Response of rice varieties to different temperature regimes

A laboratory experiment was conducted to find out the influence of temperature on germination traits of various rice varieties. Seeds of ten differentrice varieties (IR-6, DR-83, DR-92, Shahkar, Bangelo, IR-8, DR-82, Kangni-27, Shandar and Sarshar) were observed the different germination linked traits influenced by 5 different temperatures regimes (30, 35, 40, 45,50C) in germinator. The germinability and related traits of various rice varieties showed significantresponse to changes in temperatures. The highest seed germination (89.02%) was observed after 48 hours under 40C temperature. However, 35C was found to be themost suitable temperature for growth and vigor of seedlings.Whereas,in seedling growth and vigor traits, variety Bangelo showed superiority with highest shoot length, root length, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, and vigor index. It was concluded that for obtaining higher germination in rice varieties, 35C proved to be most suitable temperature, while germination and almost all other studied traits were adversely affected with increasing temperature.


Introduction
Rice (Oriza sativa L.) is principal food for about half of the world's population.Moreover, its consumption and demand has increased over the years.Itis an essential cash crop and one of the main export items of Pakistan accounting for 6.4 percent of value added in agriculture and 1.6 percent in GDP.Rice is grown in all four provinces of the country however, Punjab and Sindh count for more than 90% production in Pakistan.In Sindh province, Kangni-27, Sonhari Kangni, Kangni× Torhi, Dokri Basmati, Bangalo, Sugdasi, Lari, Motia Ratria, Irri-6 and Irri-8 are the commonly cultivated varieties.It is a selection from the local indigenous stock with high yielding quality and wide range of adaptability [1].Rice is grown in irrigated cropping systems with maximum day temperatures either close to or higher than the critical threshold ranging between 33 o C and35 o C [2, 3].Recent global climate models have presented alarming situation for the crop as an increase in mean temperature by 2-4.5 o C is predicted, moreover, the rice crop affected by water stress is also expected to double by the end of this century.In high temperature stress, rice's damage and sensitivity is determined by its growth stage.The highest sensitivity is known to be recorded at the reproductive stage [4].Seed germination is one of theessential parametersin plant development forultimately obtaininghigher seed yield.However, these traits may strongly decline as a result of drought stress that is perhaps one of the most important stresses which limitthe germination as well as growth of the seedlings [5-7].The difference response by germination of rice again higher temperature day night temperature mentioned.The higher temperature is usually linked with increased rat of respiration percentage that causes low production [8].While particular level of overlap physiological and biochemical reactions such as reduction in number of pollens on stigma germinated and reduced in pollination with increased in fertility of spikelet in higher day night temperature effects are also reported by [8,9].Influence of environment hot and cold temperature effect enhance the reactive oxygen species (ROS) production, at high elevated concentration cause oxidative destruction and death of cell [10].In order to know that plant respond sense for particular combination of stress is a important part of research [11].For instance higher level of (ROS) can increase transcriptional changes in cold and hot environmental conditions.While hormones production with signaling can also bring change during the variation of temperature stress [12].Present study was carried-out to investigate the effect of temperature stress on thegermination of rice varieties and to assess the interaction between different varieties and temperature on the related traits of rice seedlings.

Site, experimental design and treatments
Present investigation was performed during the year 2011 to 2012 under the (STL) Seed Testing Laboratory, Department of Agronomy, Faculty of Crop Production, Sindh Agriculture University, Tando Jam.To assess the influence of higher temperature stress of the germination traits of seed of various rice varieties.Experiment was arranged in completely randomized design (CRD) having three replications.Five temperature regimes were used in the experiment including 30°C, 35°C, 40°C, 45°C and 50°C, whereas tested rice cultivars were IR-6, DR-83, DR-92, Shahkar, Bangelo, IR-8, DR-82, Kangni-27, Shandar and Sarshar.

Experimentation and data recorded
In this study experimental treatments were set out in (complete randomized design with 3 replications.A set with total 50 complete randomly selected seed from all every varieties were kept in the petri dishes with 13.5 cm in diameter fifty germinator model -PL3 at different temperature as described above.The seed were moisten when it was necessary.Data were observed for several parameters including seed germination, shoot length, root length, shoot fresh weight, shoot fresh weight, root dry weight, shoot dry weight and seed vigor index.The seed germinated percentage was recorded after 48 hours, while root and plumule length and their fresh and dry weights were observed after 7 days.The physiological indices of root and shoot growth were alsodetermined.The root and the shoot length were measured through measuring scale, while their fresh weights were recorded using electronic balance.The shoot and root samples were kept separately in paper bags and dried in an electric oven maintaining 65°C temperature for 7 days.After drying, the root and shoot dry weights were recorded by an electric balance.
Seedling vigor index were collected through multiplication of germination percentage and length of seedling in cm.

Statistical analysis
The collected data were typed on computer and used for statistical analyze variance in means of treatments.The LSD (least significant difference) test was used to find out the statistical difference among the treatment with below described procedure as suggested by Gomez and Gomez (1984).

Root dry weight
It is evident from Table 2 that the highest root weight of15.50 mg was noted in variety Bangelo, followed by DR-83 (14.39 mg), Shahkar (13.70 mg), DR-82 (13.45 mg) and DR-92 (12.72 mg).However, the lowest root dry weight was observed in variety Sarshar (9.60 mg).The root dry weight 15.89 mg was increased whenseeds were kept under 35 o C followed by 30 and 40 o C with root dry weight of 13.19 and 12.87 mg; however, shoot dry weight of rice seedlings decreases with increasing the temperature further (50 o C).

Seed vigor index
The seed vigor index could show the increase in speed and growth rate of germination, observed on the basis of average germination and shoot length; the results for this trait of rice as influenced by temperature regimes are presented in Table 2.The effect of temperature, varieties, and their interaction on vigor index was significant (P<0.05).Different rice varieties, showed significant behavior for vigor index amongst each other.Highly significant data showed themaximum vigor index was recorded in Bangelo(270.54),followed by Shahkar (248.36).However,the lowest vigor index was noted for variety  , who reported that the temperature variations effect the growth parameters of the plants, and the responses of rice crop to changing temperatures particularly on root and leaf tissues, depend upon the genotypes under the stress.These results are in conformity with the [16], that temperature and different abiotic effect are reducing the factors for cultivation of crop at different marginal and sub marginal lands, while the production of crop mostly due to environmental changes.It was observed that regardless of the varieties; the shoot fresh weight was straight away higher when sown under 35 o C temperature and other than this optimum level of temperature, the shoot fresh weight was adversely affected.Apart from the temperature regimes, variety Bangelo resulted higher shoot fresh weight than rest of the varieties.Irrespective of varietal effect, the root fresh weight was linearly increased when the rice seedlings were kept under 35 o C temperature and either lower to the optimum temperature, or under higher temperature, the root fresh weight was negatively influenced [17], Risein temperature up to 50 o C decreased the shoot dry weight(14.86mg).Moreover, it was also observed that regardless of the varieties, increasing temperature did not suit well for sowing of rice over 35 o C. [18], found that both high and low temperature stresses can alter multiple aspects of cellular physiology.) and reportedthat the effect of genotype, temperature and their interactions on germination characteristics were significant and significant reductions in the germination of rice at temperature less than 16 o C and upper than 30 o C were observed.[19], argued that for stress of heat and response with expression heat influenced transcripts factors that then enhance the expression various stress linked transcripts.The root dry weight was markedly increased when the rice seedlings were sown under 35 o C temperature and reduced with increase or decrease in the temperature.Rice is relatively sensitive to high temperature and germination is generally affected by such variations [20].The above discussion clearly indicates that temperature plays very important role in the germination of seed, and varieties with diversified origin should be considered for their proper temperature requirement for seed germination, and ultimately, the higher seed yield.

Conclusions
It was concluded that for obtaining higher seed germination in rice varieties, 35 o C proved to be most suitable temperature, while germination and almost all the other growth and vigor traits studied were adversely affected with increasing temperature.Moreover, seed germination was the highest (84.28%) in variety Shandar after 48 hours, but the seedling growth in terms of fresh and dry weight of shoots and roots as well as the seed vigor was markedly higher in variety Bangelo as compared to rest of the varieties.

Authors' contributions
used 15 genotypes of rice crop with 9 fixed temperatures (12 o C, 16 o C, 20 o C, 25 o C, 27 o C, 30 o C, 33 o C, 36 o C and 40 o C Conceived and designed the experiments: M Buriro & IH Soomro, Performed the experiments: M Buriro & IH Soomro, Analyzed the data: SK Baloch & NS Soomro, Contributed materials/ analysis/ tools: MT Khan & SK Baloch, Wrote the paper: SK Baloch.

Table 2
.The vigor index was the highest (317.6) at 30 0 C. whereas, vigor index of seedlings under temperature 30 o C, 40 o C and 45 o C ranked 2 nd , 3 rd and 4 th with vigor index of 264.7, 233.9 and 179.8, respectively as shown in