1. NAFRI 15th Anniversary conference, Vientiane, Laos, 9th April, 2014
Spikelet sterility observed in popular rice variety, Thadokkham1 (TDK1), during the hot, dry season in Southern Laos
Tsutomu Ishimaru,1,2* Xaiyalath Seefong,3 Mayumi Yoshimoto,4 Lathvilayvong　Phoudalay,3 Samson　Benjamin,1 Toshihiro Hasegawa,4 and Kei-ichi Hayashi1,2
(1．IRRI，2．JIRCAS，3．PAFO, 4. NIAES)
Keywords; heat stress, panicle temperature, spikelet sterility,
TDK1, time of day of flowering (TDF)

2. Vegetative stage: White leaf tip, reduced tillering and plant height
Heat stress affects plant growth differently depending on the growth stage
Vegetative stage: White leaf tip, reduced tillering and plant height
Booting stage: reduced spikelet, low fertile pollen and spikelet
Flowering stage: low fertile spikelet
Grain filling stage: low grain weight, high chalkiness.
Yoshida, 1981

3. Effect of heat stress at anthesis
Chamber experiments revealed that high temperature over 35 oC induced spikelet sterility at anthesis
Induced over 35 oC
Most sensitive at flowering stage
Variety: Akihikari (Japonica)
(Kim et al. 1996)
How spikelet sterility at flowering stage is induced by heat stress?
(Satake and Yoshida 1978)

4. Less pollens released to stigma
Anther dehiscence is important factor for fertilization under heat stress
Yellow anther:
Less pollen release
Anther
High temperatures (35oC<)
Stigma
Aberrant dehiscence
Less pollens released to stigma
Lesser chances in fertilization
Sterility
Pollens drop from dehisced anther to stigma

5. Record-setting high temperature in the summer of 2007 in Japan
Daily maximum temperatures
Maximum temperature is normally around 34 oC, but it went up to 38.5 oC in 2007
Spikelet sterility (%) by date of full heading
Heat-induced spikelet sterility was occasionally observed in the temperate Japan

6. Heat vulnerable regions in the world
Southern Lao is one of the most heat-vulnerable regions
Maximum temperature > 35oC for 10 days during rice flowering stage
Laborte et al. 2012
Reports on actual damage by heat stress at anthesis are limited.
HISS is occuring in popular variety in Southern Laos?

7. Maximum and minimum temperature in the dry season of 2013, Savannakhet
Experimental site:Thasano Rice Research and Seed Multiplication Center
Heading period of 1st batch
Heading period of 2nd batch
Daily air temperature (oC)
Date
Temperatures in 2nd batch was much hotter than those in 1st batch

8. Shading net experiment
Variety: Thadokkham1 (TDK1)
Hypothesis
Key method:
Shading treatment at flowering stage
(Only for 5-7 days by 60% shading net)
Decrease in panicle temperature
Decrease in spikelet sterility
Idea from Wakamatsu et al. (2009)
“White-back type of chalkiness increases by the combination stress of heat and solar radiation”.
‘Hot spot’ for heat-induced sterility

9. Shading treatment reduced sterility by 13 %
Spikelet sterility under shaded and unshaded condition
**Significant at 1% level by Tuckey’s test
Shading treatment reduced sterility by 13 %

10. TDK1 flowered around noon
Anther color is white around 11:00.
*Panicles at border were used
35.5oC
Percentage of opened spikelet in a day (%)
36.5oC
Anther color was yellow around 13:00.
37.3oC
Time of day (h)
Spikelets that flowered at the later hour would be at the higher risk to be sterile.

11. Estimated panicle temperatures at late time of day of flowering at 1230h
Panicle temperatures calculated by Integrated Micrometeorology Model for Panicle And Canopy Temperature (IM2PACT)
Date
Time of day (h)
Treatment
Air temperature (oC)
Relative humidity (%)
Wind speed (m/s)
Solar radiation (W/m2)
Panicle temperature (oC)
6th March
1230
Unshaded
30.2 49
1.8
913
30.4
4th April
37.6 47
2.2
799
34.6
Shaded
320
32.6
5th April
36.5
796
33.9
318
31.7
IM2PACT; Yoshimoto et al. 2012
Panicle temperatures could reduce by 2 oC in shaded condition

12. Summary and future prospects
Heat-induced spikelet sterility at anthesis occurred in TDK1 in Southern Lao during the hot period of the dry season in 2013.
Solar radiation is considered to be related to the induction of spikelet sterility.
More detailed analyses will be required with local popular varieties (i.e. TDK8, TDK11, Thasano varieties).
Upgrading heat tolerance or heat escape is required to cope with expected future vulnerable climates in the dry season.

13. International Rice Congress, Bangkok
Thank you for your attention!
IRRI-Japan project funded by MAFF
Development of technologies for mitigation and adaptation to climate change in agriculture, forestry and fisheries by MAFF.
Abstract submission, 26th April !
International Rice Congress, Bangkok
October 27- November 1, 2014

14. Tolerance and escape strategy
Points of heat-induced sterility at anthesis
(Satake and Yoshida 1978)
Spikelets flower during 9-12AM in the cultivated rice.
→Higher risk to be exposed to high temperature during daytime
2) One-hour exposure to high temperature can cause sterility.
→Higher risk under the era of global warming
3) Flowering stage is the most sensitive for heat.
→Varietal differences. 35oC is a threshold
→N22 is known as most heat-tolerant variety
4) High temperatures one hour after flowering doesn’t cause sterility
→Heat escape with early-morning flowering (EMF) trait is effective approach
Tolerance and escape strategy

15. Development of IR64-EMF IR64 EMF IR64 Nanjing-EMF x F1 IR64 x BC1F1 F1
IR64 x
qEMF3
MAS
BC1F1
IR64 x
MAS
BC2F1
IR64 x
MAS
82 SSR markers
BC3F1
MAS
2014DS, IRRI
BC3F2
BC3F3 (IR64-EMF)
Opened spikelets (%)
Nanjing-EMF
temperate
IR64-EMF
tropics

16. Detection of QTL and development of NIL for EMF
33 oC
37 oC
40 oC
Percentage of flowered spikelets (%)
Flowering pattern
NIL for EMF
（平林ら 2010）
EMF-NIL
Nanjing 11
Higher fertility in EMF
QTL also works at IRRI (tropics)
EMF-NIL
Nanjing11

17. Global Research Network
Thasano station
TNAU
Partner
Local varieties
Shading net experiment
Meteorological data
IRRI
Phytotron test
MAS, genetics
Protocol
Global-scale collaboration toward the mitigation of heat-induced spikelet sterility