 8:00 and 18:00 every day, water was replenished and controlled by weighing method and recorded, which lasted for 7 days after reaching the expected 4 different levels of drought stress, and photosynthetic rate and chlorophyll uorescence were measured for 3 consecutive days. After that, all treatments were restored to normal watering level. Except for the obvious difference of soil moisture in the bucket, the other management was all the same. Each index was measured with the third leaf of each repeated sample. After maturing and harvesting, the economic and yield characters of naked oats were measured in the laboratory.

Photosynthetic rate
The photosynthetic rate of naked oats leaves were measured at 09:00-11:00 am in sunny day by LC pro + portable full-automatic photosynthetic measurement system produced by ADC company in UK under natural condition. Each treatment repeated 6 times. Three healthy functional leaves on naked oats were selected for the measurement. Three data were measured each time. The average value was taken as the measured value at that time. The parameters obtained were net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO 2 concentration (Ci) and transpiration rate (Tr).

Chlorophyll uorescence:
The chlorophyll uorescence value of naked oats was determined by the Fluerpen FP100 chlorophyll uorescence meter produced in Czech Republic. The parameters obtained are the initial uorescence value Fo, the maximum uorescence value (Fm), and the light energy conversion e ciency Fv/Fm of the PS center.

The yield and yield components of oat
After harvested in mature stage, 10 plants taken from each pot were tested, and plant height, ear length, spikelet number, ear grain weight, stem number, ear number, ear grain number, 1000 grain weight are measured in the Lab. After seed test, grain yield and biomass are measured, and the remaining 10 plants in the basin are combined to calculate the total 20 plants grain yield and biomass in each pot.

Statement
All experimental research and eld studies on plants in this article comply with relevant institutional, national, and international guidelines and legislation.

Effects of drought stress on photosynthetic characteristics
The photosynthetic rate under LS, MS and SS were signi cantly lower than the CK (P<0.05), and the values decreased by 9.30%, 14.08% and 20.89% respectively in 2018, for 2019 the decrease amounts were 14.43%, 16.85% and 24.55% ( Fig. 1a and b). With the increase of drought stress, the Ci decreased rst and then increased ( Fig. 1c and d). Compared with CK, the Ci decreased by 12.41% under LS, while it increased by 9.49% under SS, and the difference was not signi cantly under MS in 2018 (Fig. 1c). In 2019, the Ci under LS was signi cantly increased. The Ci under MS and SS was 4.94% and 13.09% higher than that of CK (P<0.05), respectively (Fig. 1d). With the increase of drought stress, the Tr gradually decreased (Fig. 1e and f). The Tr under LS, MS and SS in two years was signi cantly lower than CK (P<0.05), and the Tr decreased by 10.83%, 41.39% and 45.61% in 2018 (Fig. 1e), for 2019 the values were 21.85%, 29.30% and 45.77% (Fig. 1f). With the aggravation of drought stress, Gs of naked oats showed a decreasing trend ( Fig. 1g and h). In 2018, the difference of Gs under LS and CK was not signi cance, while in MS and SS the Gs decreased by 35.68% and 78.89%, respectively compared with CK (Fig. 1g). In 2019, the Gs under LS, MS and SS were 26.04%, 46.88% and 85.42% lower than CK (P<0.05) (Fig. 1h).

Effects of drought stress on photosystem
The initial uorescence value (Fo) increased with the aggravation of drought stress ( Fig. 2a and b). In 2018, the Fo under LS, MS and SS were 9.03%, 9.89% and 14.13% higher than CK (P<0.05) (Fig. 2a). While, in 2019, the Fo was only increased signi cantly under MS and SS values were 24.84% and 50.92% higher than CK (P<0.05) (Fig. 2b). Maximum uorescence value (Fm) decreased with the increase of drought stress (Fig. 2c and d). The difference of F m under LS and CK was not signi cant (P>0.05) in 2018, while the Fm under MS and SS were 8.49% and 19.73% lower than that CK (P < 0.05) (Fig. 2c). In 2019, the Fm under LS, MS and SS were 10.02%, 15.83% and 21.89% lower than CK (P<0.05) (Fig. 2d). Variable uorescence of PS decreased signi cantly under drought stress ( Fig. 2e and f) Fig. 2e and f). Drought stress decreased the activity of PS signi cantly ( Fig. 2g and h) Fig. 2g and h). Fv/Fm values under drought stress were lower than CK ( Fig. 2i and j). There was no signi cant difference between LS and CK (P>0.05), while the values under MS and SS were signi cantly CK (P<0.05), which decreased by 10.37% and 24.12% (Fig. 2i). In 2019, Fv/Fm under LS, MS and SS decreased by 13.3%, 22.44% and 36.83%, respectively (Fig. 2j).

Effects of drought stress on main economic characters
With the aggravation of drought stress, the plant height of naked oats decreased gradually (   (Table 2). Drought stress also decreased the harvest index signi cantly in both years (Table 2).

The impacts of drought stress on photosystem
Chlorophyll uorescence can re ect the primary photosynthetic reaction process including the absorption of light energy, the transmission of excitation energy and photochemical reaction (Zhang et al., 2004). The degree of damage caused by stress can be re ected by measuring chlorophyll uorescence parameters, the light utilization in PS (Miyashita et al., 2005). Under drought stress, plants maintain the balance of water budget by reducing the transpiration rate, which is an adaptive way to avoid drought (Schreiber et al., 1994). The Fo re ects the damage degree of thylakoid membrane, more serious the damage of thylakoid membrane induced the higher Fo (Schreiber and Bilger, 1993). The maximum uorescence re ects the electron transfer through PS . Lower Fm re ects the higher the degree of thermal damage (Schreiber and Bilger, 1993 (Csintalan et al., 1999;He et al., 2005), and it will be signi cantly reduced under adversity or injury conditions (Xu et al., 1992). Our results of two years showed that Fo in oat leaves increased with the increase of water stress during the critical period, which indicated that the damage degree of thylakoid in leaves increased gradually with the deepening of water stress. Fm decreased under water stress, which indicated that the electrical transmission of leaves through PS was inhibited and the degree of heat damage was deepened.
Fv/Fo and Fv/Fm in oat leaves were lower than those under CK, which indicated that water stress reduced the e ciency of capturing light energy in PS reaction center of leaves.

Effect of drought stress on economic and yield characters of naked oats
In the critical period of water requirement, the plant height, ear length, ear grain weight, stem number and

The limitation of the study
Due to the limited conditions, the experiment only controlled the soil water content, and set the degree of soil drought stress based on it, but did not measure the actual stress degree of oat plants. In addition, only photosynthesis, uorescence and laboratory test were carried out in the experiment, and the changes of physiological indexes related to drought stress of oat were not measured. The next step is to determine the stress degree of oat according to the leaf water potential, and to determine the drought stress related indicators such as superoxide dismutase (SOD) and peroxidase (POD) activities, malondialdehyde (MDA) content etc.

Conclusions
In the critical period of water requirement, drought stress reduced the photosynthetic e ciency of oat. Under light drought stress, stomatal restriction was the main factor that led to the decrease of photosynthetic rate of naked oat. Under moderate and severe drought stress, the thylakoid of naked oat leaves was damaged, the electron transfer through PS was inhibited, and non-stomatal restriction gradually became the main factor that led to the decrease of photosynthetic rate. The decrease of photosynthetic rate resulted in the inhibition of economic characters, yield components, seed yield and biomass of naked oats. In 2018, the yield of light drought stress, moderate drought stress and severe drought stress decreased by 12.7%, 26.9% and 44.1% respectively, and in 2019, the yield of moderate and severe drought stress decreased by 16.85% and 47.76% respectively.