Rice (Oryza sativa L.) is the staple food in Bangladesh and also for more than three billion people globally [1] . To feed this huge number of consumers, it needs to cultivate rice at a large scale of area and at a very higher production rate. However, the population of Bangladesh is up surging by about two million every year and expected to increase by another 30 million over the following 20 years. Thus, Bangladesh will demand about 27.26 million tons of rice by 2020 to feed the increased mass of people. On the contrary, total arable land is supposed to be shrinked down to 10.28 million hectares from 11.25 million ha due to settlement and other causes [2] . However, in spite of lots of efforts to escalate productivity, crop loss is stirring at large scale due to several earthly and man-made reasons. Estimates indicate that, more than two hundred million tons of rice is lost every year due to environmental stresses, diseases and pest incidents [3] . Among the environmental stresses, water stress is a major yield restraining factor of rainfed rice, which occupies almost half of the land used for rice production in Asia and in some countries including Bangladesh, Thailand, Laos; it is the dominant rice ecosystem. However, in global aspect, rainfed rice growing areas comprise 45% of the total cultivated rice growing area where drought in mild, moderate or severe forms occurs in most years [4] . Regrettably, highest loss in this practice also befalls due to drought, for its high requirement of water, thus causing damage to 2.32 million hectares crop per annum [5] . These water stress vulnerability scenarios are likely to be worsen in the future with predicted climate change scenarios, which will also lead to more complex interactions of water stress with other abiotic and biotic stresses. The frequency of water stress varies in timing and severity among locations and years. Among the rice cultivating seasons in Bangladesh viz. Aus, Aman and Boro, Aus rice is mostly cultivated under rainfed condition during April to August where June to July are most important as the crop passes its reproductive stage during this time yet the rainfall is very erratic in nature. Consequently, water stress upsurge occurs and affects translocation of assimilates and grain development in rice which results in loss of yield, particularly during reproductive stage when plants are highly sensitive to water stress [6] . Water stress reduces the grain filling percentage and grain weight, resulting in a significant decrease of grain yield by 11.6% to approximately 14.7%. Aus rice requires less number of days to mature compared to Aman and Boro. So, it has the capability to escape late season drought [2] . Therefore, prearranged the large area under rainfed rice cultivation system and the increasing prevalence of drought stress in a wide range of cultivation systems because of declining water table depths, improving the Aus rice varieties has a large potential for helping to achieve sustainable rice production. Hence, increasing Aus rice productivity may become substitute the gap in progress due to water scarcity as well as reduce pressure upon groundwater. Henceforth, it is necessary to find out Aus rice genotypes which exhibit the desired characters like short duration, high yield potential and tolerant towards water stress, so that they can be taken for mass scale production under rainfed condition. Use of yield as an index for adaptation to water stress in rice [7] may be considered as a reasonable approach, as grain yield is a major attribute of interest in most plant breeding programs. Therefore, selection of rice varieties and breeding lines with high levels of stress tolerance is one of the main challenges for rice research in recent times [8] . With that background problem and opportunity, this research was initiated to assess yield and yield contributing characters and recommend the ones which give higher yield in spite of rainfed condition.

Significant differences for all traits due to irrigated and rainfed treatment amongst the Aus rice varieties have been noticed. Those differences were occurred might be due to improper distribution of rainwater and due to prevailing high temperature during the crop cycle particularly early growth stages that caused a relatively higher evapotranspiration rate during the entire crop cycle. In the meantime, air temperature as well as the soil temperature was higher which made the crops difficult to uptake lesser amount of water and minerals required for plants growth and development. The impact of water regime was considerably important for varietal performance. The effect of water stress may vary with genotypes as well as growing environment of the crop. It indicates that there is sufficient scope to select the promising genotypes from the present study for yield and some other economic plant traits.

The number of effective tillers was higher under irrigated condition. Water influences to increase tiller numbers of the varieties by upholding cell turgidity and it is the fundamental input for occurring photosynthesis as well as acts as a major integral of tissue, an element in chemical reaction, a solvent for and mode of translocation for metabolites and minerals within the plant body. Besides these function, it is also essential for cell enlargement through increasing turgor pressure ultimately accelerating growth and development of the number of effective tillers. Another reason might be due to maintaining relatively higher water potential outside the plant which is necessary for absorbing water with minerals by the plant roots as well as due to receiving water from both irrigation and

rainfall. In contrast, the lowest number of effective tillers was observed under rainfed condition might be due to lack of available soil moisture and due to have reduced transpiration rate with a view to conserving water for their survival and also for having low relative water content of the cells [11] [12] [13] [14] [15] . Another reason might be due to prevalent high temperature during the period of formation of tillers which eventually increased evapotranspiration rate and lowered the differences of water potential gradient. However, water deficit caused impaired mitosis, cell elongation and expansion resulted in decreased growth of the number of tillers [16] and induced lesser number of effective tillers which affects its yield [17] [18] . Results are agreed with the findings of [19] and stated that plants produced lower number of effective tillers when subjected to water stress (Table 2).

From the Table 2 it is noticed that the highest number of non-effective tillers was observed in rainfed due to the reduction in turgor pressure under stress and finally cell growth of the tillers was severely impaired [20] . Water stress affects both expansion as well as enlargement of growth [21] of the tillers thus producing a large number of non-effective tillers. It interrupts plant net photosynthesis [19] [22] , rate of transpiration [23] , stomatal conductance [24] , water use efficiency [25] , intercellular carbon-dioxide, photosystem-II activity [26] , relative water content of the cell [25] [27] [28] and ultimately membrane stability index [29] . All of these factors reduce under water shortage in rice plant [30] [31] [32] .

The higher number of filled grains panicle⁻¹ was recorded under irrigated condition might be due to effective translocation of substantial amount of photo-assimilates from source to sinks and due to have produced profuse number of effective tillers (Table 3). On the contrary, filled grains were reduced in rainfed condition due to produce less number of effective tillers as well as reduced transpirational water loss for their subsistence. Another reason might be due to having high air and soil temperature, high evaporation during early growth stages of the crops. Similar results were found by several scientists [33] [34] [35] .

The number of unfilled grains was followed an increasing trend under rainfed treatment (Table 3) might be due to reduced photosynthetic rate of the varieties and due to the production of less number of effective tillers and filled grains as well as due to causing spikelet sterility that ultimately lead to unfilled grains [36] . Increase in spikelet sterility [29] [37] is commonly observed under water stress. Results are in agreement with the findings by some scientists [38] [39] .

Panicle length was also greater due to get ample water in addition to have adequate assimilates which was required for increasing length of panicle in irrigated state (Table 4). In contrast, Panicle length was reduced in rainfed due to shortage of photo-assimilates production resulting stunted growth and due to have lower water potential around the plants roots compared to plant body. Water stress reduces panicle length [17] [18] [39] . Results are endorsed with the findings of [40] .

Under irrigated condition test weight was higher due to uniform grain and also for having lesser number of shriveled grains as well as varieties were capable to mobilize their assimilates to the economic portion i.e. grain (Table 4). In contrast, reduction in grain size, weight [6] [14] [41] , rate of seed-setting and test weight [42] are commonly noticed under water deficit condition. Results are agreed with the findings of [35] . Thousand-grain weight and grain yield were decreased with increasing water stress levels [40] [43] [44] [45] . Water deficit at vegetative growth particularly booting stage [46] , flowering and terminal phases can interrupt commencement of floret, triggering spikelet sterility and slow grain filling, resulted in lower grain weight [36] [42] [47] .

The ultimate purpose of growing crops is to fetch greater amount of harvestable yield. Under irrigated situation adequate water allowing high stomatal conductance and transport of CO₂ into leaves, thus produced a large quantity of photosynthates (Table 6). In that condition plant produced larger leaf area, more number of effective tillers and filled grains, increased test weight and less number of unfilled grains and irrigation water as well as rainwater necessary for functioning normal metabolic activities of the plants whereas in rainfed condition plant produced smaller leaf area, effective tillers, filled grains and more number of unfilled grains. Nevertheless, chlorophyll content and rate of photosynthesis of the flag leaves declined more quickly in water deficit condition resulting reduction in grain yield [48] . Water stress reduces grain yield probably by shortening the grain filling period [38] , troublemaking leaf gas exchange properties, restraining the size of the source and sink tissues, impaired phloem loading and translocation of assimilate [49] . The reduction in yield may also be due to drought stimulated reduction in rates of CO₂ assimilation, reduced conductivity of stomata, photosynthetic pigments, tiny leaf size, disturbed plant water relations, reduction in water use efficiency, reduction in activities of enzymes of sucrose and starch synthesis and reduced photo-assimilate partitioning, leading to a reduction in plant growth and yield [50] . The amount of grain yield loss depends on the duration of shortage of water, the crop growth phase [51] as well as the existing water stress condition [29] . Grain yield of rice severely decreases under water deficit condition [22] [28] [52] [53] . Results are concurred with the findings of numerous researchers [21] [28] [37] [54] - [61] .

Under irrigated condition the maintenance of reproductive growth around the time of flowering allowed the maintenance of capability for storing of photo-assimilates later in the crop cycle, thereby increasing trend of harvest index was seen in that condition (Table 5). Water stress at flowering and grain filling resulted in lower HI than water stress at tillering and no stress control which are statistically similar with higher HI. Decrease in HI could be largely due to water stress which resulted to decrease in translocation of assimilates to the grains, which lowered grain weight and increased the empty grains. High HI indicate the efficient translocation of assimilates towards sink. Lower HI values under water stress at flowering and grain filling stages indicate that it was more harmful in translocation of assimilates towards the grains. This finding is in accord with that observed by [62] . Harvest index was higher in irrigation treatment compared to rainfed as application of water encouraged growth of large number of effective tillers, produced more amount of assimilates, less number of non-effective tillers and ultimately substantial grain yield of Aus rice. Henceforth, harvest index exhibited maximum positive direct effect on grain yield [63] . However, harvest index and yield were decreased in water stress condition [34] [35] [64] .

Yield was increased due to profuse number of effective tillers and due to the production of considerable number of filled grains panicle⁻¹. Reference [65] also reported that, higher numbers of effective tillers are positively correlated with higher yield. Reference [66] testified that, the number of effective tillers rather than total number of tillers contributes more to enhance productivity of rice plant.

All of the yield parameters had exerted decreasing trend under rainfed condition. In the present study the tolerance of rice genotypes subjected to rainfed or water stress varied remarkably. Based on the results it can be concluded that BRRI dhan27, BRRI dhan55, BRRI dhan48 and BRRI dhan43 showed the highest effective tillers, filled grains ultimately grain yield in the both growth environment. In case of yield, BRRI dhan27 ranked the top under both irrigated and rainfed conditions, yet the relative yield was the maximum in BRRI dhan55 followed by Nerika10. Therefore, based on the yield stability and absolute yield performance BRRI dhan27 may be considered satisfactorily for Aus rice cultivation, followed by BRRI dhan55, BRRI dhan48 and BRRI dhan43. Farmers could cultivate BRRI dhan27 for the purpose of higher yield under water limiting area.

Ghosh, U.K., Khan, M.A.R., Karim, M.A. and Haque, M.A. (2018) Yield Response of Direct Seeded Aus Rice Varieties under Rainfed Condition. American Journal of Plant Sciences, 9, 416-434. https://doi.org/10.4236/ajps.2018.93032