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Effect of High Temperature During Heading and Early Filling on Grain Yield and Physiological Characteristics in Indica Rice
ACTA AGRONOMICA SINICA Volume 35, Issue 3, March 2009 Online English edition of the Chinese language journal Cite this article as: Acta Agron Sin, 2009, 35(3): 512–521.
RESEARCH PAPER
Effect of High Temperature During Heading and Early Filling on Grain Yield and Physiological Characteristics in Indica Rice CAO Yun-Ying1,2, DUAN Hua1, YANG Li-Nian1, WANG Zhi-Qing1, LIU Li-Jun1, and YANG Jian-Chang1,* 1
Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou 225009, China
2
College of Life Sciences, Nantong University, Nantong 226007, China
Abstract: To disclose the physiological mechanism of heat tolerance in rice (Oryza sativa L.), this article investigated the pollen development, yield components, and some physiological parameters of indica rice genotypes under high temperature (HT) treatment during heading and early filling stages. Two heat-sensitive genotypes, Shuanggui 1 and T219, and 2 heat-tolerant genotypes, Huanghuazhan and T226, were used in pot experiments with HT treatment (mean temperature during the day above 33ºC) and natural temperature (control) treatments from 0 to 10 d and 10 to 20 d after heading. Compared with the control, the HT treatment significantly reduced the rates of pollen and spikelet fertility in the heat-sensitive genotypes, but had no significant effects on the heat-tolerant genotypes. Seed-setting rate decreased significantly in the heat-sensitive genotypes under HT, leading to a significant reduction in grain yield. Moreover, the reductions of yield of heat-sensitive genotypes were greater in the HT treatment at heading stage (0–10 d after heading) than in the HT treatment at early grain-filling stage (11–20 d after heading). However, the heat-tolerant genotypes were in minor influence under the HT treatment. The activities of enzymes from rice leaves, which were involved in antioxidant system, significantly increased in the heat-tolerant genotypes when treated with HT, whereas they were maintained at relative high levels in the control. The ATPase activity in grains was significantly reduced in the heat-sensitive genotypes, but slightly influenced in the heat-tolerant genotypes. The high temperature increased leaf temperature and malondialdehyde content in leaves, and reduced root activity and photosynthetic rate of flag leaf in all genotypes. However, variation range was much less in heat-tolerant genotypes than in heat-sensitive genotypes. The results suggest that the relative high yield in heat-tolerant genotypes under high-temperature stress is associated with low leaf temperature, high root activity, and the high levels of ATPase activity in grains, photosynthetic rate, and activities of antioxidant enzymes in leaves. Keywords:
indica rice; grain yield; heat tolerance; physiological mechanism
The global temperature has been continuously increasing in recent decades due to the explosion of population, the development of industries with emission of greenhouse gas, and excessive deforestation [1, 2]. Extreme climate, such as high temperature in summer, occurs frequently and keeps longer in many regions of the world, leading to adverse effects on crop growth and development [3, 4]. The harmfulness of high temperature to rice (Oryza sativa L.) has become a serious problem during the reproductive growth period of rice. In
recent years, high temperature has caused serious yield losses in several rice-producing provinces in China, especially in the Yangtze-Huai River Valley [5–7]. Consequently, the mechanisms of heat injury during rice growth and tolerance to high temperature stress have become interest to scientists worldwide. Flowering period is one of the most sensitive periods to heat stress in rice. Heat stress at this time could cause a serious reduction in grain yield due to pollen sterility, empty
Received: 27 June 2008; Accepted: 5 September 2008. * Corresponding author. E-mail: [email protected] Copyright © 2009, Crop Science Society of China and Institute of Crop Sciences, Chinese Academy of Agricultural Sciences. Published by Elsevier BV. All rights reserved. Chinese edition available online at http://www.chinacrops.org/zwxb/ DOI: 10.1016/S1875-2780(08)60071-1
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or unfilled grains, low grain weight, and poor seed setting [8–12]. A number of physiological parameters were also affected by heat stress, such as chlorophyll content, net photosynthetic rate, and RuBP carboxylase activity [13, 14]. Rice varieties showed genotypic differences on heat tolerance; however, the physiological and biochemical mechanisms are rarely studied. The objective of this study was to compare the physiological characteristics of heat-sensitive and heat-tolerant genotypes in response to high temperature stress during heading and flowering period, and to explain the physiological mechanism of heat tolerance.
1 1.1
Materials and methods Plant materials and growing conditions
The experiment was conducted with 4 midmaturity indica cultivars or lines in different heat tolerance at the farm of Yangzhou University during the rice growing season in 2006 and 2007. Huanghuazhan (heat-tolerant) and Shuanggui 1 (heat-sensitive), which were provided by the Guangdong Academy of Agricultural Sciences, and T226 (heat-tolerant) and T219 (heat-sensitive) were from the Huazhong Agricultural University. Seeds of all genotypes were sown on 12 to 14 May and transplanted to plastic pots on 4 to 6 June. The plastic pot was 30 cm in height and 25 cm in diameter, and loaded with 18 kg sieved sandy loam soils. The soil nutrient conditions were as follows: organic matter of 18.2 g kg1, available nitrogen of 95.2 mg kg1, available phosphorus of 22.5 mg kg1, and available potassium of 82.6 mg kg1. Before transplanting, 2 g of urea and 0.5 g of KH2PO4 were applied to each pot, and another 0.5 and 0.6 g pot1 of urea were topdressed at midtillering and panicle initiation, respectively. Each pot contained 3 hills with 2 seedlings per hill, and each genotype was planted in 160 pots. The initial heading (10% plant heading) dates were 2–4 August for Huanghuazhan and Shuanggui 1, 7 August for T226, and 17 August for T219. All pots maintained a water layer of 1–2 cm in depth during the whole growth period, except for drainage at the end of tillering. Other crop managements were conducted following the same procedure as the conventional high-yielding cultivation. 1.2
Heat stress treatment
The greenhouse was equipped with 10 far-infrared heating lamps (1.5 m above the ground) and an automatic temperaturecontrol system. The lamps were 1.5 m in length and 1500 W in rated power. The temperature of heating area from 0.8 to 1.2 m above ground was controlled within 40°C through the alternation of heating and ventilating. Temperature was recorded with an automatic temperature recorder. When the temperature was lower than the designed temperature, the far-infrared lamps worked for heating; the door and windows
were opened whenever the indoor temperature above 40°C during the daytime, and the ventilating fans were also turned on for lower down the temperature. Phototherapy lights inside the greenhouse were used to complement the light intensity in agreement with that outdoors. The concentration of carbon dioxide in the glasshouse was 385 μmol mol1, which was consistent with that outdoors. Plants approaching heading stage were moved to the greenhouse for high temperature treatment from the initial heading to 10 d after heading (T1) and from 11 to 20 d after heading (T2). Thereafter, the plants resumed their growth under normal condition outside the greenhouse till maturity stage. Each treatment contained 40 pots as replicates. The daily average temperature of glasshouse was 33.4°C/20.9°C (day/night). The outdoor condition was taken as a control with temperature of 29.9°C/20.9°C (day/night). From 18:00 hour to the following 6:00 hour everyday, the door and windows of greenhouse were opening so that the indoor temperature during the night was the same as that outdoors. During the period of heat-stress treatment, temperatures in the T1 and T2 treatments were constantly higher than those in the control, and the relative humidity in T1 and T2 was highly consistent with that in the control. Except for the rainy days on 3 August, 15 August, 19 August, and 31 August and the cloudy days on 11 August, 27 August, and 4 September, the remaining days were all clear (Fig. 1). Compared with the control, the daily average, maximum, and minimum temperatures were higher in the heat-stress treatments by 2.7–4.7, 2.9–7.2, and 0.0–0.7°C, respectively (Table 1). 1.3
Sampling and measurements
1.3.1 Pollen fertility Each 2 uniform spikelets (flowering at the same) in the upper, middle, and lower position of a panicle were sampled from plants in T1 treatment, and 2 stamens per spikelet were observed for pollen fertility using a light microscope. After dyed with 1% I2–KI solution, the fertile (plump and deep-dyed) and sterile pollen (abnormal in sharp and light-dyed or partially dyed) were counted individually in 3 fields of view with 30–50 pollen grains per field. The observation was carried out in 3 continuous days. 1.3.2 Leaf temperature On the last day of heat-stress treatment, the temperature of flag leaf (middle position) was measured with the BAU-I infrared thermometer (China Agricultural University, Beijing, China). Ten leaves were determined as replicates in a measurement. 1.3.3 Photosynthetic rate At the fifth and the tenth day of treatments, the photosynthetic rate of flag leaf was determined using LI-6400 Portable Photosynthesis System (LI-COR Biosciences, Lincoln, NE, USA) from 9:00 to 11:00 hour when the photosynthetic active radiation above the canopy was 1000–1100 Pmol m2 s1. Six leaves were measured for each treatment.
Daily minimum temperature (°C)
CAO Yun-Ying et al. / Acta Agronomica Sinica, 2009, 35(3): 512–521
Hour
Fig. 1 Daily average (A), maximum (B), and minimum temperature (C) and daily relative humidity (D) in the heat-stress treatments
Table 1 Daily temperatures during the heat-stress treatments (°C) Genotype Shuanggui 1 Huanghuazhan T226 T219
Period
Daily maximum temperature
Daily average temperature
Daily minimum temperature
(month/day)
Control
Control
Control
HT
HT
HT
8/2–8/11
34.3
38.1*
30.2
33.6*
28.2
28.4
8/12–8/21
32.3
37.8*
29.5
32.9*
26.2
26.9*
8/4–8/13
33.7
38.1*
30.1
33.2*
27.7
28.2*
8/14–8/23
33.2
38.4*
30.4
33.7*
26.6
27.1*
8/7–8/16
33.3
38.3*
30.2
32.9*
27.4
27.8*
8/17–8/26
33.8
38.8*
30.1
34.9*
27.0
27.8*
8/17–8/26
33.8
38.8*
30.1
34.8*
27.0
27.8*
8/27–9/5
27.8
35.0*
25.8
30.5*
23.6
25.0*
* Significant difference (P < 0.05) observed between the heat-stress treatment and its corresponding controls. HT: High temperature treatment.
1.3.4 Activities of antioxidant enzymes and content of malondialdehyde At the fifth and the tenth day of treatments, flag leaves were sampled for determining the activities of peroxidase (POD) [15], catalase (CAT) [15], and superoxide dismutase (SOD) [16]. Besides, the malondialdehyde (MDA) content was also determined with thiobarbituric acid [17]. Each measurement had 3 replicates. 1.3.5 ATPase activity in grains Fifty panicles with the same heading date were tagged for each treatment. Ten days after treatments, grains in the middle part of a tagged panicle were sampled and quickly frozen in liquid nitrogen for 30 s
before being stored at 20°C. The activity of ATPase measured in μmol Pi mg1 protein h1 using the method described by Yang et al. [18]. Each measurement had 3 replicates. 1.3.6 Root activity Plants from 3 pots were sampled at the fifth and the tenth day of the treatments. Roots of the samples were cleaned with tap water and distilled water, and the oxidation of alpha-naphthylamine (D-NA) was used to quantify the root activity, which was shown in μg Į-NA g1 DW h1 [19]. 1.3.7 Yield and its components Plants from 5 pots in
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each treatment were harvested to determine grain yield. The yield components, including panicles per pot, spikelets per panicle, 1000-grain weight, seed-setting rate, and spikeletfertilized rate, were measured based on 3 pots of plants. The spikelet- fertilized rate is the percentage of fertile spikelet number to total spikelet number. 1.4
Statistics
Analysis of variance was carried out in SPSS 13.0 package, and the means were tested according to Duncan’s test at the 0.05 probability level. With the help of Sigmaplot 10.0, graphs were derived from the means and standard deviations of each indicator. All data except for grain yield were presented on the average of 2 years owing to similar results in the both experimental years.
2
Results
2.1 Effects of heat stress on pollen sterility and spikelet-fertilized rate The effects of heat stress on pollen fertile rate varied among genotypes (Fig. 2-A). Heat-tolerant genotypes Huanghuazhan and T226 received relative small influences under the heat stress with pollen fertility reductions by 5.9% and 9.7%, respectively, when compared with the corresponding controls. In the heat-sensitive genotypes Shuanggui 1 and T219, the reductions were 28.2% and 14.2%, respectively. The fertilization rates of all genotypes reduced under heat stress. The heat-sensitive genotypes had greater reduction than the heat-tolerant genotypes (Fig. 2-B). The fertilization rate of Shuanggui 1 was significantly lower than that of the control under T1 treatment, but the difference between Huanghuazhan and the control was not significant. Genotypic variations were also observed within the same heat-response type. For instance, Huanghuazhan had smaller reduction in fertilization
Control
Fig. 2
rate than T226. If the reduction of fertilization rate under heat stress was considered as an index to evaluate the heat tolerance, the tolerance to high temperature of the 4 genotypes were in the order of Huanghuazhan > T226 > T219 >Shuanggui 1. 2.2 Effects of heat stress on grain yield and its components Similar to fertilization rate, the yield reduction was higher in the heat-sensitive genotypes than in the heat-tolerant genotypes. The T1 treatment showed larger effects than the T2 treatment (Table 2). Compared with the control, the yields of the heat-sensitive genotypes reduced by 24.4–27.5% and 6.6–13.0% in T1 and T2 treatments, respectively; whereas, the yield reductions in the heat-tolerant genotypes were 5.6–14.6% and 3.9–9.2%, respectively. In terms of the yield components, the panicle number per pot and spikelet number per panicle were not affected significantly by the heat treatment because the stress duration was at 0 to 20 d after heading. The 1000-grain weight was also affected in minor degree with no significant differences between genotypes and their corresponding controls, excluding the 2.5% of promotion in Huanghuazhan under the T1 condition. However, the seed-setting rate was significantly lower in the heat-stress treatment than in the control, with the reductions of 4.3–24.4% in the heat-sensitive genotypes and 1.1–8.4% in the heat-tolerant genotypes (Table 2). 2.3 Effects of heat stress on leaf temperature and physiological characteristics 2.3.1 Leaf temperature The leaf temperature of rice increased significantly under heat stress, and the heat-sensitive genotypes varied greater than the heat-tolerant genotypes. In the T1 treatment, the leaf temperatures of heat-sensitive genotypes Shuanggui 1 and T219 were 2.0°C and
T1
Effects of heat stress during heading and early grain filling on pollen fertility (A) and spikelet fertilized rate (B) T1: Heat stress from initial heading to 10 d after heading. SG: Shuanggui 1; HHZ: Huanghuazhan. Bars superscripted by a different letter in the same genotype are significantly different at P < 0.05.
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Table 2 Genotype Treatment
Effects of high temperature during heading and early grain filling on grain yield and its components Panicles per pot 2006
2007
Spikelets per panicle 2006
2007
Seed-setting rate (%) 2006
1000-grain weight (g)
Yield (g pot1)
Relative yield (%) 2006
2007 100.0
2007
2006
2007
2006
2007
Heat-sensitive genotype SG
T219
Control
27.6 a
28.0 a
186.4 a
186.2 a
89.4 a
91.4 a
21.2 a
21.6 a
97.5 a
102.9 a
100.0
T1
25.9 a
25.5 a
180.9 a
176.9 a
76.6 c
78.6 c
20.6 a
21.0 a
73.9 c
74.6 c
75.6
72.5
T2
27.3 a
27.3 a
185.2 a
183.2 a
85.6 b
83.6 b
21.0 a
21.4 a
90.9 b
89.5 b
93.2
87.0 100.0
Control
25.6 a
26.0 a
186.7 a
184.7 a
68.5 a
68.6 a
27.0 a
27.2 a
88.4 a
89.6 a
100.0
T1
26.3 a
25.9 a
181.4 a
177.4 a
51.5 c
55.5 c
26.7 a
26.3 a
65.6 c
67.1 c
74.2
74.9
T2
26.1 a
26.3 a
189.4 a
187.4 a
63.3 b
61.3 b
26.5 a
26.9 a
82.9 b
81.3 b
93.4
90.7 100.0
Heat-tolerant genotype HHZ
T226
Control
29.0 a
29.2 a
186.5 a
184.5 a
91.5 a
93.5 a
20.1 b
20.3 b
99.5 a
102.3 a
100.0
T1
28.9 a
29.1 a
176.3 a
178.3 a
89.8 a
87.8 a
20.6 a
20.8 a
94.3 a
94.8 a
94.4
92.7
T2
28.7 a
29.1 a
185.1 a
189.1 a
90.4 a
88.4 a
20.0 a
19.8 b
96.0 a
96.3 a
96.1
94.1
101.4 a 102.1 a
Control
23.1 a
22.9 a
195.6 a
199.6 a
95.1 a
93.1 a
23.6 a
24.0 a
100.0
100.0
T1
22.7 a
22.5 a
192.2 a
194.2 a
87.3 b
85.3 b
23.0 a
23.4 a
87.6 b
87.2 b
86.4
85.4
T2
22.7 a
23.1 a
198.6 a
194.6 a
88.1 b
92.1 b
23.2 a
23.6 a
92.1 b
97.7 b
90.8
95.7
In each genotype, values followed by the same letter within a column are not significantly different (P < 0.05) among treatments. Abbreviations as in Fig. 2.
by 16.1% in Shuanggui 1 and 15.4% in T219 under the T1 condition, and by 7.9% and 6.6% under the T2 condition, respectively. In contrast, the photosynthetic rate reduced by 6.6% for Huanghuazhan and 7.2% for T226 in the T1 treatment, and 2.6% for Huanghuazhan and 4.6% for T226 in the T2 treatment. 2.3.3 Activities of antioxidant enzymes and content of MDA The antioxidant enzymes showed higher activities in all genotypes after high temperature treatment, but variation was observed among genotypes. In the heat-sensitive genotypes, the activities of antioxidant enzymes varied insignificantly, whereas in the heat-tolerant genotypes, the
1.0°C higher than those of the controls, respectively; but the leaf temperatures of heat-tolerant genotypes Huanghuazhan and T226 were only 0.3°C and 0.1°C higher than those of the controls. In the T2 treatment, the differences of leaf temperature were 3.5°C and 6.8°C for Shuanggui 1 and T219, and 1.9°C and 2.5°C for Huanghuazhan and T226, respectively (Fig. 3). 2.3.2 Photosynthetic rate in flag leaf Under the heat stress, the net photosynthetic rate of flag leaf decreased gradually in all genotypes, but the value of reduction was greater in heat-sensitive genotypes and in earlier stress treatment (Fig. 4). For instance, when the rice plants exposed to high temperature for 5 d, the photosynthetic rate decreased
C5 T5
C10 T10
Fig. 3 Effect of high temperature during heading and early grain filling on leaf temperature Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. T1: Heat stress from initial heading to 10 d after heading; T2: Heat stress from 10 to 20 d after heading. C5 and C10: Samples from control at the 5th and the 10th day; T5 and T10: Samples from T1 or T2 treatment at the 5th and 10th day; SG: Shuanggui 1; HHZ: Huanghuazhan.
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C5 T5
Fig. 4
Effect of high temperature during heading and early grain filing on the photosynthetic rate of the flag leaf Bars superscriped by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
activities of antioxidant enzymes significantly increased. Five days after heat stress in T1 treatment, the activities of POD, SOD, and CAT increased by 67.5, 93.4, and 41.3% in Huanghuazhan and by 66.2, 53.1, and 16.9% in T226, respectively. At the end of the stress in T2 treatment, the activities of POD, SOD, and CAT increased by 97.4, 89.8, and 16.2% in Huanghuazhan and by 59.9, 51.8, and 11.6% in T226, respectively (Fig. 5-A to F). This indicates that the heat-tolerant genotypes could maintain higher antioxidant capacity than the heat-sensitive genotypes under heat stress. In contrast to the activities of antioxidant enzymes, the lipid peroxidation, which was indicated by MDA content, significantly increased in flag leaves of the heat-sensitive genotypes, but was stable in the heat-tolerant genotypes with no significant differences from the corresponding controls (Fig. 5-G and H). Under the heat-stress conditions, the MDA contents in Shuanggui 1 and T219 increased by 33.0–56.5% and 14.7–46.0%, respectively. The results indicated that heat-tolerant genotypes had higher antioxidant capacity under high-temperature stress. 2.4
ATPase activity in grains
Compared with the controls, the ATPase activity decreased significantly in the heat-sensitive genotypes in T1 treatment, but was not significantly different in T2 treatment. In the heat-tolerant genotypes, except for a significant increment in Huanghuazhan in T2 treatment, heat stress resulted in minor variations of ATPase activity (Fig. 6). 2.5
C10 T10
Root activity
Heat stress significantly reduced the root activity, i.e., root oxidation power, with various degrees among genotypes or treatments. In the T1 treatment, there were no significant
reductions in root activity in the heat-tolerant genotypes 5 d after heat stress, but the root activity significantly decreased by in 20.8% Shuanggui 1 and by 17.7% in T219. Ten days after heat stress, no significant reduction was observed in all genotypes. In the T2 treatment, the root activity varied with no significant difference to the control in heat-sensitive genotypes throughout the stress period; however, it significantly increased 5 d after treatment by 26.8% in Huanghuazhan and by 22.1% in T226, and reduced to the level of control 10 d after treatment (Fig. 7). These results indicated that the heat-tolerant genotypes had higher root activity than the heat-sensitive genotypes under heat stress.
3
Discussion
Grain yield response to heat stress imposed at heading or flowering proved to be dependent on rice genotypes. The yield reduction of the heat-sensitive genotypes was significantly greater than that of the heat-tolerant genotypes, which mainly resulted from the poor fertilization and low seed-setting rate in the heat-sensitive genotypes. Besides, we observed that high temperature during heading or flowering stage had little effect on the development of pistil or female gametophyte (data not shown), suggesting that high temperature mainly affects the development of male gametophyte (pollen). Therefore, we propose that pollen fertility acts as an index for heat-tolerance breeding and selecting in rice. In this study, we explained that high activity of protective enzymes in the antioxidant system in plants might be one of the physiological mechanisms for heat tolerance in rice. In earlier studies, high-temperature injury was caused by the excessive production of reactive oxygen radicals, the low activities of antioxidant enzymes, and the membrane damage in
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C5 T5
Fig. 5
C10 T10
Effects of high temperature during heading and early grain filing on activities of POD, SOD, CAT, and content of MDA in leaves Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
plants [20–23]. We also observed aggravation of lipid peroxidation under heat stress, which was represented by the increment of MDA content in leaves. However, we found that heat stress induced higher activities of POD, SOD, and CAT in the heat-tolerant genotypes than the controls in the initial 20 d
after heading. However, activities of the enzymes in the heatsensitive genotypes maintained the same levels as the controls. The result indicates that heat-tolerant rice alleviates heat damages through increasing activities of protective enzymes in the antioxidant system to remove free radicals in plant.
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C10
T10
Fig. 6 Effect of high temperature during heading and early grain filling on ATPase activity in grains Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
C5 T5
C10 T10
Fig. 7 Effect of high temperature during heading and early grain filling on root activity Bars superscriped by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
The high ATPase activity in grains under heat stress is an important characteristic for heat tolerance in rice. ATPase activity in grains proves to be highly associated with matter translocation and grain-filling degree [18]. We observed that the ATPase activity was significantly higher in grains of heattolerant genotypes than that in the controls, but significantly lower in grains of the heat-sensitive genotypes. Thereby, even under high-temperature stress, the heat-tolerant genotypes maintained relatively high capabilities for translocating assimilation from source organ to sink organ and unloading the assimilation in the sink organ, resulting in good pollen development and grain filling. Root activity directly affects the growth of aboveground parts and yield formation in plant [24]. The higher level of root
activity in the heat-tolerant genotypes under high temperature is favorable for grain yield formation compared with that in the heat-sensitive genotypes. This is probably an important reason for the high levels of photosynthetic rate in leaves and ATPase activity in grains of heat-tolerant genotypes. We, therefore, consider root activity as another physiological indicator important to heat tolerant evaluation. In this study, the leaf temperature in the heat-tolerant genotypes was significantly lower than that in heat-sensitive genotypes under the high-temperature conditions. The lower leaf temperature under heat stress is believed to be able to reduce the consumption of breath and to keep normal physiological functions of leaves. We wonder whether the difference of transpiration rate between sensitive and tolerant
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genotypes is involved in the physiological basis of leaf temperature. This will be further investigated in future.
4
[8]
Conclusions
High-temperature stress during heading or early grain filling reduced grain yield of rice mainly because of low seed-setting rate. The low seed-setting rate resulted from the sterile pollen in early developmental stages. The effects of high temperature on the fertilization and grain filling varied among genotypes and the developmental stages of plant when exposed to the stress. The negative effects were greater if heat stress occurs at early heading stage than at early filling stage. Under high-temperature stress, the heat-tolerant genotypes display stronger root activity, more active antioxidative defense system in leaves, higher ATPase activity in grains, and lower leaf temperature than the heat-sensitive genotypes.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (30671225 and 30771274) and the Natural Science Foundation of Jiangsu Province, China (BK2006069 and BK2007071). The authors thank Professors ZHOU Shao-Chuan from the Rice Research Institute of Guangdong Academy of Agricultural Sciences and MOU Tong-Min from the Huazhong Agricultural University for providing the rice seeds.
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high temperature stress on activated oxygen metabolism of the leaves of the heavy panicle type of medium indica hybrid rice. Seed, 2005, 24(3): 25–27 (in Chinese with English abstract) [23] Luo G H, Wang A G, Guo J Y. Effects of some exogenous factors on superoxide dismutase activity in soybean seedlings.
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RESEARCH PAPER
Effect of High Temperature During Heading and Early Filling on Grain Yield and Physiological Characteristics in Indica Rice CAO Yun-Ying1,2, DUAN Hua1, YANG Li-Nian1, WANG Zhi-Qing1, LIU Li-Jun1, and YANG Jian-Chang1,* 1
Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou 225009, China
2
College of Life Sciences, Nantong University, Nantong 226007, China
Abstract: To disclose the physiological mechanism of heat tolerance in rice (Oryza sativa L.), this article investigated the pollen development, yield components, and some physiological parameters of indica rice genotypes under high temperature (HT) treatment during heading and early filling stages. Two heat-sensitive genotypes, Shuanggui 1 and T219, and 2 heat-tolerant genotypes, Huanghuazhan and T226, were used in pot experiments with HT treatment (mean temperature during the day above 33ºC) and natural temperature (control) treatments from 0 to 10 d and 10 to 20 d after heading. Compared with the control, the HT treatment significantly reduced the rates of pollen and spikelet fertility in the heat-sensitive genotypes, but had no significant effects on the heat-tolerant genotypes. Seed-setting rate decreased significantly in the heat-sensitive genotypes under HT, leading to a significant reduction in grain yield. Moreover, the reductions of yield of heat-sensitive genotypes were greater in the HT treatment at heading stage (0–10 d after heading) than in the HT treatment at early grain-filling stage (11–20 d after heading). However, the heat-tolerant genotypes were in minor influence under the HT treatment. The activities of enzymes from rice leaves, which were involved in antioxidant system, significantly increased in the heat-tolerant genotypes when treated with HT, whereas they were maintained at relative high levels in the control. The ATPase activity in grains was significantly reduced in the heat-sensitive genotypes, but slightly influenced in the heat-tolerant genotypes. The high temperature increased leaf temperature and malondialdehyde content in leaves, and reduced root activity and photosynthetic rate of flag leaf in all genotypes. However, variation range was much less in heat-tolerant genotypes than in heat-sensitive genotypes. The results suggest that the relative high yield in heat-tolerant genotypes under high-temperature stress is associated with low leaf temperature, high root activity, and the high levels of ATPase activity in grains, photosynthetic rate, and activities of antioxidant enzymes in leaves. Keywords:
indica rice; grain yield; heat tolerance; physiological mechanism
The global temperature has been continuously increasing in recent decades due to the explosion of population, the development of industries with emission of greenhouse gas, and excessive deforestation [1, 2]. Extreme climate, such as high temperature in summer, occurs frequently and keeps longer in many regions of the world, leading to adverse effects on crop growth and development [3, 4]. The harmfulness of high temperature to rice (Oryza sativa L.) has become a serious problem during the reproductive growth period of rice. In
recent years, high temperature has caused serious yield losses in several rice-producing provinces in China, especially in the Yangtze-Huai River Valley [5–7]. Consequently, the mechanisms of heat injury during rice growth and tolerance to high temperature stress have become interest to scientists worldwide. Flowering period is one of the most sensitive periods to heat stress in rice. Heat stress at this time could cause a serious reduction in grain yield due to pollen sterility, empty
Received: 27 June 2008; Accepted: 5 September 2008. * Corresponding author. E-mail: [email protected] Copyright © 2009, Crop Science Society of China and Institute of Crop Sciences, Chinese Academy of Agricultural Sciences. Published by Elsevier BV. All rights reserved. Chinese edition available online at http://www.chinacrops.org/zwxb/ DOI: 10.1016/S1875-2780(08)60071-1
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or unfilled grains, low grain weight, and poor seed setting [8–12]. A number of physiological parameters were also affected by heat stress, such as chlorophyll content, net photosynthetic rate, and RuBP carboxylase activity [13, 14]. Rice varieties showed genotypic differences on heat tolerance; however, the physiological and biochemical mechanisms are rarely studied. The objective of this study was to compare the physiological characteristics of heat-sensitive and heat-tolerant genotypes in response to high temperature stress during heading and flowering period, and to explain the physiological mechanism of heat tolerance.
1 1.1
Materials and methods Plant materials and growing conditions
The experiment was conducted with 4 midmaturity indica cultivars or lines in different heat tolerance at the farm of Yangzhou University during the rice growing season in 2006 and 2007. Huanghuazhan (heat-tolerant) and Shuanggui 1 (heat-sensitive), which were provided by the Guangdong Academy of Agricultural Sciences, and T226 (heat-tolerant) and T219 (heat-sensitive) were from the Huazhong Agricultural University. Seeds of all genotypes were sown on 12 to 14 May and transplanted to plastic pots on 4 to 6 June. The plastic pot was 30 cm in height and 25 cm in diameter, and loaded with 18 kg sieved sandy loam soils. The soil nutrient conditions were as follows: organic matter of 18.2 g kg1, available nitrogen of 95.2 mg kg1, available phosphorus of 22.5 mg kg1, and available potassium of 82.6 mg kg1. Before transplanting, 2 g of urea and 0.5 g of KH2PO4 were applied to each pot, and another 0.5 and 0.6 g pot1 of urea were topdressed at midtillering and panicle initiation, respectively. Each pot contained 3 hills with 2 seedlings per hill, and each genotype was planted in 160 pots. The initial heading (10% plant heading) dates were 2–4 August for Huanghuazhan and Shuanggui 1, 7 August for T226, and 17 August for T219. All pots maintained a water layer of 1–2 cm in depth during the whole growth period, except for drainage at the end of tillering. Other crop managements were conducted following the same procedure as the conventional high-yielding cultivation. 1.2
Heat stress treatment
The greenhouse was equipped with 10 far-infrared heating lamps (1.5 m above the ground) and an automatic temperaturecontrol system. The lamps were 1.5 m in length and 1500 W in rated power. The temperature of heating area from 0.8 to 1.2 m above ground was controlled within 40°C through the alternation of heating and ventilating. Temperature was recorded with an automatic temperature recorder. When the temperature was lower than the designed temperature, the far-infrared lamps worked for heating; the door and windows
were opened whenever the indoor temperature above 40°C during the daytime, and the ventilating fans were also turned on for lower down the temperature. Phototherapy lights inside the greenhouse were used to complement the light intensity in agreement with that outdoors. The concentration of carbon dioxide in the glasshouse was 385 μmol mol1, which was consistent with that outdoors. Plants approaching heading stage were moved to the greenhouse for high temperature treatment from the initial heading to 10 d after heading (T1) and from 11 to 20 d after heading (T2). Thereafter, the plants resumed their growth under normal condition outside the greenhouse till maturity stage. Each treatment contained 40 pots as replicates. The daily average temperature of glasshouse was 33.4°C/20.9°C (day/night). The outdoor condition was taken as a control with temperature of 29.9°C/20.9°C (day/night). From 18:00 hour to the following 6:00 hour everyday, the door and windows of greenhouse were opening so that the indoor temperature during the night was the same as that outdoors. During the period of heat-stress treatment, temperatures in the T1 and T2 treatments were constantly higher than those in the control, and the relative humidity in T1 and T2 was highly consistent with that in the control. Except for the rainy days on 3 August, 15 August, 19 August, and 31 August and the cloudy days on 11 August, 27 August, and 4 September, the remaining days were all clear (Fig. 1). Compared with the control, the daily average, maximum, and minimum temperatures were higher in the heat-stress treatments by 2.7–4.7, 2.9–7.2, and 0.0–0.7°C, respectively (Table 1). 1.3
Sampling and measurements
1.3.1 Pollen fertility Each 2 uniform spikelets (flowering at the same) in the upper, middle, and lower position of a panicle were sampled from plants in T1 treatment, and 2 stamens per spikelet were observed for pollen fertility using a light microscope. After dyed with 1% I2–KI solution, the fertile (plump and deep-dyed) and sterile pollen (abnormal in sharp and light-dyed or partially dyed) were counted individually in 3 fields of view with 30–50 pollen grains per field. The observation was carried out in 3 continuous days. 1.3.2 Leaf temperature On the last day of heat-stress treatment, the temperature of flag leaf (middle position) was measured with the BAU-I infrared thermometer (China Agricultural University, Beijing, China). Ten leaves were determined as replicates in a measurement. 1.3.3 Photosynthetic rate At the fifth and the tenth day of treatments, the photosynthetic rate of flag leaf was determined using LI-6400 Portable Photosynthesis System (LI-COR Biosciences, Lincoln, NE, USA) from 9:00 to 11:00 hour when the photosynthetic active radiation above the canopy was 1000–1100 Pmol m2 s1. Six leaves were measured for each treatment.
Daily minimum temperature (°C)
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Hour
Fig. 1 Daily average (A), maximum (B), and minimum temperature (C) and daily relative humidity (D) in the heat-stress treatments
Table 1 Daily temperatures during the heat-stress treatments (°C) Genotype Shuanggui 1 Huanghuazhan T226 T219
Period
Daily maximum temperature
Daily average temperature
Daily minimum temperature
(month/day)
Control
Control
Control
HT
HT
HT
8/2–8/11
34.3
38.1*
30.2
33.6*
28.2
28.4
8/12–8/21
32.3
37.8*
29.5
32.9*
26.2
26.9*
8/4–8/13
33.7
38.1*
30.1
33.2*
27.7
28.2*
8/14–8/23
33.2
38.4*
30.4
33.7*
26.6
27.1*
8/7–8/16
33.3
38.3*
30.2
32.9*
27.4
27.8*
8/17–8/26
33.8
38.8*
30.1
34.9*
27.0
27.8*
8/17–8/26
33.8
38.8*
30.1
34.8*
27.0
27.8*
8/27–9/5
27.8
35.0*
25.8
30.5*
23.6
25.0*
* Significant difference (P < 0.05) observed between the heat-stress treatment and its corresponding controls. HT: High temperature treatment.
1.3.4 Activities of antioxidant enzymes and content of malondialdehyde At the fifth and the tenth day of treatments, flag leaves were sampled for determining the activities of peroxidase (POD) [15], catalase (CAT) [15], and superoxide dismutase (SOD) [16]. Besides, the malondialdehyde (MDA) content was also determined with thiobarbituric acid [17]. Each measurement had 3 replicates. 1.3.5 ATPase activity in grains Fifty panicles with the same heading date were tagged for each treatment. Ten days after treatments, grains in the middle part of a tagged panicle were sampled and quickly frozen in liquid nitrogen for 30 s
before being stored at 20°C. The activity of ATPase measured in μmol Pi mg1 protein h1 using the method described by Yang et al. [18]. Each measurement had 3 replicates. 1.3.6 Root activity Plants from 3 pots were sampled at the fifth and the tenth day of the treatments. Roots of the samples were cleaned with tap water and distilled water, and the oxidation of alpha-naphthylamine (D-NA) was used to quantify the root activity, which was shown in μg Į-NA g1 DW h1 [19]. 1.3.7 Yield and its components Plants from 5 pots in
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each treatment were harvested to determine grain yield. The yield components, including panicles per pot, spikelets per panicle, 1000-grain weight, seed-setting rate, and spikeletfertilized rate, were measured based on 3 pots of plants. The spikelet- fertilized rate is the percentage of fertile spikelet number to total spikelet number. 1.4
Statistics
Analysis of variance was carried out in SPSS 13.0 package, and the means were tested according to Duncan’s test at the 0.05 probability level. With the help of Sigmaplot 10.0, graphs were derived from the means and standard deviations of each indicator. All data except for grain yield were presented on the average of 2 years owing to similar results in the both experimental years.
2
Results
2.1 Effects of heat stress on pollen sterility and spikelet-fertilized rate The effects of heat stress on pollen fertile rate varied among genotypes (Fig. 2-A). Heat-tolerant genotypes Huanghuazhan and T226 received relative small influences under the heat stress with pollen fertility reductions by 5.9% and 9.7%, respectively, when compared with the corresponding controls. In the heat-sensitive genotypes Shuanggui 1 and T219, the reductions were 28.2% and 14.2%, respectively. The fertilization rates of all genotypes reduced under heat stress. The heat-sensitive genotypes had greater reduction than the heat-tolerant genotypes (Fig. 2-B). The fertilization rate of Shuanggui 1 was significantly lower than that of the control under T1 treatment, but the difference between Huanghuazhan and the control was not significant. Genotypic variations were also observed within the same heat-response type. For instance, Huanghuazhan had smaller reduction in fertilization
Control
Fig. 2
rate than T226. If the reduction of fertilization rate under heat stress was considered as an index to evaluate the heat tolerance, the tolerance to high temperature of the 4 genotypes were in the order of Huanghuazhan > T226 > T219 >Shuanggui 1. 2.2 Effects of heat stress on grain yield and its components Similar to fertilization rate, the yield reduction was higher in the heat-sensitive genotypes than in the heat-tolerant genotypes. The T1 treatment showed larger effects than the T2 treatment (Table 2). Compared with the control, the yields of the heat-sensitive genotypes reduced by 24.4–27.5% and 6.6–13.0% in T1 and T2 treatments, respectively; whereas, the yield reductions in the heat-tolerant genotypes were 5.6–14.6% and 3.9–9.2%, respectively. In terms of the yield components, the panicle number per pot and spikelet number per panicle were not affected significantly by the heat treatment because the stress duration was at 0 to 20 d after heading. The 1000-grain weight was also affected in minor degree with no significant differences between genotypes and their corresponding controls, excluding the 2.5% of promotion in Huanghuazhan under the T1 condition. However, the seed-setting rate was significantly lower in the heat-stress treatment than in the control, with the reductions of 4.3–24.4% in the heat-sensitive genotypes and 1.1–8.4% in the heat-tolerant genotypes (Table 2). 2.3 Effects of heat stress on leaf temperature and physiological characteristics 2.3.1 Leaf temperature The leaf temperature of rice increased significantly under heat stress, and the heat-sensitive genotypes varied greater than the heat-tolerant genotypes. In the T1 treatment, the leaf temperatures of heat-sensitive genotypes Shuanggui 1 and T219 were 2.0°C and
T1
Effects of heat stress during heading and early grain filling on pollen fertility (A) and spikelet fertilized rate (B) T1: Heat stress from initial heading to 10 d after heading. SG: Shuanggui 1; HHZ: Huanghuazhan. Bars superscripted by a different letter in the same genotype are significantly different at P < 0.05.
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Table 2 Genotype Treatment
Effects of high temperature during heading and early grain filling on grain yield and its components Panicles per pot 2006
2007
Spikelets per panicle 2006
2007
Seed-setting rate (%) 2006
1000-grain weight (g)
Yield (g pot1)
Relative yield (%) 2006
2007 100.0
2007
2006
2007
2006
2007
Heat-sensitive genotype SG
T219
Control
27.6 a
28.0 a
186.4 a
186.2 a
89.4 a
91.4 a
21.2 a
21.6 a
97.5 a
102.9 a
100.0
T1
25.9 a
25.5 a
180.9 a
176.9 a
76.6 c
78.6 c
20.6 a
21.0 a
73.9 c
74.6 c
75.6
72.5
T2
27.3 a
27.3 a
185.2 a
183.2 a
85.6 b
83.6 b
21.0 a
21.4 a
90.9 b
89.5 b
93.2
87.0 100.0
Control
25.6 a
26.0 a
186.7 a
184.7 a
68.5 a
68.6 a
27.0 a
27.2 a
88.4 a
89.6 a
100.0
T1
26.3 a
25.9 a
181.4 a
177.4 a
51.5 c
55.5 c
26.7 a
26.3 a
65.6 c
67.1 c
74.2
74.9
T2
26.1 a
26.3 a
189.4 a
187.4 a
63.3 b
61.3 b
26.5 a
26.9 a
82.9 b
81.3 b
93.4
90.7 100.0
Heat-tolerant genotype HHZ
T226
Control
29.0 a
29.2 a
186.5 a
184.5 a
91.5 a
93.5 a
20.1 b
20.3 b
99.5 a
102.3 a
100.0
T1
28.9 a
29.1 a
176.3 a
178.3 a
89.8 a
87.8 a
20.6 a
20.8 a
94.3 a
94.8 a
94.4
92.7
T2
28.7 a
29.1 a
185.1 a
189.1 a
90.4 a
88.4 a
20.0 a
19.8 b
96.0 a
96.3 a
96.1
94.1
101.4 a 102.1 a
Control
23.1 a
22.9 a
195.6 a
199.6 a
95.1 a
93.1 a
23.6 a
24.0 a
100.0
100.0
T1
22.7 a
22.5 a
192.2 a
194.2 a
87.3 b
85.3 b
23.0 a
23.4 a
87.6 b
87.2 b
86.4
85.4
T2
22.7 a
23.1 a
198.6 a
194.6 a
88.1 b
92.1 b
23.2 a
23.6 a
92.1 b
97.7 b
90.8
95.7
In each genotype, values followed by the same letter within a column are not significantly different (P < 0.05) among treatments. Abbreviations as in Fig. 2.
by 16.1% in Shuanggui 1 and 15.4% in T219 under the T1 condition, and by 7.9% and 6.6% under the T2 condition, respectively. In contrast, the photosynthetic rate reduced by 6.6% for Huanghuazhan and 7.2% for T226 in the T1 treatment, and 2.6% for Huanghuazhan and 4.6% for T226 in the T2 treatment. 2.3.3 Activities of antioxidant enzymes and content of MDA The antioxidant enzymes showed higher activities in all genotypes after high temperature treatment, but variation was observed among genotypes. In the heat-sensitive genotypes, the activities of antioxidant enzymes varied insignificantly, whereas in the heat-tolerant genotypes, the
1.0°C higher than those of the controls, respectively; but the leaf temperatures of heat-tolerant genotypes Huanghuazhan and T226 were only 0.3°C and 0.1°C higher than those of the controls. In the T2 treatment, the differences of leaf temperature were 3.5°C and 6.8°C for Shuanggui 1 and T219, and 1.9°C and 2.5°C for Huanghuazhan and T226, respectively (Fig. 3). 2.3.2 Photosynthetic rate in flag leaf Under the heat stress, the net photosynthetic rate of flag leaf decreased gradually in all genotypes, but the value of reduction was greater in heat-sensitive genotypes and in earlier stress treatment (Fig. 4). For instance, when the rice plants exposed to high temperature for 5 d, the photosynthetic rate decreased
C5 T5
C10 T10
Fig. 3 Effect of high temperature during heading and early grain filling on leaf temperature Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. T1: Heat stress from initial heading to 10 d after heading; T2: Heat stress from 10 to 20 d after heading. C5 and C10: Samples from control at the 5th and the 10th day; T5 and T10: Samples from T1 or T2 treatment at the 5th and 10th day; SG: Shuanggui 1; HHZ: Huanghuazhan.
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C5 T5
Fig. 4
Effect of high temperature during heading and early grain filing on the photosynthetic rate of the flag leaf Bars superscriped by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
activities of antioxidant enzymes significantly increased. Five days after heat stress in T1 treatment, the activities of POD, SOD, and CAT increased by 67.5, 93.4, and 41.3% in Huanghuazhan and by 66.2, 53.1, and 16.9% in T226, respectively. At the end of the stress in T2 treatment, the activities of POD, SOD, and CAT increased by 97.4, 89.8, and 16.2% in Huanghuazhan and by 59.9, 51.8, and 11.6% in T226, respectively (Fig. 5-A to F). This indicates that the heat-tolerant genotypes could maintain higher antioxidant capacity than the heat-sensitive genotypes under heat stress. In contrast to the activities of antioxidant enzymes, the lipid peroxidation, which was indicated by MDA content, significantly increased in flag leaves of the heat-sensitive genotypes, but was stable in the heat-tolerant genotypes with no significant differences from the corresponding controls (Fig. 5-G and H). Under the heat-stress conditions, the MDA contents in Shuanggui 1 and T219 increased by 33.0–56.5% and 14.7–46.0%, respectively. The results indicated that heat-tolerant genotypes had higher antioxidant capacity under high-temperature stress. 2.4
ATPase activity in grains
Compared with the controls, the ATPase activity decreased significantly in the heat-sensitive genotypes in T1 treatment, but was not significantly different in T2 treatment. In the heat-tolerant genotypes, except for a significant increment in Huanghuazhan in T2 treatment, heat stress resulted in minor variations of ATPase activity (Fig. 6). 2.5
C10 T10
Root activity
Heat stress significantly reduced the root activity, i.e., root oxidation power, with various degrees among genotypes or treatments. In the T1 treatment, there were no significant
reductions in root activity in the heat-tolerant genotypes 5 d after heat stress, but the root activity significantly decreased by in 20.8% Shuanggui 1 and by 17.7% in T219. Ten days after heat stress, no significant reduction was observed in all genotypes. In the T2 treatment, the root activity varied with no significant difference to the control in heat-sensitive genotypes throughout the stress period; however, it significantly increased 5 d after treatment by 26.8% in Huanghuazhan and by 22.1% in T226, and reduced to the level of control 10 d after treatment (Fig. 7). These results indicated that the heat-tolerant genotypes had higher root activity than the heat-sensitive genotypes under heat stress.
3
Discussion
Grain yield response to heat stress imposed at heading or flowering proved to be dependent on rice genotypes. The yield reduction of the heat-sensitive genotypes was significantly greater than that of the heat-tolerant genotypes, which mainly resulted from the poor fertilization and low seed-setting rate in the heat-sensitive genotypes. Besides, we observed that high temperature during heading or flowering stage had little effect on the development of pistil or female gametophyte (data not shown), suggesting that high temperature mainly affects the development of male gametophyte (pollen). Therefore, we propose that pollen fertility acts as an index for heat-tolerance breeding and selecting in rice. In this study, we explained that high activity of protective enzymes in the antioxidant system in plants might be one of the physiological mechanisms for heat tolerance in rice. In earlier studies, high-temperature injury was caused by the excessive production of reactive oxygen radicals, the low activities of antioxidant enzymes, and the membrane damage in
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C5 T5
Fig. 5
C10 T10
Effects of high temperature during heading and early grain filing on activities of POD, SOD, CAT, and content of MDA in leaves Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
plants [20–23]. We also observed aggravation of lipid peroxidation under heat stress, which was represented by the increment of MDA content in leaves. However, we found that heat stress induced higher activities of POD, SOD, and CAT in the heat-tolerant genotypes than the controls in the initial 20 d
after heading. However, activities of the enzymes in the heatsensitive genotypes maintained the same levels as the controls. The result indicates that heat-tolerant rice alleviates heat damages through increasing activities of protective enzymes in the antioxidant system to remove free radicals in plant.
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C10
T10
Fig. 6 Effect of high temperature during heading and early grain filling on ATPase activity in grains Bars superscripted by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
C5 T5
C10 T10
Fig. 7 Effect of high temperature during heading and early grain filling on root activity Bars superscriped by different letters for the same genotype are significantly different at P < 0.05. Abbreviations as in Fig. 3.
The high ATPase activity in grains under heat stress is an important characteristic for heat tolerance in rice. ATPase activity in grains proves to be highly associated with matter translocation and grain-filling degree [18]. We observed that the ATPase activity was significantly higher in grains of heattolerant genotypes than that in the controls, but significantly lower in grains of the heat-sensitive genotypes. Thereby, even under high-temperature stress, the heat-tolerant genotypes maintained relatively high capabilities for translocating assimilation from source organ to sink organ and unloading the assimilation in the sink organ, resulting in good pollen development and grain filling. Root activity directly affects the growth of aboveground parts and yield formation in plant [24]. The higher level of root
activity in the heat-tolerant genotypes under high temperature is favorable for grain yield formation compared with that in the heat-sensitive genotypes. This is probably an important reason for the high levels of photosynthetic rate in leaves and ATPase activity in grains of heat-tolerant genotypes. We, therefore, consider root activity as another physiological indicator important to heat tolerant evaluation. In this study, the leaf temperature in the heat-tolerant genotypes was significantly lower than that in heat-sensitive genotypes under the high-temperature conditions. The lower leaf temperature under heat stress is believed to be able to reduce the consumption of breath and to keep normal physiological functions of leaves. We wonder whether the difference of transpiration rate between sensitive and tolerant
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genotypes is involved in the physiological basis of leaf temperature. This will be further investigated in future.
4
[8]
Conclusions
High-temperature stress during heading or early grain filling reduced grain yield of rice mainly because of low seed-setting rate. The low seed-setting rate resulted from the sterile pollen in early developmental stages. The effects of high temperature on the fertilization and grain filling varied among genotypes and the developmental stages of plant when exposed to the stress. The negative effects were greater if heat stress occurs at early heading stage than at early filling stage. Under high-temperature stress, the heat-tolerant genotypes display stronger root activity, more active antioxidative defense system in leaves, higher ATPase activity in grains, and lower leaf temperature than the heat-sensitive genotypes.
Acknowledgments This study was financially supported by the National Natural Science Foundation of China (30671225 and 30771274) and the Natural Science Foundation of Jiangsu Province, China (BK2006069 and BK2007071). The authors thank Professors ZHOU Shao-Chuan from the Rice Research Institute of Guangdong Academy of Agricultural Sciences and MOU Tong-Min from the Huazhong Agricultural University for providing the rice seeds.
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