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B. ARPITHA SHANKAR*, N.P. ESWARA REDDY AND A. SRIVIDHYA
Department of Molecular Biology and Biotechnology, IFT, ANGRAU, RARS, Tirupati – 517 502, Chittoor Dt., Andhra Pradesh
Heat is one of the major factors that considerably limit rice production. Here we report a novel temperature induction response (TIR) technique which was standardized for Rice crop. By adapting TIR technique 74 genotypes were screened for thermo tolerance. Out of 74 genotypes, 14 genotypes exhibited thermo tolerance due to induced high temperature. These geno-types have intrinsic heat tolerance and they can be explored as donor source in breeding program aimed for global warming.
Thermo tolerance, lethal temperature, sub – lethal temperature, rice genotypes, seedling survival.
Rice is the most important staple food around the world. India is the second largest producer of Rice. Rice occupied an area of about 43.86 million ha with production of 11,267 million tons during 2015-16. Rice is important kharif and rabi crop. Due to increasing temperatures by Global warming, plants are prone for recurrent heat and drought stresses which affect the crop growth and yield. Plants adopt to high temperature stress by inherent basal level tolerance as well as acquired tolerance to severe temperature stress. Acquired thermo tolerance is quite rapid and has been shown to be induced during cell acclimation to moderately high temperature periods (Larkindale et al., 2005; Massie et al.,2003). Temperature affects a broad spectrum of cellular components and metabolism, and temperature extremes impose stresses of variable severity that depend on the rate of temperature change, intensity, and duration. The ability to withstand and to acclimate to supra-optimal temperatures results from both prevention of heat damage and repair of heat-sensitive components (Sung et al., 2003; Senthil Kumar et al., 2007). Seedlings exposed to a sub lethal temperature prior to challenge with severe temperature have better growth recovery than those seedlings challenged directly to severe temperature stress. The global rise in temperature will also increase the severity of other environmental stresses such as floods and drought. The variation in rainfall will lead to more frequent floods and droughts (Yildiz and Terzi, 2008) which are the most important constraints for deep water
and aerobic cropping systems, respectively. Both these extreme conditions (drought and flood), if exceed certain critical period, will have substantial consequences on rice and may lead to complete failure of the rice crop when occur at sensitive stages either in the form of water shortage or excessive submergence. And thus, the changing climate may enforce a shift in the cropping pattern in most parts of the world most probably making rice the most suitable choice for areas with increased water availability but becoming less appropriate for farmers in areas with decreased wetness. So there is a need to adopt a multi-faceted approach while studying the impact of high-temperature stress, also focusing on other environmental stresses, which may be equally detrimental for rice productivity. Acquired tolerance for a specific abiotic stress has been shown to give cross protection for other stresses such as salinity, chilling temperatures, and drought. Therefore, evaluating the relative performance of rice genotypes for high temperature tolerance using TIR technique is the main objective.
Experimental Details and Treatments Experimental Details:
The experiment was conducted at Phenotyping laboratory, Institute of Frontier Technology, Regional Agricultural Research Station, Tirupati, using the standardized TIR (Temperature Induction Response) protocol.
Highly thermo tolerant rice genotypes were screened from 74 rice germplasm obtained from Nellore, Marteru
, some land races and African lines (Nerica) including proven varieties for heat tolerance such as N22, Dular and Nipponbare which were used as check genotypes. This approach of TIR involves first the identification of challenging temperature and induction temperature and later standardizing them before being used for screening germplasm for intrinsic tolerance. Phenotyping of rice genotypes for thermo tolerance using TIR technique was established in this laboratory (Sudhakar et al., 2012) and same protocol is used in this study.
Treatments:
Rice seeds were washed with distilled water 2- 3 times and were kept for germination at room temperature. After 42 hours, seedlings which have attained 0.5 cm uniform size were selected and sown in aluminum trays containing blotter paper wetted with water. These trays with seedlings were subjected to sub-lethal temperatures (gradual temperature increasing for every half an hour from 38°C to 55°C for 4 hours in the environmental chamber – ‘LABLINE’ –(Humididty Controlled Oven). Later these seedlings were exposed to lethal temperatures (55ÚC) (induced) for 2 hours. Another sub set of seedlings were exposed directly to lethal temperatures (non induced). Induced and non induced rice seedlings were allowed to recover at room temperature for one week. A control tray was maintained at room temperature, without exposing to sub-lethal and lethal temperatures.
The following parameters were recorded from the seedlings:
a) Per cent seedling survival =
Number of seedlings survived at the end of recovery 100 Total number of seedlings sown in the tray
b) Per cent reduction in root growth =
high temperature tolerance. The genotypes showed significant genetic variability for per cent survival of seedlings, per cent reduction in root and shoot growth respectively. The per cent survival of seedlings varied from 0 to 100 per cent. Among the 74 rice genotypes screened 14 (FR 13A, Swarna Sub 1A, NLR3238, MTU1001, Jagannadh, MTU1061, Konark, Vajram, Satya,Minghui, VL Dhan16, BPT1235, JGL3855, Basmathi386) genotypes showed highest thermo tolerance in terms of 80-100 per cent seedlings survival and no or very little reduction in root and shoot growth. Of these, two genotypes namely FR13A, and SwarnaSub1A were out performed the highly tolerant check Dular in the study both for RRL (Relative Root Length) (62.53% and 47.56%) and RSL (Relative Shoot Length) (106.20% and 40.74%), respectively. However, the other two checks N22 and Nipponbare showed 100% survival, the growth performance was poor over control. The genotypes NLR3238, MTU1001, Jagannadh, MTU1061 and Konark also showed better performance over Dular.
These results are in conformity with several studies, which showed that acclimated plants survive upon exposure to a severe stress, which otherwise could be lethal and is considered to be as thermo tolerance (Srikanth Babu et al., 2002). Results of this study indicated that the effect of TIR on other genotypes revealed variable results. Such acquired tolerance was variably recorded in other rice genotypes, where either survival of seedlings was affected or root growth alone was affected or only shoot growth was affected. This technique of exposing young seedlings to sub-lethal and lethal temperature has been validated in many crop species (Senthil Kumar et al., 2007). This novel temperature induction response technique has been demonstrated to reveal genetic variability in intrinsic stress tolerance at cellular level. The present study also revealed that the Thermo Induced Response (TIR) technique can very well be used in rice crop.
Actual root growth of treated seedlings
Actual root growth of control seedlings – Actual root growth of control seedlings 100
c) Per cent reduction in shoot growth =
Actual shoot growth of control seedlings – Actual shoot growth of treated seedlings 100 Actual shoot growth of control seedlings
Using this technique it was proved that sufficient genetic variability was present among rice genotypes for
The above results suggest that the TIR technique is a powerful and constructive technique to identify genetic variability in high temperature tolerance in rice within a short period of time and it is suitable for screening a large number of genotypes. The identified 14 genotypes of rice can be used as donor source for developing high temperature tolerant rice genotypes to resist global rise temperature.
