Genotypic variation among 20 rice cultivars/landraces in response to cadmium stress grown locally in West Bengal, India

Highlights

• TThe studied rice genotypes grown locally in West Bengal exhibit differential response towards Cd stress.

• Cd toxicity negatively affects plant growth, maturation period and crop yield.

• Rice cultivars that are able to tolerate Cd stress, are less affected as compared to the susceptible genotypes.

• The studied tolerant cultivars accumulated Cd in grains but the Cd content was acutely lower than the PTMI limit.

Abstract

Cadmium (Cd) is a hazardous soil contaminant and causes environmental toxicity when present beyond the allowable limit in soil. It can alter growth and metabolism in both plants and animals even at very low concentration. Being sessile in nature, plants try to evade this harmful effect by adopting various defence mechanisms including activation of antioxidants and other metal homeostasis mechanisms. This study shows the varietal Cd stress tolerance capacity of rice cultivars commonly grown in West Bengal, which is a rice biodiversity region in India. Seven days old rice (Oryza sativa L.) seedlings were treated with 10 μM CdCl₂ for another 7days and different physiological and biochemical stress parameters were studied to compare the varietal stress responses. Principle component analysis (PCA) and root tolerance index (RTI) revealed that rice cultivars I.E.T-4786, Jamini and Netiya, Maharaj showed divergent stress responses towards susceptibility and tolerance. Histochemical localization of hydrogen peroxide (H₂O₂), superoxide (O₂˙^-) and pot experiment were performed in these four cultivars (I.E.T-4786-Jamini and Netiya-Maharaj) to elucidate the different Cd stress tolerance. Histochemical analysis, agronomic traits and grain Cd content analyses showed that I.E.T-4786 and Jamini were susceptible with no Cd accumulation in grain, whereas cultivars Netiya and Maharaj were stress tolerant and Cd accumulators. In addition, health risk assessment was monitored for dietary intake of Cd through Cd accumulating rice and non Cd accumulating rice genotypes were identified. Thus, the study identified the Cd tolerant and sensitive cultivars grown locally.

Introduction

Rice is one of the principle sources of calorie, consumed by more than 3 billion people and third highest growing crop among all the cultivated cereals through-out the world (Gao et al., 2018). Asia, including India has been the major contributor for the global food security, where 90% of lands are being used for rice cultivation that meet the demand of around 60% population (Fahad et al., 2019). Since 1970, West Bengal is the home of large numbers of rice varieties (more than 5000 genotypes) and is well known as a rice diversity region (Sinha and Mishra, 2013). These cultivars vary in their morphological, physiological and yield parameters (Sadimantara et al., 2014). Cultivars vary in their maturation period and resistance to several biotic as well as abiotic stress factors. Similarly, metal uptake capacity also varies from variety to variety (Grant et al., 2008). Cd uptake, translocation and accumulation not only differs from one plant species to another but intra-species variability is also reported, several reports have been made on the intra-species variation of Cd accumulation in rice (Sebastian and Prasad, 2015; Wang et al., 2014; Liu et al., 2007). Uptake and accumulation of Cd was found to be dose and genotype dependent, Pinson et al. (2015) studied 1763 rice accessions from different geographical zones and observed 40 fold variations in grain Cd uptake.

It is a common practice among the farmers to use fertilizers mainly phosphate fertilizers in high amounts, to increase the agricultural productivity, which contributes majorly to heavy metal (especially Cd) contamination in agricultural lands (Roberts, 2014). Apart from phosphate fertilizers several other factors like application of pesticides, sewage sludge and irrigation with waste water also lead to increase in heavy metal concentration in agricultural soil (Alves et al., 2016; Majumdar et al., 2018). Cd is one of the most widespread and potent toxic element (Gallego et al., 2012), a metal cation (Cd²⁺) that can easily be dissolved in water and taken up by plants through roots. As Cd has long biological half-life, it cannot be degraded easily and thus, remains within the system. Cd is toxic, non-nutritive, having adverse effects on both plants and animals (Zhang et al., 2018).

Being highly mobile Cd is easily taken up by the rice plants (Oryza sativa L.), which is preferred as a model plant to study Cd translocation (Uraguchi et al., 2009). Cd acts as a negative key inducer of growth-related parameters in rice, causing growth impairment, chlorosis, tissue damage and cell death. Cd is also able to disrupt many non-enzymatic and enzymatic pathways (Rizwan et al., 2016). Earlier reports have shown that Cd generated ROS can either enhance or diminish cellular antioxidant levels in species, in a variety dependent manner (Benavides et al., 2005). In most cases activity of SOD, CAT and GPOD increase to scavenge cellular ROS, depending on the tolerance potential of the cultivar. Cd induced ROS causes lipid peroxidation leading to membrane damage. In the mature plants, Cd toxicity can negatively affect the crop yield (Wahid and Ghani, 2008). Cd contaminated rice is a threat to human health (Xue et al., 2014), as it causes osteomalacia and kidney dysfunction.

Though it is known that phosphate fertilizer application causes Cd contamination of the arable land, till date no data is available from West Bengal regarding this. Preliminary studies by our group have detected the presence of Cd in varying concentration in different districts of West Bengal. Cd commonly mobilizes and accumulates in the rice grain, but no detailed report is available regarding grain Cd accumulating potential of local cultivars from West Bengal. Therefore, it is very much essential to screen the local cultivars/landraces for Cd accumulation in grain. The main objective of the present study is to investigate the varietal difference among the rice cultivars and to co-relate the tolerance potential of the cultivars with metal accumulation capacity in grains. Genotypes with no/low Cd accumulating grains as well as able to withstand Cd stress, will be beneficial as it is able to tolerate Cd stress without compromising plant productivity and posing no threat to human health.

Based on morphological, physiological and biochemical stress parameters (root shoot length, photosynthetic pigment content, lipid peroxidation, proline contents and activities of antioxidant enzymes) two pairs of rice genotypes, with contrasting stress responses were chosen for further studies. Pot experiment was conducted with these genotypes till maturity to study the agronomic traits (plant final height, leaf length, leaf breath, total tiller number, effective tiller numbers) and grain Cd uptake under moderate Cd stress.