Genetic variation in kernel traits under lead and tin stresses in spring wheat diverse collection

Highlights

• Investigated the effect of Pb and Sn on wheat kernels to be used as criteria in indirect selection for Pb and Sn tolerance.

• Kernel length was found to be the best selection criteria for the tolerance of both metals.

• Tolerance to Pb and Sn are controlled by different systems in the plant based on the physiological parameters.

• One Egyptian genotype, Gimmiza_11 was selected as a high tolerant genotype to tin.

• No genotype was found to be highly tolerant to Pb and searching for other tolerant genotypes is urgently needed.

Abstract

Pollution with heavy metals is a common problem worldwide that affects the growth of important crops and human health due to the consumption of contaminated crops. In the recent study, a set of 103 diverse spring wheat genotypes were evaluated for their tolerance to two important metals (lead and tin). The effect of these metals on kernel traits was measured to select tolerant and high-yielding genotypes. A great reduction in kernel length, kernel diameter, kernel width, and thousand kernel weight were found due to the effect of both metals. However, a highly significant correlation between the studied kernel traits was found suggesting their efficiency to indirectly select high yielding genotypes under heavy metals stress. In addition, the effect of the studied heavy metals on the physiological process was investigated and different responses to each metal were found suggesting that there are two different tolerant systems that the plant used to resist lead and tin. The selection of superior genotypes was done based on eight indices calculated for each studied kernel trait. Superior genotypes are those that were found to be tolerant based on all kernel traits. Based on this, one genotype, Gimmiza_11, was found to be highly tolerant to Sn and could be used as a parent in breeding for Sn tolerance. However, one genotype, BeniSweif_7, was found to be moderately tolerant to Pb based on only three kernel traits. Hence, searching for other sources of Pb tolerance in spring wheat is urgently needed. No genotype was found to be tolerant to both metals.

Introduction

Pollution with heavy metals is a common problem worldwide due to the increased population and industrialization, excessive use of pesticides and fertilizer, and rapid progress in chemical and petrochemical industries (Chen et al., 2013; Murtaza et al., 2008). In Egypt, a high concentration of harmful heavy metals was reported in the soil of Delta, Asyut, and El-Minya (El-Bady and Metwally, 2019; Mekky et al., 2019; Salman et al., 2019; Shokr et al., 2016). Lead (Pb) is one of these harmful heavy metals that exists with a concentration exceeding the allowed level for human health in soils and irrigation water including wastewater and Nile river water (Elgharably and Mohamed, 2016; Hassan and Elhassan, 2016; Naggar et al., 2014). Furthermore, tin (Sn) contamination, an important component of plastic products and canned food, has been reported widely in agricultural soils and water irrigation (Noubissié et al., 2017, 2016). However, there is not enough quantification and identification of tin in soils to provide a full risk assessment. Generally, the uptake of heavy metals usually occurs by different plant species which alter the plant growth, could cause cell death, and reduce crop yield (Ashfaque et al., 2016; FL et al., 2015). The effect of heavy metals on plant growth could be illustrated by the physiological changes that occur in the plant cell such as increasing peroxidation and membrane lipids and decreasing the activity of antioxidant enzymes (Rizvi et al., 2020). Consumption of such contaminated plant tissues with heavy metals affects human health and causes severe diseases like kidney dysfunction, diabetes, osteoporosis, high blood pressure, cancer, and infertility (Debnath et al., 2019; Morias et al., 2012). For non-smoking people, the main source of heavy metals is contaminated food; mainly cereals, potatoes, and vegetables (WHO, 2011; Wu et al., 2016).

Wheat (Triticum aestivum L.) is the third most widely grown cereal globally. It is the most important cereal crop in Egypt and is the main energy source in the Egyptian diet. Bread is the main staple food in the typical Egyptian diet especially for families living under the poverty level. The consumption of this cereal per Egyptian capita is among the highest in the world. Due to this high consumption of wheat per capita, Egyptian human health is threatened with a high concentration of heavy metals (Bermudez et al., 2011). Pb was found to be among the highest concentrated heavy metal in the kernels of wheat plants irrigated with sewage water (Hassan et al., 2013; Khan et al., 2018; Yu et al., 2016). So, it is very important to reduce these heavy metals content in wheat parts, especially grains to reduce its effect on human health. Many approaches are possible to reduce the concentration of heavy metals in wheat plants such as; using polyelectrolyte-coated fly ash and sophorolipids produced by Starmerella bombicola (Olabemiwo et al., 2017; Qi et al., 2018), reducing plant uptake by increasing the PH of the soil (Naidu et al., 1994) and removing accumulated heavy metals from grains during food processing. However, these approaches are slow, costly and affect the concentration of other useful metals in wheat grains like Calcium (Ca) and Magnesium (Mg). On the other hand, producing wheat varieties that have reduced concentrations of heavy metals combined with high yield and high quality by using breeding programs is a more effective approach (Zaid et al., 2018]; Mondal et al., 2021). Therefore, selection criteria should be effective in improving both targets simultaneously.

Wheat grain yield is a complex trait that is controlled by multiple genes/QTLs interacting with each other and with the environment. Therefore, in recent decades there is an indirect trend to increase grain yield by increasing its components. Kernel length (KL), kernel width (KW), and thousand kernel weight (TKW) have been reported as important components associated with kernel weight in durum and bread wheat (Arriagada et al., 2020; Xin et al., 2020). Previous studies reported that TKW is mainly affected by KL and KW hence both are very important components in increasing grain yield (Campbell et al., 1999). In addition, TKW, KL, and KW have been reported as easy and cheap predictors of milling quality as larger kernels usually have higher flour yield than those smaller kernels (Berman et al., 1996; Marshall et al., 1986; Wiersma et al., 2001). A positive correlation was reported between KL, KW, and kernel area with flour-water dough quality and endosperm separation index and friability (Baker et al., 1999; Breseghello and Sorrells, 2006; Morgan et al., 2000). As a result, selection for higher values of kernel traits under Pb and Sn conditions will help in breeding for high yield, high end-use quality, and tolerant genotypes.

Different selection indices have been used in selecting tolerance genotypes to the different stresses such as mean productivity (MP) (Rosielle and Hamblin, 1981), geometric mean productivity (GMP) (Fernandez, 1992), tolerance (TOL) (Rosielle and Hamblin, 1981), stress susceptibility index (SSI) (Fischer and Maurer, 1978), harmonic mean (HM) (Bidinger et al., 1987), stress tolerance index (STI) (Fernandez, 1992), yield index (YI) (Gavuzzi et al., 1997), yield stability index (YSI) (Bouslama and Schapaugh, 1984), and relative stress index (RSI) (Fischere and Wood, 1979). These indices differ in their ability to distinguish tolerant and susceptible genotypes (Belay et al., 2021; Moursi et al., 2020; Negarestani et al., 2019; Pour-Aboughadareh et al., 2020; Sallam et al., 2018, 2015; Thabet et al., 2021). Previous studies reported that select resistant genotypes based on a single index might be problematic. Thus, it is better to select resistant genotypes based on all indices together. The iPASTIC orders the genotypes based on all indices and provides an average rank for each one of them. Using this rank is very helpful in selecting the resistant/tolerant genotypes (Pour-Aboughadareh et al., 2019).

The objectives of this study are (1) study the genetic variation in kernel traits in spring wheat core collection under controlled and heavy metals conditions (Pb and Sn), (2) identifying the effect of Pb and Sn on kernel traits and some physiological parameters in the flag leaf, and (3) Select resistant genotypes to each metal based on kernel traits.