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Impact of Low Light Stress on Physiological Characters, Yield and Yield Attributes of Rice (Oryza sativa L.)

N. Veronica1,*, P. Venkata Ramana Rao2
1Department of Plant Physiology, Regional Agricultural Research Station, Acharya N.G. Ranga Agricultural University, Maruteru-534 122, Andhra Pradesh, India.
2Department of Plant Breeding, Regional Agricultural Research Station, Acharya N.G. Ranga Agricultural University, Maruteru-534 122, Andhra Pradesh, India.

Background: Keeping in view of low light effect on rice yield and quality, identification of low light tolerant genotypes as donor for plant breeding program is needed. 

Methods: Field experiments were conducted for low light tolerance during kharif 2021 at Regional Agricultural Research Station, Maruteru with an aim to screen and identify low light tolerant cultivars. The impact of low light was investigated on physiological, yield and yield attributes of eighteen rice genotypes. 

Result: Low light treatment resulted and increase in mean days to 50% flowering and days to maturity. An increment of both plant height and leaf area was noted under low light stress in all the tested genotypes. Yield and yield attributes such as panicle number, grain number per panicle and test weight were also affected. The reduction in total dry matter and harvest index was recorded on exposure to low light condition. Under low light grain yield was maximum in IET 29032 followed by IET 30408 and Gayatri and these were identified as low light tolerant genotypes. These genotypes had a higher panicle number. Genotype Gayatri showed maximum 1000 grain weight and minimum reduction in harvest index was evident in IET 29032 and IET 30408. The chlorophyll a pigment content reduced under low light while majority of the genotypes showed an increase in chlorophyll b pigment. Gayatri followed by IET 29032 and IET 30408 retained higher chlorophyll content under low light stress. Lesser reduction in chlorophyll content (less than 13%) was evident in IET 30408, Gayatri, Swarnaprabha, IET 29031 and IET 29032.

Rice, the staple food for more than half of the world’s population and its production is influenced by environmental factors such as solar radiation, water and temperature. Of this, light/solar radiation is a very critical and essential natural resource and the main driver for vital physiological processes such as photosynthesis and photoperiodism. Both light intensity and light duration are important and affect the plant growth and development. In the changing climate scenario caused due to global warming, overcast and cloudy weather particularly during the wet season has become a prevalent problem. Earlier it was a minor problem but in due course of time it has become a major abiotic constraint for rice. It has been reported that 95% of rice is produced during wet season and low light intensity hinders the productivity of the crop. It has been reported that rice plant requires on an average about 1500 bright sunshine (BSS) hours for the period from transplanting to maturity (Suvendhu et al., 2017). Reduction in the light intensity as well as sunshine hours hampers the physiological efficiency and ultimately the yield and quality of rice.
Low light affects different traits at all the stages of rice growth such as plant height, root and shoot growth as well as tiller number (Xiu et al., 2013). It also significantly influences the gas exchange parameters majorly the photosynthetic rate and photosynthetic pigment content. Besides this it results in alteration in antioxidant activities and accumulation of carbohydrate reserves as well as its assimilation and partitioning (Liu et al., 2014). Low light during grain filling stage was reported to cause decreased starch synthase activity, resulting in poor grain filling and thus reducing the yield (Li et al., 2006). A reduction in spikelet number and 1000 grain weight also was apparent (Goto et al., 2009; Liu et al., 2006; Ren et al., 2003).
An approach to overcome the low light problem is to identify cultivars for low light tolerance with substantial grain yield that can be utilized in breeding program to develop cultivars to increase the production and productivity of rice during wet season. This would certainly ensure future global food security upto a certain extent. It is also imperative to identify physiological traits that can be used as indicators to screen under low light stress. So keeping these in view, the present study was taken up with the aim to evaluate rice germplasm under low light stress and to identify low light tolerant rice genotypes that can be used as a donor in future breeding programmes.
Plant material
A field experiment was conducted at Regional Agricultural Research Station, Maruteru with eighteen rice genotypes during kharif 2021. The list of genotypes used along with their designation was given in Table 1. The seeds were sown in nursery beds and 25 days old seedlings were transplanted into main field in two sets. One set was considered as control and in the other set low light stress was imposed. The experiment was laid out in split plot design with three replications. Spacing of 20 × 15 cm was maintained. The recommended dose of Nitrogen (N), Phosphorus (P) and Potassium (K) (100:60:60 kg ha-1) was applied. All packages of practices recommended for irrigated transplanted rice were followed.

Table 1: List of genotypes utilised in the experiment.

Imposition of stress
Low light stress was imposed in one set of genotypes one month after transplanting by erecting 50% shade net structure that was constructed using bamboo poles as support. The light intensity was measured using Lux meter 3-4 times during the cropping season. The crop was left under low light conditions until maturity.
Experiment details
The experimental data pertaining to morpho-phenological, physiological and yield parameters was recorded under treated and control plant.
Morpho-phenological parameters
The number of days taken from sowing till 50% of plants to flower in each plot for every genotype was noted as days to 50% flowering and was expressed in days. Similarly, days taken to physiological maturity was noted and expressed as days to maturity. Plant height was noted at flowering from the base of root shoot junction till the tip of leaf/panicle. Leaf area was recorded at flowering and expressed in cm2.
Chlorophyll content
It was estimated in flag leaf at reproductive stage (1 week after anthesis). For this flag leaf was cut into small pieces and 25 mg of leaf sample was weighed. Chlorophyll was extracted by placing the sample in 80% acetone solution as per the methodology described by Porra et al., (1989). The absorbance was measured using a UV-VIS spectrophotometer. Chlorophyll a and chlorophyll b were measured at 663.2 nm and 646.8 nm respectively and the chlorophyll content was expressed in mg g-1 fresh weight (mg g-1 FW). Chlorophyll a content, chlorophyll b content and the total chlorophyll content was calculated according to Lichtenthaler and Wellburn, (1983).  
Harvest parameters
Yield and yield attributes such as panicle number /m2, grain number per panicle, 1000 grain weight, grain yield, total dry matter and harvest index were recorded. At physiological maturity, panicles were threshed from a demarcated area of one square meter in all the genotypes. The number of panicles were counted and expressed as panicle number/m2. Later they were threshed, cleaned and the weight of grains was recorded and expressed as grain yield in g/m2. Five panicles were selected at random in every genotype and all the spikelets were separated from the panicle and filled grains were further separated and expressed as grain number per panicle. A sample of 1000 seeds at random were taken from every genotype under both the conditions and weighed in gm. After harvest the shoot was dried and shoot biomass was recorded. The ratio of economic yield to total biological yield × 100 was computed as harvest index and expressed in %.
Statistical analysis
Two-way analysis of variance (ANOVA) was performed using Statistix 8.1 package. Statistical significance of the parameter means was determined by performing Fisher’s LSD test to test the statistical significance.
Morpho-phenological parameters
Low light stress treatment resulted in the increase in mean days to 50% flowering by 3 days with respect to control. The difference in days was maximum in IR 8 and IET 30408 (4 days). In other tested genotypes it varied from 2-3 days (Table 2). Similarly, the mean days to maturity increased by 2 days. Maximum increase in number of days was seen in IET 27538 (6 days) and interestingly in the genotype Gayatri there was no difference (Table 2). There was an increase in plant height under low light stress. Mean plant height increased by 9 cm.  and maximum recorded in IET 27547 (13 cm) and minimum in IET 29032 and IET 27538 (4 cm) (Table 2). An increased response to leaf area was noted in all the genotypes under low light stress. Maximum leaf area under low light stress was recorded in Gayatri (41.2 cm2) and minimum was in Swarna sub 1 (30.2 cm2) (Table 2). Maximum increase in leaf area was in Swarna prabha when compared to control. This might may be due to the reason increased leaf area would capture more light. A similar increase of leaf area under 50% light intensity followed by 75% and 100% in rice was reported by Deepali et al., (2022).

Table 2: Effect of low light stress on morpho-phenological traits in rice genotypes.

Low light impacts the morphology and physiology of plants. Gbadamosi et al., (2014) reported an increase in both plant height and leaf area under low light conditions in rice. Similar response was noted in this study too where the mean plant height increased by 9 cm. Increased leaf length and leaf width ultimately increases the leaf area in response to low light (Ren et al., 2002; Ding et al., 2004). In the present investigation, the mean leaf area increased by 5.45% under low light stress however among the genotypes the increased varied from 2.2% to 11.2%. Similarly, an increase of leaf area by 5.76% under 50% of natural light was reported by Chonan, (1967).
Grain yield and yield attributes
Low light hampered the grain yield and yield attributes in Rice. A reduction in panicle number under low light stress was noted and mean reduction was 13.5%. Under low light, higher number of panicles (number/m2) was noted in IET 29032 (418) followed by Gayatri (385) and IET 30408 (374) whereas lowest was noted in IET 29031 (264) and IET 27538 (275) (Table 3). The grain number per panicle reduced under low light stress by 12.5%. Under stress, higher number of grain per panicle was noted in IET 30410 (176) followed by Varshadhan (122), IET 29032 (122), IET 29031 (119) and IET 30408 (115). Lowest grain number was in Swarnaprabha (70) followed by IET 27547 (76) (Table 3). The average 1000 grain weight of all the tested entries reduced from 23.2 g in control to 20.4 g under low light stress conditions and the highest reduction was observed in IET30410 (29.7%) followed by IET 30409 (21.3%). Highest 1000 grain weight was noted in Gayatri (23.2 g), IET 27547 (22.8 g) and IET 29100 (22.8 g) under low light while lowest test weight was noted in IET 30410 (15.6 g) (Table 3). The mean grain yield reduced from 547 g/m2 to 347 g/m2. The grain yield under low light stress was maximum in IET 29032 (484 g/m2) followed by IET 30408 (452 g/m2) and Gayatri (426 g/m2). The grain yield was lowest in IET 27538 (256 g/m2), IR 8 (274 g/m2) and IET 30410 (279 g/m2) (Table 4). When imposed to low light stress, the total dry matter at harvest reduced from 1353 g/m2 to 978 g/m2. Highest total dry matter was recorded in IET 29032 (1255 g/m2) and IET 30408 (1182 g/m2) under stress. Lowest total dry matter was recorded in IET 29031 (803 g/m2) and IR 8 (815 g/m2) (Table 4). Harvest index dropped from 40.5% to 35.6%. Lesser reduction in harvest index was noted in IET 29031 (1.8%), IET 29032 (4.0 %), Swarnaprabha (5.4%) and IET 30408 (5.7%). On the other hand, higher reduction in harvest index was noted in IET 27538 (30.4) followed by Chiranj (24.7%) (Table 4).

Table 3: Impact of low light stress on panicle number/m2, grain number per panicle and 1000 grain weight in rice genotypes.


Table 4: Impact of low light stress on grain yield, total dry matter and harvest index in rice genotypes.

Low light significantly affects the yield and yield attributes. It has been stated that the yield of a plant has a direct correlation with radiation use efficiency (Hao et al., 2016). In the present study, around 36.5% reduction in the mean grain yield was noted which may be attributed to a lower panicle number and lesser 1000 seed weight in the genotypes grown under low light conditions. Previous studies revealed that when rice plants were subjected to low light stress from transplanting to booting stage, 39.56% drop in grain yield was noted with a corresponding reduction in grains per panicle produced (Liu et al., 2009). The reason being mainly in impairment of translocation of assimilated by the source which includes leaves, culm and leaf sheath to the sink that is the developing grain. Reduction in grain number and weight under low light was also reported by Mu et al., (2010) and Singh, (2005). In this study, IET 29032, IET 30408 and Gayatri recorded a higher grain yield and these genotypes had a higher number of panicle. Lesser reduction in harvest index was evident in IET 29032 and IET 30408. In the present experiment, low light has resulted in significant reduction in the total dry matter in all the genotypes. There was a strong correlation between grain yield and dry matter production in low light. Hence, the reduction in grain yield has resulted in lesser biomass accumulation. In rice when low light was imposed from heading stage, a considerable reduction in total dry mass accumulation, grain filling and test weight ultimately resulting in the reduction of grain yield was reported by Liu et al., (2009).
Chlorophyll content (mg/g FW)
The mean chlorophyll a content reduced from 3.04 to 2.21 mg/g FW under low light stress. Even after reduction, the higher chlorophyll a content under low light stress was noted in IET 30408 (2.56 mg/g FW) followed by IET 28281 (2.51 mg/g FW). Lower chlorophyll a content was noted in IET 30411 (1.86 mg/g FW) (Fig 1a). The mean content of chlorophyll b increased from 1.04 mg/g FW in control to 1.14 mg/g FW under low light stress. Majority of the genotypes showed an increased chlorophyll b response under low light stress. Gayatri (1.8 mg/g FW) followed by IET 29032 (1.53 mg/g FW), IET 29031 and IET 30409 (1.35 mg/g FW) and IET 30408 (1.2 mg/g FW) retained higher chlorophyll b content under low light stress. Lowest chlorophyll b content was recorded in IET 28281 (0.85 mg/g FW) and Swarna sub 1 (0.86 mg/g FW) (Fig 1b). The mean total chlorophyll of all the tested genotypes reduced from 4.08 to 3.35 mg/g FW under low light stress. The higher chlorophyll content was noted in Gayatri (4.22 mg/g FW) followed by IET 29032 (3.93 mg/g FW) and IET 30408 (3.76 mg/g FW). Lowest was noted in IET 30411 (2.91 mg/g FW) and IET 27538 (2.93 mg/g FW). Lesser reduction in chlorophyll content under low light stress was evident in IET 30408 (2.3%), Gayatri (3.7%), Swarnaprabha (3.9%), IET 29031 (6.1%) and IET 29032 (12.9%). On the other hand, a higher reduction was seen in Swarna sub 1 (36.1%) and IET 28276 (29.2%) (Fig 1c).

Fig 1: Impact of low light stress on chlorophyll content in rice genotypes.

Chlorophylls are the most important organelles that are involved in the vital photosynthetic activity mainly by absorbing and transmitting the captured solar energy and ultimately converting it into electrochemical energy (Wang, 2011). The response of plants varies under low light stress in response to chlorophyll pigment production (Zhu et al., 2008; Liu et al., 2009). It was reported that an exposure to 15d of low light stress from initial heading stage varieties that were tolerant to low light stress exhibit higher chlorophyll b (Zhu et al., 2008). Similar response was noted in the highest grain yielding genotypes under low light stress viz., Gayatri, IET 29032 and IET 30408. The mechanism being that these genotypes try to capture as much as solar light as possible by increasing the chlorophyll b molecules as well as the leaf area under low light stress indicating the morphological and physiological adaptation of rice plants when subjected to low light stress (Ren et al., 2002). This strategy helps the tolerant genotypes to minimize the grain yield penalty under low light regime (Liu et al., 2012).
Among different abiotic stresses, low light is one of the important abiotic constraints that hamper rice production due to changing climate. So, the present study was conducted to identify the genotypes having tolerance to low light stress. IET 29032, IET 30408 and Gayatri were identified as low light tolerant genotypes. These genotypes also maintained higher chlorophyll b content under low light stress. Hence, these lines can be utilised as genetic stocks or can be used as donors in the crop improvement programmes.
This experiment is a part of AICRIP programme. So ICAR- AICRP for providing material for the experiment is highly acknowledged.
The authors declare that they have no conflict of interest.

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