Comparison of Rice Yield and Evaluation Methods for Drought Resistance under Drought Stress in the Whole Growth Period and Flowering Period

Qian ZHU　Wuyun FANG　Yiru LI　Yueming YI　Cunli GONG　Jingde WU　Shimei WANG

Abstract[Objectives] This study was conducted to improve the drought resistance of rice and cultivate new varieties of drought-tolerant rice to effectively alleviate the reduction in rice yield caused by drought. [Methods]Twenty two indica and japonica rice varieties were tested to compare the yield differences under drought stress in the whole growth period and under drought stress in the flowering period. [Results] Drought stress in the whole growth period and flowering period significantly reduced rice yield， as well as the number of filled grains per plant， the number of filled grains per panicle and grain weight per plant. To evaluate the drought tolerance of rice varieties， relative yield could be used as the index under both drought stresses in the whole growth period and the flowering period. [Conclusions]This study provides a theoretical reference for drought-tolerant rice breeding and screening.

Key wordsRice; Drought stress; Drought resistance; Yield

Rice is one of the important food crops in the world， and it is the main food for more than half of the worlds population. The 2020 United Nations World Water Development Report pointed out that with population growth and economic development， global water consumption is growing steadily at a rate of about 1% per year. Globally， the area of dryland is projected to expand significantly. The freshwater resources available for agricultural production will continue to decrease， and drought caused by water shortages will severely limit the normal growth and development of plants. The water consumption for rice production accounts for 70% of the total agricultural water consumption. The average annual loss of rice yield due to drought is about 143-250 kg/hm2， and in severe years， it reaches 2 700 kg/hm2. Improving the drought resistance of rice and cultivating new varieties of drought-tolerant rice can effectively alleviate the bottleneck of rice yield reduction caused by drought.

Drought will affect the morphology， physiology and cells of rice to varying degrees， manifested as leaf curling， limited photosynthetic capacity， decreased root activity， damaged membrane structure， disrupted nucleic acid metabolism， imbalanced hormone metabolism in the body， inhibited growth and development， reduction in seed setting rate， etc.， eventually leading to a reduction in production. The drought resistance of rice is related to the genetic characteristics of varieties， soil environment， morphological characters， physiological indexes， and the time and intensity of drought. The drought resistance of rice is therefore divided into four types： drought avoidance， drought tolerance， drought recovery and drought escape. Therefore， in the evaluation of drought resistance of rice， the identification methods of drought resistance are relatively complex， and the evaluation indexes are different. According to statistics， agronomic indexes related to rice drought resistance mainly include agronomic traits， such as plant height， panicle length， leaf shape， leaf water potential， stomata， leaf cuticle， root morphological traits， seed setting rate and yield. They also include physiological and biochemical indexes， such as chlorophyll content， proline content， relative water content， stomatal resistance， plasma membrane permeability， MDA， SOD， CAT， amylase activity and abscisic acid. These indexes directly or indirectly reflect the differences in rice drought resistance， and there are differences in accuracy and difficulty in the actual measurement process. Mau et al. believed that using yield as an index for drought resistance identification is an effective method to select genotypes that combine drought resistance with high-yield potential. In addition， the most closely related， most influential， and most direct index with dry rice yield is seed setting rate， which can intuitively reflect the drought resistance of rice. At present， under controllable drought stress conditions， yield and seed setting rate are commonly used indexes for evaluating the drought resistance of rice in production. However， whether different degrees of drought stress in different growth period will have an impact on the evaluation results needs to be further explored.

Materials and Methods

Experimental materials

In the experiment， 16 indica rice varieties （lines） including Xinliangyouhan 168， Wanhanliangyou 25， Kenxuan 92， Hanyou 737， and Hanyou 73 and 6 japonica rice varieties （lines） including Handao 502 and Lvhanjing 1 were selected. A total of 22 varieties （lines） were used as test materials.

Experimental design

The experiment was carried out in 2021 at the experimental base of Rice Research Institute of Anhui Academy of Agricultural Sciences， in Guohe Town， Lujiang County. The seeds were sown two times， on May 16 and May 26， respectively， and the seedlings were transplanted for 30 d.

Drought treatment in the whole growth period： A field test was adopted. The transplanting was performed according to a specification of 17 cm×20 cm， 5 rows per plot， 10 plants per row. The test adopted randomized block arrangement， and each treatment was repeated three times. Two treatment methods， i.e.， paddy field treated with normal water management and dry field was watered enough when transplanting seedlings， and after the seedlings were alive， keep the soil in a slightly dry state （soil water potential -25--15 kPa） using drip irrigation equipment. The dry field was surrounded by cement walls， and had a field surface 1.5 m above the ground. Daily open-air management was adopted， and the field was provided with a mobile greenhouse， which could be covered in case of rain.

Drought treatment in the flowering period： A pot experiment was adopted and carried out in a mobile greenhouse. It was managed in a daily open-air mode. During the treatment period， the mobile greenhouse could be covered when it was raining. The pots used were 27 cm high， and had an upper diameter of 24 cm and a bottom diameter of 21 cm， and there were drainage holes on the side at the bottom， which could freely control the removal and retention of water in the pots. Each pot was filled with 10 kg of dry prepared nutrient soil. The nutrient contents of the soil were as follows： organic matter content 18.6 g/kg， total nitrogen 1.4 g/kg， total phosphorus 0.7 g/kg， available phosphorus 64.0 mg/kg， available potassium 377.0 mg/kg， alkali-hydrolyzable nitrogen 186.9 mg/kg， and the pH value was 5.4. The experiment consisted of two treatments， control and drought， and each treatment was repeated in 5 pots with 1 plant in each pot. Both treatments were carried out with normal irrigation management before flowering， and drought treatment was carried out in the flowering period. Since the main spike began to be exposed from the bracts （about 1cm）， the control group was kept with a shallow water layer. In the drought treatment group， the rubber stopper at the bottom of each pot was removed， and the soil was dried naturally. When the leaves of the whole rice plant were severely curled and could not be recovered the next morning， the plants were re-watered. The drought treatment time was the number of days from pulling out the rubber stopper to re-watering （the number of treatment days was adjusted appropriately according to temperature， humidity， plant biomass and leaf rolling degree）. During the treatment period， the soil moisture status was monitored in real time， and the leaf rolling status of the plants was recorded. After the treatment， all treatment groups were given normal irrigation management until the plants were mature and harvested.

Yield and major yield-related traits under drought and control treatments were determined at the maturation stage.

Statistical analysis methods

ANOVA and correlation analysis were performed using Excel and SPSS 22.0.

Relative seed setting rate （%）=Seed setting rate under drought treatment of variety / Seed setting rate under control treatment of variety：

Drought resistance index （DH1）=（X1/CK1）/（X2/CK2）

X1： Yield of the test variety under drought stress; X2： yield of the test variety under control treatment; CK1： yield of the control variety under drought stress; CK2： yield of the control variety under control treatment.

Results and Analysis

Effects of drought stress in the whole growth period on rice yield and yield components

In different water treatments， rice plant height， effective panicles per plant， panicle length， filled grains per plant， the total number of grains per plant， the total number of grains per panicle， filled grains per panicle， seed setting rate， 1 000-grain weight and grain weight per plant had no significant differences between different sowing dates （Table 1）， while the differences among varieties and between treatments reached the extremely significant level （P<0.01）. Overall， drought stress during the whole growth period significantly reduced the plant height， effective panicles per plant， panicle length， filled grains per plant， the total number of grains per plant， the total number of grains per panicle， filled grains per panicle， seed setting rate， 1 000-grain weight and grain weight per plant. In addition， plant height， effective panicles per plant， panicle length， filled grains per plant， the total number of grains per plant， seed setting rate， 1 000-grain weight and grain weight per plant had extremely significant interaction between variety and treatment， indicating that the yield and its main components in different rice varieties had different changes under drought stress.

The results of correlation analysis showed （Table 2） that under different water treatments in the whole growth period， rice yield was significantly positively correlated with plant height， effective panicles per plant， panicle length， filled grains per plant， the total number of grains per plant， the total number of grains per panicle， filled grains per panicle， seed setting rate and 1 000-grain weight; there was a significant positive correlation between seed setting rate and the number of effective panicles， filled grains per plant and filled grains per panicle; 1 000-grain weight had a significant positive correlation with yield， but had little correlation with other parameters; and there was a significant positive correlation between the number of effective panicles and yield， filled grains per plant， the total number of grains per panicle， filled grains per panicle， the total number of grains per plant and seed setting rate.

Effects of drought stress in the flowering period on rice yield and yield components

Under different water treatments in the flowering period， the differences in plant height， effective panicles per plant， seed setting rate， 1 000-grain weight and grain weight per plant between different sowing dates， varieties and treatments all reached the extremely significant level （P<0.01， Table 3）. Sowing date had little effect on grains/filled grains per plant and grains/filled grains per panicle under different water treatments in the flowering period， but affected 1 000-grain weight and grain weight per plant. Delayed sowing date reduced 1 000-grain weight and grain weight per plant. Overall， drought stress in the flowering period significantly decreased plant height， effective panicles per plant， filled grains per plant， filled grains per panicle， seed setting rate， 1 000-grain weight and grain weight per plant. In addition， the number of filled grains per plant， the total number of grains per plant， the number of filled grains per panicle， the total number of grains per panicle， seed setting rate and grain weight per plant had extremely significant interaction between variety and treatment， indicating that there were differences in the changes of yield and its main components of different rice varieties under drought stress in the flowering period. The interaction among sowing date， variety and treatment was small， and only grain weight per plant showed a significant difference for the interaction among the three.

The results of correlation analysis showed （Table 4） under different water treatments in the flowering period， grain weight per plant were in a significant positive correlation with the number of effective panicles per plant， panicle length， the number of filled grains per plant， the total number of grains per plant， the total number of grains per panicle， the number of filled grains per panicle and seed setting rate. There was a significant positive correlation between seed setting rate and the number of effective panicles， the number of filled grains per plant and the number of filled grains per panicle. In addition， the number of effective panicles had a significant positive correlation with yield， the number of filled grains per plant， the number of filled grains per panicle， the total number of grains per plant and seed setting rate.

Comparison of yield and evaluation methods for different varieties under drought treatment in the whole growth period and flowering period

Changes of relative seed setting rate and relative yield are commonly used parameters for the evaluation of drought resistance of rice varieties. The twenty two varieties were subjected to different water treatments in the whole growth period （Table 5）. With relative seed setting rate as the evaluation standard， the relative seed setting rates of different rice varieties were between 72.1% and 112.2%. Among indica rice， IR26， Hanliangyou Yesi， Quanguangyou Simiao， Xinliangyou 128 and Yinliangyou Jietian Simiao had higher relative seed setting rates， ranking the top 5. The relative seed setting rates of the japonica varieties Handao 502， Longke 15077 and Suken 118 were all above 90%. After drought treatment， the relative yields of different rice varieties （lines） changed between 32.48% and 91.69%. With relative yield as the evaluation standard， the relative yields of the indica rice Quanguangyou 601， Hanliangyou Yesi， Quanguangyou 532， Wanhanliangyou 25 and Lvhanliangyou 21 were all greater than 60%; and the relative yields of japonica rice Lvhanjing 1 and Handao 502 were all higher than 60%.

The twenty two varieties were subjected to different water treatments in the flowering period （Table 6）. With relative seed setting rate as the evaluation standard， the relative seed setting rates of different rice varieties were between 54.77% and 85.50%. The five varieties （lines） with higher relative seed setting rates in indica rice were Hanliangyou Yesi， Luliangyou 1187， Hanyou 737， Quanguangyou 532 and Wanhanliangyou 25， and the japonica rice varieties with relative seed setting rates higher than 80% were Suken 118 and Longke 15077. The relative yields ranged from 35.04% to 97.13% for different rice varieties after drought treatment in the flowering period.  With relative yield as the evaluation standard， the relative yields of the indica rice Quanguangyou Simiao， Quanguangyou 532， Wanhanliangyou 25， Hanliangyou Yesi， Lvhanliangyou 21 and Hanyou 737 were all greater than 60%， and five japonica rice varieties （lines） with relative yields higher than 60% were Lvhanjing 1， Handao 502， Wanken Hannuo 122， Suken 118， Longke 15077 and Xinxiangnuo 286 from high to low.

To sum up， through the whole growth period drought treatment and the flowering period drought treatment of different rice varieties， the results showed that comparing the two drought stress treatments， the effect of the whole growth period drought stress on rice seed setting rate was smaller than that of the flowering period drought stress， but the effects of the two treatment methods on yield were relatively consistent. With relative yield as the evaluation standard， under the drought stress during the whole growth period and the stress during the flowering period， the relative yields of the indica rice Hanliangyou Yesi， Wanhanliangyou 25， Lvhanliangyou 21 and Quanguangyou 532 were all higher than 60%， which was higher than the control variety Hanyou 73. In addition， under the drought treatment in the flowering period， with relative seed setting rate as the evaluation standard， Hanliangyou Yesi， Wanhanliangyou 25 and Quanguangyou 532 were also higher than the control variety. It could be seen that with relative yield as the evaluation standard， the consistency was high under the drought stress treatments in the whole growth period and the flowering period， and the evaluation standard using relative seed setting rate also had certain consistency under the stress during the flowering period， but the evaluation standard using relative seed setting rate under the drought stress in the whole growth period was not accurate.

Conclusions and Discussion

Under different growth periods and different water treatments， there were significant differences between rice varieties and treatments used in this study. Overall， drought stress in the whole growth period and flowering period significantly reduced the number of filled grains per plant， the number of filled grains per panicle and grain weight per plant. Drought stress in the whole growth period also significantly reduced plant height， effective panicles， panicle length， the total number of grains per plant and the total number of grains per panicle， and drought stress in the flowering period significantly reduced the seed setting rate of rice. It could be seen that both drought during the whole growth period and drought during the flowering period caused rice yield reduction， but the physiological mechanisms of rice response to drought stress were different.   Under the condition of drought stress in the whole growth period， rice could resist drought stress by reducing tillering and the number of spikelets. When drought stress occurred in the flowering period， the number of spikelets was basically formed because young panicles have been differentiated， so the occurrence of drought stress at this time had little effect on the total number of grains per plant of rice plants， but reduced seed setting rate by affecting the later development of young panicles and reducing the formation of grains， resulting in reduced yield. Under drought stress treatments in different growth period， rice yield was significantly positively correlated with the number of effective panicles per plant， panicle length， the number of filled grains per plant， the total number of grains per plant， the total number of grains per panicle， the number of filled grains per panicle and seed setting rate. There was also a significant positive correlation between rice yield and plant height and 1 000-grain weight under the drought stress in the whole growth period.

The number of filled grains per plant， total number of grains per plant， seed setting rate， 1 000-grain weight and grain weight per plant had extremely significant interaction between variety and treatment， indicating that there were differences in the changes of yield and main components of different rice varieties under drought stress. Different rice varieties were subjected to drought treatments during the whole growth period and drought during the flowering period， and the results showed that comparing the two drought stress treatments， the effect of drought stress in the whole growth period on the seed setting rate of rice was smaller than that under the drought stress in the flowering period， and the seed setting rate of rice under drought stress during the whole growth period was significantly higher than that under drought stress during the flowering period. The effects of the two treatments on yield were relatively consistent. With relative yield as the evaluation standard， under drought stress during the whole growth period and stress during flowering， the relative yields of the indica rice Hanliangyou Yesi， Wanhanliangyou 25， Lvhanliangyou 21 and QuanGuangyou 532 were higher than 60%， and ranked in the forefront， and they were higher than the control variety Hanyou 73.

It is worth noting that the relative yields of the varieties （lines） with higher relative yields under drought stress in the whole growth period were higher than those under drought stress in the flowering period， while for the varieties （lines） ranked lower in relative yield， the relative yields under drought stress in the whole growth period were lower than those under drought stress in the flowering period. In addition， under the drought treatment in the flowering period， with relative seed setting rate as the evaluation standard， Hanliangyou Yesi， Wanhanliangyou 25 and Quanguangyou 532 were also higher than the control variety， which was relatively consistent with the evaluation with relative yield as the evaluation index under the two drought treatments. It could be seen that with relative yield as the evaluation standard， the consistency was high under the drought stress treatments during the whole growth period and the flowering period， and the evaluation standard using relative seed setting rate also had certain accuracy under the stress during the flowering period， but the evaluation standard using relative seed setting rate under the drought stress during the whole growth period was not accurate.

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Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU