Phytotoxic Effect of Cadmium Induced Stress on LAI, CGR and Yield of Rice Plant (Oryza sativa L.)

Cadmium is a heavy metal that occurs naturally in the soil as well as in the atmosphere in the least concentrations. As per the degree of toxicity is concerned, it ranks 7th most toxic HMs in the universe (Jaishankar et al., 2014). It is assumed that the artificial source of cadmium accumulation in the soil and water is fertilizers and agrochemicals which are consistently used on the crop to boost the harvest. Due to the increase in industrialization and urbanization, the deposition of cadmium (Cd) along with other harmful salts increases in soils while its translocation within plant organs leads to threats to food safety (Anjum et al., 2016; Siddique et al., 2018; Rizwan et al., 2012 and Satpathy et al., 2014).

As per the area under rice cultivation is concerned, it was 321.79 lakh ha in which West Bengal, UP, Punjab, Haryana, Telangana and Tamil Nadu are the main states of rice production while as per the report published (Toi 2018)  out of all the district of India, 24 districts are severely affected with cadmium toxicity. Rice is known for easily uptake and translocation of Cd in roots and shoots of rice plant hence is considered one of the sensitive plants among the monocot species because it is a probability of chance that cadmium can enter to food chain easily (Guo et al., 2018 and Song et al., 2015 and Aziz et al., 2015).

The adverse effect of Cadmium toxicity started from seed germination and seedling growth stages while carrying forward up to the crop’s yield. During the growing period of the rice plant, various morpho-physiological and biochemical changes appear due to cadmium toxicity that often results in the degradation of chlorophyll, protein, fat and carbohydrate synthesis, consequently leading to the death of the plant (Siddique and Dubey, 2017; Siddique et al., 2018; Guo et al., 2018; Dong et al., 2019 and Srivastava et al., 2014). Although several studies about Cd toxicity to the rice plant have been done, further studies are required to make it clear about overcoming cadmium stress.

  

 

Leaf area, LAI and CGR
 
Morphological changes were observed under the influence of externally imposed heavy metal stress via cadmium chloride on leaf area, leaf area index (LAI), crop growth rate (CGR), grain yield, biological yield and harvest index (HI%) in rice plants. It was observed from data as the intensity of stress increased from least to higher concentrations of CdCl₂, the leaf area and leaf area index (LAI) hill^-1 gradually decreased as compared to control. However, as the days after transplanting (DAT) is a concern, the maximum leaf area and LAI was recorded at 75 DAT during an entire set of treatment which was 855.8, 821.2, 787.6, 721.6, 678.4 660 cm2 hill^-1 and 5.71, 5.47, 5.25, 4.81, 4.52, 4.40 hill-1 (Table 1 and Fig 1). Similarly, % reduction of leaf area and LAI was calculated for each parameter corresponding to the HLCT and control and found that out of all DAT, a maximum% reduction was recorded at 50 DAT 38.47% and 38.38% (Table 1, Fig 1 and 4). The statistical analysis of the morphological parameters like leaf area and LAI were found highly significant at each DAT While the comparison of the mean value of each treatment indicated about the leaf area had no significant difference among the T4 and T5 at 25, 50 and 75 DAT. However, to check whether the treatments were significant or nonsignificant at a particular DAT for LAI, five subsets were received as per the DMRT test (p>0.05) which ranged from a to e in which similar alphabets had nonsignificant differences while dissimilar had significant among them (Fig 1). Crop growth rate (CGR) was also influenced by externally imposed CdCl₂ treatments as other parameters. The intensity of stress due to CdCl₂ also poses the reverse impact as the concentrations increased from least to higher concentrations. Statistical analysis carried out through SPSS also indicated the status of CGR. The scrutiny of ANOVA indicated that the parameter CGR had a significant difference. As per the days after transplanting (DAT) is a concern, the maximum crop growth rate was recorded between 50-75 DAT in each set of treatments, i.e. 1.717, 1.712, 1.646, 1.615, 1.476 and 1.254 mg cm² day^-1 as compared to 75-100 DAT and 25-50 DAT Similarly, % reduction of crop growth rate was calculated and found that out of all the intervals, the maximum % reduction was recorded 35.0 mg cm² day^-1 at 75-100 DAT followed by 26.79 and 20.89 mg cm² day^-1 at 50-75 and 25-50 DAT. The scrutiny of the mean data among the treatments corresponding with CD found that CGR had nonsignificant differences among the T₀, T₁, T₂ and T₃ at 50-75 DAT while T₃, T₄ and T₅ had nonsignificant at 75-100 DAT (Fig 2). 





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Grain yield, biological yield and harvest index
 
Data pertaining in (Fig 3) shows that as the concentrations of CdCl2 increased from least to higher concentrations, gradually declined in the grain yield hill^-1, biological yield hill^-1 (g) and harvest index (%)  were noticed compared to control. Among the treatments of CdCl2, the maximum amount of grain yield and biological yield was recorded in (T₁) 12.88 and 28.83 g hill^-1, while the minimum was recorded in (T₅) 8.87 and 22.31 g hill^-1. Similarly, the maximum HI % was also recorded (T₁) at 44.68% and the minimum was recorded (T₅) at 39.79%. However, % reduction was also calculated to know the severity of externally imposed stress on grain yield biological yield and harvest index which was 31.87, 28.7 and 11.10 % respectively (Fig 4). The presented in (Fig 3) were also analyzed through ANOVA and found that the parameters grain yield, biological yield and HI% had highly significant differences at (p>0.05) while the close analysis of mean data of each parameter indicated that grain yield yields and biological yield had significant differences among the treatments of CdCl₂. However, in the case of HI%, only T₄ and T₅ had significant differences while from T₀ to T₃ recorded nonsignificant differences among them. Leaves area plant^-1 and LAI both are positively linked together because the cumulative efforts of both the parameters laid the healthy foundation for the better growth and development of subsequent stages of the plant while the crop growth rate reveals the impact of growth within a certain period. Reduction in morphological growth in terms of leaf area, leaf area index (LAI) and CGR were observed throughout the study due to externally imposed CdCl₂ stress. The bounded growth of leaf area due to the interference of CdCl2 reflected in LAI depends on cell division and cell enlargement. Our results are positively correlated with the finding of (Hatamian et al., 2020 and Fellet and Marchiol, 2011), who reported that cadmium stress had a detrimental effect on leaf morphology due to the restricted growth of cells and their expansion (Nagajyoti and Sreekanth, 2010). CGR is an output of dry matter accumulation of plant which was also affected adversely due to the uptake and translocation of CdCl₂ in the plant (Xue et al., 2013 and Siddique and Dubey, 2017). Harvest index is an efficient indicator of partitioning of photosynthate from source to sink consequently reflected in grain yield. Cadmium had a negative impact on the functioning of chlorophyll and its related enzyme activity while under the severe conditions it destroy the chloroplast consequently it suppress the portioning efficiency of photosynthate which in turn reduced grain yield (Lemoine et al., 2013). Enough evidence is available that indicates the negative relationship between Cd2+ and chlorophyll content because Cd2+ had a detrimental effect on chloroplast (Song et al., 2019; Siddique et al., 2018 and Latif, 2008). Enough shreds of evidence are available in favor of cadmium-induced stress suppressing the process of photosynthesis by destroying chlorophyll structure, damaging the pigment system and reducing Rubisco activity, while the negative response of root growth and biomass production is also affected (Kupper et al., 2010; Santos et al., 2018; Rascio et al., 2008; Siddique et al., 2017; Song et al. 2019 and Sun et al. 2016).

As per the findings of our results, it is concluded that externally imposed cadmium stress shows a deleterious effect on the leaf area, LAI and CGR, biological yield, grain yield and HI%. The detrimental effect was calculated in terms of % reduction over the control and found a certain time (DAT) during the life cycle at which % reduction was recorded maximum. As a consequence of CdCl₂ toxicity, leaf area, growth analysis parameters, grain yield and other attributes like biological yield and HI % were adversely affected because the plant’s reproductive growth depends on vegetative growth in which the number of leaves, leaf area and LAI is one of the important parameters that help in stabilizing plant for further growth.