INTRODUCTION
Rice is the most important crop in Asia, where approximately 90% of the global rice production is consumed (FAO, 2008). In Vietnam, rice cultivation was accounted more than three-quarters of the country’s total annual harvested agricultural area (Thang et al., 2006). Unfortunately, Vietnam is considered one of the countries expected to be severely affected by climate change (IPPC, 2011). Under the impact of climate change, extreme weather such as high temperature, rain fluctuation is going to engendering which poses huge threats to the agriculture production (FAO, 2011). The warming trend and increasing temperature extremes have been observed across most Asia region over past century (IPCC, 2014).
Heat stress causes serious adverse effects on rice production (Shi et al., 2017). In tropical zone, high temperature is already one of the major abiotic stress limiting rice productivity, with relatively higher temperature reduction in grain weight and quality (Krishnan et al., 2011 and Norvie et al., 2014). The sterility induced by high temperature was observed in dry season crops in Vietnam, Cambodia and Thailand (Matsuo et al., 1995). In the report of Thuy & Shaitoh (2017), the Vietnamese rice cultivars yield decreased significantly under the high temperature. The greatest impact on Vietnamese rice grain yield is spikelet sterility caused by high temperature. Heat stress in flowering stage is the most environmental factor to induce spikelet sterility of indica rice (Satake & Yoshida, 1978).
The ideal temperature ranges for maintaining the rice yield is from 27 to 32°C, temperature above 32°C negatively affect all stage of rice plant growth and development (Aghamolki et al., 2014). The temperature over 38°C was recorded in temperate zone such as China, Japan…which the duration increased year by year (Wang et al., 2019 and Ishimaru et al., 2015). The high temperature usually reaches over 38°C from mid-April to mid-May in Philippines (Norvie et al., 2014). In Vietnam, days with temperature above 35°C are predicted to increase by 10-20 days in a large part of country (ADB, 2013). Unfortunately, the spikelet fertility of both indica and japonica rice has shown that less than one hour of exposure to temperature at or above 33.7°C during anthesis can cause sterility (Jagadish et al., 2007). A decrease 10% in rice yield has been found to be associated with every 1°C increased in temperature (ADB, 2013). Under the impact of climate change, the warming trend has been more rapid in the southern parts than the central and northern regions of Vietnam. The sustainability of rice production in Vietnam would be damaged under the severe impact of high temperature caused by climate change (McSweeney et al., 2006). Unfortunately, Vietnam is one of the largest exporter of rice in the world, with Mekong Delta region contribute more than 50% of Vietnam rice output (Thuy & Anh, 2015). By DES DINAS-COAT model, the rice yield in Vietnam will loss 2,4 % by 2020 and 11.6% by 2070 under the effect of climate change (Teng et al., 2016). Therefore, understanding potential impacts from high temperature at flowering stage is necessary for scientist and local authorities in designing and planning mitigation and adaption plans. This study aimed to clarify the negative effect on flowering stage and adaptive strategies to high temperature caused by climate change in Mekong Delta, Vietnam.
RICE CROP SEASONS AND TEMPERATURE TRENDS IN MEKONG DELTA, VIETNAM
The impact of changes in climatic condition can be viewed through temperature and water stress during the crop cycle (Thuy & Anh, 2015). The mean temperature in Vietnam is predicted to increase by 0.8 to 2.7°C by 2060s (McSweeney et al., 2006). The warmer temperature can be seen in both the average maximum and minimum temperature. Temperature in Mekong Delta region will increase 0.5-0.6°C during the dry season (December-May) and 0.6- 0.7°C during the wet season (June-November) by 2030s (Mart & Peter, 2011). Moreover, the extreme maximum and minimum temperature of the hottest day in year will also be warmer. The temperature over 35°C predicted to extend to about 2 months longer in the 2030s in Mekong Delta (Tuan & Chinvanno, 2011).
For increasing rice production, many regions in Mekong delta change from 2 crop seasons to 3 crop seasons per year; Winter-Spring (W-S) which is plated in Mid-November and harvested in February, Summer-Autumn (S-A) which is planted in mid-March and harvested in late June, Autumn-Winter (A-W) which is planted in mid-July and harvested in October are so popular in many provinces of Mekong Delta (Caitlin et al., 2019;Mart & Peter, 2011). The average of grain yield in Mekong Delta in Winter- Spring season is 7.17 ton/ha. While, the lower grain yield in S-A and A-W crop season were observed at 5.17 ton/ha and 5.33 ton/ha, respectively (Gioi et al., 2015 and Thanh et al., 2013). There is a negative relationship between rice yield and seasonal average maximum temperature observed in Vietnam. This is an important natural condition effect to decrease the rice grain yield in S-A and A-S crop season (Chung et al., 2015). Fig. 1
NEGATIVE EFFECTS OF HEAT STRESS ON RICE FLOWERING STAGE
Rice flowering stage is the most sensitive stage to high temperature (Satake & Yoshida, 1978). The range of high temperature and major effect on rice at flowering stage was summaries in Table 1. The optimum temperature range for the normal development of rice fluctuates between 27 and 32 °C (Thuy and Shaitoh, 2017). In the report of Satake & Yoshida (1978) and Jagadish et al. (2007), the threshold temperature for successful for flowering was defined 33°C. Above 33°C, a lot of negative effects in the flowering stage were reported in previous studies. Heat stress at meiosis stage significantly reduced anther dehiscence and pollen fertilizer rate (Cao et al., 2008). During the flowering stage, anther dehiscence inhibition and low pollen shedding onto the stigma are two main reasons for spikelet fertility reduction under heat stress (Wang et al., 2019). The low fertility was caused by delayed anther dehiscence and reduced the viabilities of both exerted sigma and pollen under high temperature (Cao et al., 2019). In the study of Matsui & Omasa (2002), the spikelet sterility reach 50% and 35% in heat susceptible and moderate cultivars, respectively when the temperature at flowering time is 37°C. The spikelet sterility reached 100% in 41°C at flowering time stage (IRRI, 1979). The impact of high night temperature at flowering stage is more devastating than high day temperature. Heat stress, particularly at night, had a severe effect on pollen germination lead to spikelet sterility (Mohamed & Tarpley, 2009;Shah et al., 2015). Fig. 2
ADAPTIVE STRATEGIES FOR RICE FLOWERING STAGE TO THE EFFECT OF HIGH TEMPERATURE CAUSED BY CLIMATE CHANGE IN MEKONG DELTA, VIETNAM
The sustainability of rice production in Mekong Delta can be damaged by warming trend. Thus the adaptive strategy to climate change is the most important target to maintain agriculture and food security in Vietnam (Thuy & Anh, 2015). Rice plants possess different mechanisms such as avoidance, defense and tolerance to overcome heat stress through transpiration cooling (Raju et al., 2017). Heat defense is the process of morphological growth relation and leaf transpiration to reduce the panicle temperature and prevent damage from high temperature. Rice responds to heat stress by adjusting various physiochemical mechanism, growth inhibition, leaf rolling, alteration in basic physiological process (respiration, photosynthesis and ROS) that help to decrease pollen sterility (Shad et al., 2018). Heat avoidance involves an adjustment of spikelet flowering time or early morning flowering (Yaliang et al., 2019). Heat stress tolerant cultivars have various mechanisms for dealing with high temperature stress to maintain the normal life activity (Tanamachi et al., 2016 and Yaliang et al., 2019).
Heat tolerant cultivars (HTC)
The selection of HTC can effectively reduce yield loss by high temperature. In tolerant genotypes, the large amount of pollen on the stigma appeared to compensate for reduced pollen growth under high temperature at flowering stage (Mackill et al., 1982). Anthers of high temperature tolerant cultivars dehisce more easily than those of susceptible cultivars and contribute to pollination under high temperature condition (Satake & Yoshida, 1978 and Mackill et al., 1982). The locule walls of anther in HTC were thicker and better development which promote the swelling of pollen grains by retaining water in the locules (Matusi et al., 2001).
Some cultivars of International Rice Research Institute (IRRI) showed adaptive ability to high temperature at anthesis stage. Cultivar; IR86991-146-2-1-1 was found more tolerant to heat stress at flowering stage with higher yield and pollen viability (80-100%) (Masuduzza et al., 2016). In Vietnam, HTC were bred by Cuu Long Delta Rice Research Institute and imported from IRRI that tested to adapt to Mekong Delta region, especially in Summer- Autumn crop season (high temperature crop season). Some cultivars such as “OM221”, “OM124” and “OM178” gave both high grain yield and heat tolerant score (Gioi et al., 2016).
Early morning flowering trait (EMF)
The flowering time during the day is important because spikelet sterility is induced by high temperature during or 1-3h after anthesis in rice (Satake & Yosida, 1978). Oryza species show a wide variability in flowering time of day, for which some wild type flower as early as 0600h (Oryza officinalis) or as late as 1700h (Oryza australiensis), and a few flower during the night (Jagadish et al., 2015). In general, the indica rice peak flowering before 1200h and japonica rice peak flowering after 1200h (Yaliang et al., 2019). The difference in spikelet flowering habit is an important factor in indica rice for the better heat resistance than japonica rice (Wang et al., 2019). In response to increasing day temperature, the EMF is effective in heat escape because it shifts the time of flowering to earlier in the morning when it is cooler (Ishimaru et al., 2016).
Identification of genes responsible for the EMF would be crucial for heat stress avoidance mechanism (Sheedy et al., 2005). This is a potential trait for breeding to incorporate into Oryza sativa now that interspecific can make (Jone et al., 1997). In the report of Raju et al (2017), a near-isogenic of early morning flowering line (IR64+ qEMF3) effectively minimized the spikelet sterility by 71% during dry season under field condition compared to tropical and subtropical cultivars. Thus, EMF trait introgression to currently growing popular rice cultivars improve their resilience to heat stress episodes coinciding with flowering which is good adaption.
Rice cropping adjustment
Rice cropping adjustment is an effective strategy to avoid heat stress occurring in the flowering and grain filling time (Lur et al., 2009). There are four ways to adjust the rice cultivation mitigate the effect of high temperature in flowering stage. The first method is the alterative sowing date of single crop season by reducing to two crop seasons per year system such as W-S and S-W. Normally AW cropping season is not suitable to rice cultivation for high temperature during flowering time. The change for cultivation management is also needed to determine the proper timing of transplanting (Lur et al., 2009). Therefore, rice transplanting time can be changed as April or May then rice can flower during Jun to July. In that case rice can avoid high temperature during flowering time and also can have good ripening characters compared to A-W cropping. By simulating model, the altering transplanting date could increase 20-22 % yield for Vietnam rice production in the future (Sangum et al., 2016). But it may not be simple as sowing time greatly depends on the preceding crop and the farmers have to reconsider their cropping pattern of the whole year to accommodate such adjustments (Zhu et al., 2013). The second method is planting early maturing rice cultivars in 3 crop seasons per year system to fit the flowering stage in cooler time can reduce the heat injury loss (Wang et al., 2019).
In addition, high temperature monitor and warming system should be improved to achieve optimal heat stress management efficiencies (Wang et al., 2019). For changing crop’s time, planting and harvesting dates should vary year to year base on the weather and local management decisions (Caitlin et al., 2019).
Finally, sustainable change from rice mono-cropping to rice-fish farm is also an ideal model for adapting with climate change in Mekong Delta region (SRD, 2013). Some rice-fish farms in Mekong Delta conducted in hot and flooding season (August-December) for aquaculture showed the benefit for an economic as well as an ecology (Hakan, 2001). This model can adapt to flooding and avoid to heat stress in Mekong Delta (SRD, 2013).
Crop management
Beside, adopting different methods like sowing, water and nutrient management could also mitigate the effect of high temperature on rice performance (Amamullah et al., 2017). For reducing the canopy temperature in flowering stage, adjust the rice plant and microclimate in the field can also alleviate heat damage. Increasing row spacing between rice plants is beneficial for air circulation in paddy field lead to decrease the canopy temperature (Wang et al., 2019).
Moreover, some previous studies show the applying fertilizer management also mitigate the effect of high temperature at flowering stage (Wang et al., 2019;Wu et al., 2013). Applied biochar and phosphorus fertilizer before transplantation alleviated the damaging effect of high temperature on the attributes of pollen germination, anther dehiscence and greater pollen retention and germination (Shah et al., 2015). In the studies at northern Vietnam, biochar addition could reduce CH4 emission from the paddy field. Therefore, biochar is a potential fertilizer for mitigating the impact of climate change in Vietnam rice cultivation (Ali et al., 2016). Micronutrient fertilizers (Silicon, KH2PO4, ZnSO4, Na2SeO3) and natural abscisic acid can increase the capacity of spikelet fertilization under heat stress condition (Wu et al., 2013;Wang et al., 2019).
CONCLUSIONS
For maintaining the rice yield production under the warming trend caused by climate change in one of the main challenge for rice cultivation in Mekong Delta in this century. The prevented damage by high temperature at flowering stage is a key factor to achieve this target. Rice cultivation practices must be based on actual knowledge of systems that are already in place, but with emphasis on new adjustment to make their function with much greater efficiency. Context specific adaption is a necessity. Because the impact of climate change impact will be different from place to place in Mekong Delta region. There are some options to mitigate the effect of heat stress at flowering stage; that (i) heat tolerant cultivars, (ii) early morning flowering trait to avoid the heat in the noon, (iii) rice cropping adjustment, and (iv) crop management which were discussed in this review study. These methods provide a basic background on breeding strategy with a greater tolerance cultivar and mitigation strategy to alleviate the effect of high temperature at anthesis stage, and therefore provide an efficient way to adapt for future circumstance in Mekong Delta, Vietnam.
적 요
베트남 메콩 델타 지역은 베트남에서도 쌀 생산이 활발히 이루어지는 지역으로 꼽히고 있으나 기후온난화에 따른 고온 현상 증가로 쌀생산량 감소의 우려가 증대되고 있다. 이에 고 온에 따른 벼 피해발생 현황 및 대응 기술에 대한 정보를 제 공하고자 한다.
베트남의 평균기온은 2060년 까지 약 2.7°C 증가, 35°C 이 상의 폭염은 2030년까지 2달 이상 길어질 것으로 예측되고 있 으며 기온상승에 따라 기온이 높은 작기의 벼 생산량이 감소 되고 있다.
벼 개화기 온도가 33°C 이상으로 높을 시, 화분의 감수분열 에 영향을 미쳐 화분의 활력이 저하되며 수술에서 약의 개열 이 감소되어 화분의 방출이 감소될 뿐만 아니라 화분이 암술 의 주두에 부착하여 발아 시 발아율을 낮추어 불임 발생이 증 가하게 된다.
베트남의 벼 연구소는 고온에 대비하여 베트남 벼 생산의 안정화를 위해서 개화기에 내열성을 가지는 품종 육성 뿐 만 아니라 이른 아침 에 개화할 수 있는 품종을 육성하고 있다. 또한 재배 적인 측면에서 고온 피해를 줄일 수 있도록 벼 재 배시기를 조절하여 기존 3기작에서 고온피해가 큰 시기의 재 배를 제외한 2기작으로 재배하는 반면 파종 및 이앙시기를 조 절하여 벼의 생육량을 충분히 확보하여 2기작에 따른 수량감 소를 최소화하는 연구가 필요하다. 더불어 벼를 재배하지 못 하는 무더운 계절에는 농업인의 소득보존을 위해 논을 이용한 물고기 양식 기술 도입하는 것도 필요하며, 시비 방법 등의 개선을 통한 벼 생육량 조절, 물관리 기술 개발을 통한 포장 미세기상변화 등의 연구가 필요할 것으로 판단된다.
