Evaluation of elite rice genotypes for physiological

Current Plant Biology xxx (xxxx) xxx–xxx

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Evaluation of elite rice genotypes for physiological and yield attributes under aerobic and irrigated conditions in tarai areas of western Himalayan region Rohit Joshia, a b

⁎,1

, Balwant Singhb, Alok Shuklaa

Department of Plant Physiology, College of Basic Sciences and Humanities, G.B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, 263 145, India National Research Centre on Plant Biotechnology, New Delhi, 110012, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Aerobic rice Drought stress Grain yield Irrigation practices Oryza sativa Paddy

All the irrigated rice systems are currently facing a worldwide challenge for producing higher yield with lower water availability. Aerobic rice is considered to be promising for rice production under water constrained environments where it can be grown under non-flooded and unsaturated soil. All practices for aerobic rice cultivation must start by first identifying promising rice varieties that are expected to produce higher grain yield under such conditions. Therefore, we conducted a field experiment with an experimental design of split-plot in the Tarai region of the Western Himalayas, India, in two irrigation regimes i.e., of continuous flooding and of aerobic condition, using four high-yielding rice genotypes: DRRH-2, PA6444, KRH-2 and Jaya. A grain yield of 743 to 910 g/m2 was obtained on a typical freely draining soil i.e., under aerobic conditions. Further, DRRH-2 showed enhanced panicle number, spikelet number, filled grain number under aerobic conditions, resulting in the higher grain yield of 910 g m/m2. We conclude from our studies that the higher productivity of rice depends upon the improved sink capacity (grain number x grain weight) of the genotype, and that this acts as a major factor limiting yield potential under aerobic and flooded conditions.

1. Introduction Rice is one of the principal food crops of world and accounts for almost 60% of the global energy consumption [1]. Flood-irrigated rice utilizes 45% of the total fresh water, accounting for almost two to three times of that consumed by other cereals [2]. However, by the end of the 21st century, decreasing water resources due to anthropogenic and natural factors will reduce the sustainable production of flood-irrigated rice, a heavy user of water [3,4]. Thus, rice production needs to be increased besides the availability of water, using sustainable water saving technologies and judicious water management practices, in order to feed the increasing global population [4,5,6,7]. Aerobic rice cultivation is an alternative strategy for the conventional methods to deal with water security in the tropical as well as in the sub-tropical agriculture. In the aerobic system, rice is usually directly dry seeded in the non-flooded as well as in the non-puddled fields mimicking the upland conditions, with adequate fertilizer application combined with supplementary irrigation during insufficient rainfall [2,8,9]. This technology utilizes reduced surface runoff, seepage, percolation and evaporation leading to substantial water saving [10].

⁎

1

Lafitte et al. [11] reported that several lowland genotypes survive well in irrigated aerobic soils with occasional flooding. However, under aerobic conditions, even high-yielding lowland rice varieties have shown severe yield loss [12]. Therefore, information using morphophysiological and yield traits to identify and select superior yield performing aerobic rice genotypes is vital for developing aerobic rice cultivars. However, analysis of correlation between the physiological conditions and the yield of rice showed enhanced grain yield under aerobic conditions [1,13,14,15]. In addition, China Agricultural University (Beijing, China) has developed high- yielding aerobic rice cultivars labelled as “Han Dao” that are being widely grown by the farmers there [16]. In India, 23.3% of the gross cropped area is occupied by rice, which contributes to 43% of the total food grain production and 46% of the total cereal production of India [17]. Moreover, out of the ten million hectares of cultivated rice area in the Indo-Gangetic Basin of India, almost 2.6 million hectares receive either temporal or erratic rains, and are affected by insufficient or irregular surface and ground water supplies during the Kharif (i.e., the autumn) season [18]. To meet the increasing food demand under varying climatic conditions, it is essential

Corresponding author at: Research Scientist, (Stress Physiology and Molecular Biology Laboratory), School of Life Sciences, Jawaharlal Nehru University, New Delhi, 110067, India. E-mail address: [email protected] (R. Joshi). Google Scholar Profile: http://scholar.google.co.in/citations?user=1X4WDRsAAAAJ&hl=en.

https://doi.org/10.1016/j.cpb.2018.05.001 Received 18 March 2018; Received in revised form 12 May 2018; Accepted 12 May 2018 2214-6628/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

Please cite this article as: Joshi, R., Current Plant Biology (2018), https://doi.org/10.1016/j.cpb.2018.05.001


2

Irrigated 130 1969

KRH2

PA6444

Jaya (IET-723)

2.

3.

4.

T(N)1 x T-141

2001

135

Irrigated

Semi tall (110 cm), non-lodging, non-shattering, long bold grains with high amylose (27%), low ASV (2.2). Semi tall (100-120 cm), compact and erect, medium slender grains with intermediate amylase. Semi dwarf (82 cm) long bold and white grains, moderately susceptible to bacterial leaf blight, sheath blight, rice tungro virus, gall midge and resistant to blast, Yield: 5-6 t/ha.

DRRH1 1.

6CO2/6MO5

Irrigated 125-130 1996

Ecosystem Duration (days) Year of Notification Parentage Name of Variety Sl. No.

Table 2 Parental lines and their significant characteristics of the rice varieties tested in the experiment.

Field experiment was carried out at the Norman Borlaug Crops Research Center, Pantnagar, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India (29 °N, 79°29′E and 243.8 m m.s.l.) during the Kharif season. The soil was silty clay loam with a pH of 7.38, 202.35 kg/ha total N, 20.13 kg/ha P, 178.61 kg/ha K, and 32.5 meq 100 g−1 cation exchange capacity (Table 1). Four genotypes DRRH-2 (IR68897 A/DR 714-1-2R), PA6444 (6CO2/6MO5), KRH-2 (IR 58025 A/KMR-3R) and Jaya (T(N)1xT-141) were used for aerobic treatment, and the flooding was taken as the control (Table 2). Weekly weather data on wind speed (Km/h), minimum and maximum relative humidity (%), minimum and maximum temperature (°C), sunshine (hours/day) and rainfall (mm) during the cropping season were obtained from the Department of Agrometeorology of the University (Fig. 1). Seedlings were raised in dry nursery beds with alternate day irrigation treatment. Transplanting was done after 25 days in 2 × 3 m plots with a total area of 544 m2. The row-to-row distance was maintained at 20 cm and the plant-to-plant distance at 10 cm during transplanting. Two-meter distance was retained between the aerobic and the flooded fields to avoid water flow by seepage. Fifteen cm high earthern bunds were mounded to avoid runoff loss in flooded plots and runoff gain in aerobic plots [19]. Regular doses of phosphorus (45 kg P ha−1 as single super phosphate), potassium (60 kg K ha−1 as muriate of potash), and zinc (30 kg Zn ha−1 as zinc sulfate) were applied in all the plots. Nitrogen in the form of urea was applied at three developmental stages (50 kg N ha−1 after 15 days of transplanting, 25 kg N ha−1 at active tillering and 25 kg N ha−1 at the panicle initiation stage). Manual weeding was done to keep the plots weed-free and the recommended doses of pesticides were applied for optimum crop protection. Surface flooding was applied for irrigation through channels connected to the sub surface pressurized pipe system lifting ground water.

Salient Features

2.1. Experimental design

IR 58025 A × IR 40750 IR 58025 Ax KMR-3R

2. Materials and methods

Grains are long slender, yield: 7.3 t/ha

to develop new rice cultivars with improved water use efficiency and those that can be grown under the Himalayan ecosystem. Therefore, in our current research, we have estimated the influence of continuous flooding and aerobic conditions that differentially affect the different rice genotypes during their growth and yield in the Indo-Gangetic Basin of India. Further, our research aims to provide quantitative information on the productivity of these high-yielding genotypes under flooded and aerobic situations in order to determine whether sustainable yield can be obtained under aerobic conditions by improving water management practices.

Irrigated

7.38 0.72 202.35 20.13 178.61 2.64 1.38 0.27 Dark Grayish Brown/ Dark Grey 12.83 63.75 29.48 Loam/ Silty Clay Loam

125 - 130

pH Organic C (%) Available N (kg/ha) Available P (kg/ha) Available K (kg/ha) Mn (ppm) Bulk density (g/cm3) Electrical conductivity (EC) Soil color Sand % Silt % Clay % Soil texture

1996

Mean value

Andhra Pradesh

Parameters

Recommended for cultivation

Table 1 Physico-chemical characteristics of experimental soil.

Tamil Nadu, Pondicherry, Karnataka, U.P., Bihar, West Bengal, Tripura, Orissa, Maharashtra, Rajasthan and Goa Uttar Pradesh, Tripura, Orissa, Andhra Pradesh, Karnataka, Maharashtra and Uttarakhand All India

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Fig. 1. Mean maximum and minimum temperatures (°C), maximum and minimum relative humidity (%), wind speed, sunshine and total rainfall (mm) during the crop growing season.

thousand grains, from the bulk harvest of each replication, was taken on a digital balance. Leaf area index was measured at the time of flowering. The relative water content, on a percentage basis, was calculated using the equation of Schonfeld et al. [25].

2.2. Plant sampling The samples were taken in three replicates and the dates of panicle exertion, grain filling and physiological maturity were recorded manually. Plant height was recorded from 30 to 90 DAP (days after planting) at the interval of 15 days from the base of the top most fully expanded leaf to the soil level till panicle initiation and thereafter the data was recorded from the soil base to the base of panicle (Neck-node joint). Soil bulk density was measured using a cylindrical metal sampler [20] and the soil moisture content was measured as described by Reynolds [21]. Leaf relative water content (RWC) was determined by measuring the turgid weight of 0.5 g fresh leaf samples by soaking for 4 h in water, followed by oven drying till a constant weight was achieved [22].

2.5. Data analysis All the recorded data were analyzed statistically using Split Plot Design. The effect of treatment (aerobic vs. flooded) was determined by the analysis of variance of the complete data set from three replications [26]. Critical differences were calculated at 1% probability level, wherever the treatment differences were significant for interpretations. Correlation coefficient was determined using SPSS (Ver.16). 3. Results

2.3. Physiological analysis 3.1. Growth parameters

Free proline concentration (μg g−1 FW) was determined from 30 DAT (days after transplanting) to 90 DAT at the interval of 15 days through a rapid determination method as described earlier [23]. Various photosynthesis-related parameters, such as the net photosynthetic rate (NPR), transpiration rate (TR), and stomatal conductance, were measured from 30 DAT to 90 DAT at the interval of 15 days, using a LI6400XT portable photosynthesis instrument [7]. Total chlorophyll content, chlorophyll a, chlorophyll b and chlorophyll a/b ratio was calculated from 30 DAT to 90 DAT at the interval of 15 days [22].

During the cropping season, the soil moisture content was found to be always less under aerobic conditions than under control conditions, and the soil bulk density was found to be always higher under aerobic conditions than under control conditions (Table 3). Significantly higher plant height was observed under flooded than under aerobic condition (Fig. 2a). The decline in plant height in all the genotypes under the aerobic condition is probably due to the constraint in cell elongation, which must have led to reduced internodal length. Similarly, all the genotypes showed reduced relative water content (Fig. 2b). Total free proline content was also observed to be increased in all the genotypes with the highest in KRH-2 under aerobic conditions (Fig. 2c). Proline is a well known compatible osmolyte produced in water-stressed plants [27]. With the increase in water stress the proline levels were also found to increase in all the genotypes in our experiments (Fig. 2c). Similarly, during aerobic situations, all the genotypes displayed significantly reduced photosynthetic rates in comparison to those under flooded conditions (Fig. 2d). This paralled the decrease in total chlorophyll as well chlorophyll a/b ratios under aerobic conditions (Fig. 3a–d). These results may suggest that the chlorophyll loss from the

2.4. Yield component analysis Tiller number and panicle number per hill were measured to evaluate tiller numbers/m2. Panicles were hand-threshed manually to separate filled spikelets from the unfilled ones. At maturity, the number of filled grains per m2, grain yield per m2 and the total dry matter per m2 were determined and expressed at 14% moisture content, as described earlier [12,24]. A total number of spikelets per m2 and harvest index (100× filled spikelet weight/above ground total biomass) were recorded for each replication at the time of grain filling. The weight of 3


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Table 3 Soil moisture content and soil bulk density of different varieties of rice under aerobic and irrigated conditions at different time intervals at 15 and 30 cm depth. DAP

Soil moisture content (%)

Soil bulk density (g/cc)

15 cm

30 cm

Irrigated 30 DAP 45 DAP 60 DAP 75 DAP 90 DAP Mean S.Em. ± CD at 5%

39.80 38.40 37.40 45.80 39.70 40.22 0.143 0.413

± ± ± ± ±

Aerobic 1.62 0.40 0.35 0.29 0.17

18.50 18.80 18.70 18.60 18.40 18.60

± ± ± ± ±

1.04 0.40 0.35 0.29 0.17

15 cm

Mean

Irrigated

29.15 28.6 28.05 32.2 29.05

34.30 36.30 32.60 40.20 32.80 35.24 0.160 0.463

± ± ± ± ±

Aerobic 1.44 0.40 0.35 0.29 0.17

23.80 24.60 21.40 23.30 21.50 22.92

± ± ± ± ±

1.44 0.40 0.35 0.29 0.17

30 cm

Mean

Irrigated

Aerobic

29.05 30.45 27.0 31.75 27.15

1.46 ± 1.47 ± 1.47 ± 1.46 ± 1.47 ± 1.466 0.0035 0.0099

1.67 1.67 1.67 1.67 1.67 1.67

0.04 0.02 0.02 0.01 0.03

± ± ± ± ±

0.01 0.03 0.02 0.02 0.01

Mean

Irrigated

Aerobic

1.578 1.568 1.567 1.567 1.567

1.47 ± 1.47 ± 1.47 ± 1.47 ± 1.47 ± 1.47 0.0033 0.0096

1.67 1.67 1.67 1.67 1.67 1.67

0.03 0.03 0.02 0.01 0.03

± ± ± ± ±

Mean 0.01 0.03 0.02 0.02 0.01

1.568 1.569 1.568 1.569 1.569

*The data are the mean of four replications in each case.

conditions, all the genotypes showed a delay in achieving 50% flowering under aerobic conditions (Table 4). DRRH-2 attained 50% flowering as well as maturity prior to the other genotypes. Similarly, total dry matter (TDM) was also reduced significantly under aerobic conditions as compared to that under flooded conditions. However, KRH-2 showed the highest TDM followed by PA-6444. In contrast, the lowest TDM was recorded in Jaya under aerobic conditions (Table 4). Data obtained on the yield attributes and the yield (Table 5) further confirmed that flood-irrigation gave notably higher values as compared to the aerobic conditions. DRRH-2 showed higher number of filled grains/ m2 in comparison to that in all other genotypes. Thousand grain weight and the grain yield of all the genotypes were reduced significantly under the aerobic condition. This led to a severe decline in the harvest index in all the genotypes under aerobic condition. The rice genotype DRRH-2 had the highest grain yield as well as the harvest index under both flooded and aerobic conditions. When we correlated the mean growth related traits with the yield related traits, we observed that in aerobic environment, the days to 50% flowering and days to maturity were negatively correlated with productive tillers, spikelet

leaves subjected to drought stress is mainly in the mesophyll cells since these cells are farther from the vascular supply of water than the bundle sheath cells, and hence they develop a greater cellular water deficit which leads to greater loss of chlorophyll. Alternatively, it is so because the mesophyll chloroplasts contain more light harvesting Chl a/b protein, which is labile even under mild water stress [28]. In addition, both the transpiration rate (Fig. 2e) and the stomatal conductance (Fig. 2f) were also found to be reduced under aerobic conditions in all the genotypes in comparison to those grown under flooded conditions. 3.2. Yield attributes and yield Significantly higher tiller number/m2, panicle number/m2 and spikelet number/m2 were observed in DRRH-2 and PA-6444 under aerobic conditions as compared to that under flooded conditions (Table 4). In contrast, KRH-2 and Jaya showed a drastic reduction in tiller number/ m2, panicle number/m2 and spikelet number/m2 under aerobic conditions in comparison to those under flooded conditions. Inspite of achieving maturity at the same time under both the above-mentioned

Fig. 2. Assessment of physiological parameters of field-grown rice varieties under irrigated and aerobic conditions. Bar graph depicting the comparison of (A) plant height (cm), (B) relative water content (%), (C) proline content (μg g−1 FW), (D) photosynthetic rate (μmolCO2 m−2 s−1), (E) transpiration rate (mmol m-2 s-1), and stomatal conductance (mmol m-2 s-1). Values are the mean ± standard deviation (n = 3). 4


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Fig. 3. Evaluation of photosynthetic parameters of field-grown rice varieties under irrigated and aerobic conditions. Bar graph depicting the comparison of (A) chlorophyll a (mg g−1 FW), (B) chlorophyll b (mg g−1 FW), (C) total chlorophyll (mg g−1 FW) and (D) chlorophyll a/b. Values are the mean ± standard deviation (n = 3).

the inland valley areas that are benefited from the flooding soils during submergence in monsoon [29]. However, due to the varying climatic conditions, rainfall is becoming more erratic with the passage of time, resulting in water stress at various stages of rice development [2]. In contrast, as a primary source of calories for more than 2 billion people, it cannot be substituted just because of water shortage. In fact, higher production of rice using less water is the need of the day to feed the increasing human population [3]. Development of productive and sustainable cultivation strategies under water-stress is the original concept for the so-called aerobic rice [30]. Several studies have pointed to the sub optimal growth of rice in aerobic soil through evaluation of drought tolerant medium-yielding varieties for the explicit purpose of reducing the risk of having low yield. In the temperate climate, aerobic

fertility and the grain yield, whereas the total yield was positively correlated with productive tillers, spikelet numbers and the leaf area index (LAI) at flowering (Table 6). 4. Discussion Water is an essential commodity for rice production, thus plentiful rainfall and high water table allows successful irrigated lowland rice cultivation in the Northern Region of India. Historically, lowland rice (wetland rice or paddy rice) cultivation practices have continued for centuries in Asia in flooded or flooding prone areas during wet/monsoon season. Most suitable areas for wetland rice cultivation are the large rain-fed low-lying regions including the Indo-Gangetic plains and

Table 4 Tiller number/m2, panicle number/m2, days to 50% flowering, days to maturity and total dry matter production (g/m2) of different varieties of rice under aerobic and irrigated conditions. Treatments

Irrigated DRRH2 KRH2 PA6444 Jaya Mean Aerobic DDRH2 KRH2 PA6444 Jaya Mean S.Em. ± CD at 5%

Tiller number/m2

Panicle number/m2

Spikelet number/m2

Days to 50% flowering

Days to maturity

Total dry matter production (g/m2)

395.63 408.33 375.19 416.40 398.80

± ± ± ±

8.14 0.54 0.31 4.21

337.65 364.15 310.63 327.44 334.96

± ± ± ±

0.15 0.26 4.29 7.96

63.28 86.48 80.21 77.23 76.80

± ± ± ±

0.36 1.40 0.46 0.51

90.0 95.0 100.0 105.0 97.5

120.0 125.0 130.0 135.0 127.5

2950.00 2550.00 2850.00 2362.50 2678.0

± ± ± ±

88.39 20.41 35.36 21.65

427.21 379.27 437.52 381.42 406.36 1.470 6.560

± ± ± ±

8.36 0.36 4.38 0.32

387.36 327.36 329.48 316.54 340.18 0.781 3.480

± ± ± ±

3.80 8.35 0.40 3.93

91.35 84.32 79.39 64.41 79.87 0.247 1.100

± ± ± ±

1.07 0.96 0.40 0.55

101.2 108.2 110.2 115.5 108.78

132.0 136.0 139.0 149.0 139.0

2212.50 2450.00 2443.75 2350.00 2364.0 20.32 85.55

± ± ± ±

119.68 17.67 25.77 20.41

*The data are the mean of four replications in each case. 5


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Table 5 Filled grain number/m2, thousand grain weight (g), grain yield (g/m2), harvest index (%) and LAI at flowering of different varieties of rice under aerobic and irrigated conditions. Treatments Irrigated DRRH2 KRH2 PA6444 Jaya Mean Aerobic DDRH2 KRH2 PA6444 Jaya Mean S.Em. ± CD at 5%

Filled grain number/m2

Thousand grain weight (g)

Grain yield (g/m2)

Harvest index (%)

LAI at flowering

50.13 69.95 62.26 62.50 61.21

± ± ± ±

0.17 0.54 0.63 0.51

23.52 25.60 27.58 33.44 27.54

± ± ± ±

0.48 0.36 0.44 0.35

1301.25 1102.50 1280.00 1031.25 1179.0

0.44 ± 0.43 ± 0.45 ± 0.44 ± 43.99

0.01 0.03 0.06 0.04

9.98 8.42 8.73 7.66 8.69

± ± ± ±

0.17 0.50 0.01 0.08

58.42 54.46 59.46 50.40 55.69 0.231 1.030

± ± ± ±

0.76 0.41 0.47 0.62

22.65 22.79 21.35 25.43 23.05 0.157 0.698

± ± ± ±

0.69 0.47 0.43 0.73

910.00 792.50 765.00 743.75 802.8 5.244 23.38

0.41 ± 0.32 ± 0.31 ± 0.32 ± 34.13 0.114 0.507

0.01 0.07 0.03 0.02

7.55 ± 7.05 ± 8.24 ± 6.96 ± 7.45 0.087 0.389

0.20 0.01 0.09 0.01

± ± ± ±

± ± ± ±

21.45 1.44 9.35 13.44

5.40 1.44 0.82 3.15

*The data are the mean of four replications in each case.

Table 6 Correlation coefficients (r) between growth-related and yield-related traits under aerobic conditions. Traits

Tiller number/m2

Panicle number/m2

Spikelet number/m2

Days to 50% flowering

Days to maturity

Total dry matter

Filled grain number/m2

Thousand grain weight (g)

Panicle number/m2 Spikelet number/m2 Days to 50% flowering Days to maturity Total dry matter Filled grain number/m2 Thousand grain weight (g) Grain yield (g/m2) Harvest index (%) LAI at flowering

0.54 0.47 −0.49 −0.47 −0.26 0.90* −0.72 0.42 0.37 0.93*

0.78 −0.92* −0.76 −0.82 0.58 −0.32 0.98** 0.97** 0.23

−0.96** −0.99** −0.31 0.75 −0.72 0.83 0.65 0.32

0.97** 0.57 −0.68 0.55 −0.95** −0.84 −0.26

0.274 −0.76 0.74 −0.81 −0.62 −0.34

−0.10 −0.24 −0.78 −0.92* 0.10

−0.92* 0.52 0.36 0.87

−0.30 −0.07 −0.79

Grain yield (g/m2)

Harvest index (%)

0.96** 0.11

0.01

* Significance at = 0.05. ** Significance at = 0.01.

free amino acid content, and in reactive oxygen species (ROS); further, there was a shortened grain filling period, indicating that water deficit enhanced the leaf senescence [35]. We note that this reduced photosynthetic rate does not recover even after irrigation. Similarly, a decrease in leaf area is due to the extreme sensitivity of rice towards soil moisture content showing significant changes between saturation and field capacity [36]. Further, leaf rolling is a primary indicator of any rice genotype for its ability to maintain water status under water stress [37]. However, soil moisture content in the aerobic plots during the wet season also remains in saturation, due to frequent rainfall and shallow water table in this Indo-Gangetic region. In addition, free proline content has been reported to enhance in rice kernels during ripening under osmotic stress [38]. It is well known that proline acts as an osmoprotectant and its overproduction provides enhanced tolerance against osmotic stress [6]. Proline protects thylakoid membranes and stabilizes proteins and DNA against free radical induced photodamage by quenching singlet oxygen and scavenging OH• radicals. Water deficit primarily affects the sterility of panicles and spikelets and the final grain weight. Few of these spikelets could be filled, leading to high sterility and low harvest index under reduced water levels. The yield gap between the flooded and aerobic rice mainly occurs because of the variation in total dry matter accumulation, sink capacity (spikelets/panicle), panicles/m2 and the thousand grain weight [35]. However, higher sink size (spikelet/panicle) is the most critical factor for reduced productivity under aerobic conditions [2]. In contrast, several reports have shown severe yield penalty under aerobic conditions in high-yielding lowland rice cultivars [2,12,39]. This

rice cultivars show less yield penalty in comparison to those in tropical regions [31]. Recommendation of specific aerobic rice cultivars, suitable for local conditions, in addition to effective crop management practices that can reduce the yield penalty in comparison to the flooded rice, is the key towards successful cultivation of aerobic rice. Keeping water saving strategy in mind, our study, presented here, was conducted by using aerobic rice technology to determine whether few promising high yielding rice genotypes, recommended for lowland cultivation, can give higher productivity under aerobic conditions. The yield barrier between the aerobic and the flooded rice also depends upon the physical properties of the soil, soil moisture content and other environmental conditions. In addition, the lack of soil tillage may induce dryness in the soil, making superficial soil layer more compact under aerobic conditions. However, this compact layer surrounding the seed can be broken by using a chisel with the cutting disk of the no-till planter that allows application of the fertilizer at effective soil depth [32]. The reduction in plant height (Fig. 2a) may be due to the inhibition of stem growth which might, in turn, be due to inhibition of cell length or cell division because of “pressurized” roots under water limited conditions. With an increase in the severity of stress, the hybrids as well as inbred varieties show reduced plant height and number of tillers resulting in yield penalty [33]. Because of a poorer root system, leaf water potential in rice plants decreased significantly with the depletion of soil moisture. In such plants, the reduced leaf water potential drastically decreases leaf expansion, photosynthetic rate, dry matter production and grain filling period [34]. This decline was associated with the down regulation of PS II activity and a concomitant increase in 6


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publication to the memory of Professor Alok Shukla, who died in February 2017 while preparing this article.

decrease is probably due to water stress after panicle initiation, which causes spikelet sterility and reduction in spikelet number and in translocation of assimilates to the grain [31,40]. In addition, water stress-induced lack of root pressure would dehydrate the panicles and reduce the number of endosperm cells, resulting in a decline in the sink size per kernel and in the total dry matter content [41,42]. Field studies, however, has demonstrated that under aerobic conditions, the yield is decreased in all the rice genotypes. Xiaoguang et al. [43] and Grassi et al. [44] have shown 20–30% decline in yield under aerobic conditions in comparison to irrigated lowland genotypes. This decline in the yield may be due to reduced biomass and grain yield as a result of water stress occurring particularly during flowering to anthesis stage [18]. Similarly, lower harvest values indicate that water stress at booting and flowering stages severely affects translocation of assimilates towards the grains during grain filling stage. Heat waves and dry weather conditions prevail during the end of May to June, in most parts of India; this increases spikelet sterility. Exposure to 41 °C for 4 h at the flowering stage causes irreversible damage and plants become completely sterile [45]. The low grain yield under aerobic conditions has been shown to be due to the prevalence of high atmospheric temperature during the study that affected almost all the growth stages of rice from panicle emergence to ripening and harvesting [45–47]. The most salient feature of our study was the identification of the genotypes showing improved yield under aerobic situations i.e., combined with rainfall and slight irrigation from the sowing to the harvest stage. These cultivars can be grown in areas with inadequate water availability (i.e., with water shortage) for lowland rice production. In comparison with this type of rice, water consumption by cultivars, used in our study, is much reduced than their yields showing higher water use efficiency. Better performance of DRRH-2 and KRH-2 under aerobic situation was also evident from their higher yield, plant height and biomass in comparison to Jaya, the control variety, which has been reported to be the most suitable variety under water stress conditions. The physiological and yield parameters of these varieties can further be used for breeding under aerobic environment. Further studies are required to standardize the nutrient uptake and fertilizer response under rainfed hill ecosystems and to optimize the aerobic rice crop management practices. Earlier reports [18,43,48] and the current study confirm that sustainable yield can be achieved by scheduling irrigation depending upon the sensitivity of a variety towards water stress. Water saving could also be achieved by reducing irrigation cycles, termed as “dry saving” [18]. Farmers can utilize this dry saving by increasing the irrigated area to increase total productivity, thus, reducing their production cost. Similar results can also be achieved either by shifting the transplanting date to reduce evaporation or by growing shorter duration genotypes or through aerobic cultivation. However, by breeding drought resistant traits of upland varieties with high yielding traits of lowland varieties, new genotypes with enhanced yield under aerobic situations could also be achieved. Thus the development of aerobic rice cultivars is a novel approach from the socio-economic point of view. These aerobic rice varieties can also be grown in upland irrigated areas with less water availability or in completely abandoned areas to meet the growing demand of food globally with the burgeoning population and shrinking resources.

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Conflict of interest Authors declare that there is no conflict of interest between all the authors and each one has read and approved the manuscript. Acknowledgements RJ and AS acknowledge funds received from All India Coordinated Research Project (AICRP) on rice, ICAR, Govt. of India. The authors also thank Professor Govindjee, University of Illinois at Urbana-Champaign, for critical reading of the manuscript. RJ and BS dedicate this 7


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