Quantifying the agronomic performance of new grain sorghum hybrids for enhanced early-stage chilling tolerance

Highlights

• Constant chilling conditions in chambers better related to field grown sorghum seedlings.

• Early planting has potential for extending grain-filling duration in grain sorghum.

• Poor plant stand under early-stage chilling confounds yield estimates.

• A promising early-stage tannin free chilling tolerant sorghum hybrid identified.

Abstract

Enhancing chilling tolerance to attain uniform emergence with better seedling vigor under temperatures <15 °C will enable earlier planting of sorghum. Early-stage chilling tolerant sorghum has the potential to better utilize residual soil moisture and enhance yield through extended vegetative and grain-filling periods. This study comprised of 12 genotypes, including four new hybrids developed from different parental combinations (two B-lines and three R-lines), parents and three checks (two inbred lines - RTx430 and SQR; and one commercial hybrid). The major objective of the study was to phenotype the above mentioned genotypes over two years under field conditions and in controlled environment chambers, to quantify the level of early-stage chilling tolerance and the impact of stress on yield related parameters and grain quality. An improvised phenotyping approach termed as “field-like” was devised for controlled environment chambers to replicate temperature conditions prevailing under field conditions. Systematic testing resulted in enhanced early-stage seedling vigor significantly correlating with constant chilling stress conditions compared to the improvised “field-like” treatment, strengthening the reliability of routinely followed constant chilling stress imposition under chamber conditions. Averaged across genotypes, early planting resulted in longer duration to flower and accumulated a higher number of growing degree units (GDU) during grain filling. Our most promising tannin-free hybrid (ARCH11192A/ARCH12012R) with enhanced chilling tolerance took 23 days (100 GDU) longer to reach flowering compared to regular planting, with the grain-filling period extended by three days (14 GDU). Reduction in grain protein and an increase in starch content with early planting indicated extended vegetative stage possibly providing additional carbon for the developing grains. In summary, developing tannin-free early-stage chilling tolerant hybrids can create opportunities to extend vegetative and grain-filling duration, paving the way to enhance grain sorghum productivity.

Introduction

Grain sorghum (Sorghum bicolor (L.) Moench) is a warm-season crop and is grown in arid and semi-arid regions of the world. Sorghum originated from semi-arid tropics of Africa (Smith and Frederiksen, 2000) and is known for its tolerance to drought and heat stress, which makes it an ideal crop to be grown in hot and arid regions of the world (Doggett, 1988; Blum, 2004; Pennisi, 2009). Sorghum is one of the major agricultural commodities that has a significant impact on the economy of the Great Plains of the USA, India, and African countries (Leff et al., 2004; Nagaraj et al., 2013; Hariprasanna and Rakshit, 2016). Due to sorghum’s high water-use efficiency, ability to maintain productivity under low input levels, and suitability for cropping rotation in the US Great Plains region (Saballos, 2008), 76 % of US grain sorghum area is in Kansas and Texas (USDA-NASS, 2019). In spite of sorghum’s tolerance, extreme environmental conditions occurring during pre- and post-flowering phases were shown to cause significant yield losses (Assefa et al., 2010; Prasad et al., 2015; Tack et al., 2017). Introducing early-stage chilling tolerance and planting earlier under US Midwestern conditions can help reduce the cumulative impact of heat and drought stresses during the sorghum growing period (Chiluwal et al., 2018). Shifting to earlier planting of sorghum can have other benefits such as efficient utilization of spring residual soil moisture and early canopy cover for improved water conservation by reducing evaporation (Burow et al., 2011; Moghimi et al., 2019). However, due to sorghum’s tropical adaptation, the crop is highly sensitive to chilling stress (Peacock, 1982; Rooney, 2004).

Sorghum in Kansas is currently planted during late May or early June since soil temperatures >18 °C are required for optimum seed germination and emergence (Stoffer and Riper, 1963; Chiluwal et al., 2018). Sorghum, when planted early (soil temperatures <15 °C) is associated with challenges that include poor seedling emergence and seedling vigor, which negatively affects yield (Yu and Tuinstra, 2001; Cisse and Ejeta, 2003; Burow et al., 2011; Kapanigowda et al., 2013; Maulana and Tesso, 2013; Chiluwal et al., 2018). Developing sorghum hybrids that can maintain good plant stand with improved growth under early-stage chilling temperatures is critical to shift the growing period earlier and to provide opportunities for expanding sorghum cultivation into more temperate and high elevated regions of the US and the world. Using natural field conditions provides realistic conditions to test genetically diverse germplasm for early-stage chilling, but an integrated approach using controlled environment facilities and field testing provides a more robust phenotyping approach (Chiluwal et al., 2018). However, a persistent challenge faced by the research community is to devise an acceptable methodology that can better connect the findings from controlled environments to field conditions (Poorter et al., 2016), which also applies to early-stage chilling response in sorghum. Recently, Chiluwal et al. (2018) proposed an approach to improve the relevance of controlled environment findings to field conditions by varying chamber temperature settings to replicate actual field conditions, which is further modified and systematically tested in this study.

It is well known that by producing F₁ hybrids, heterosis can be exploited to enhance the effectiveness of the traits of interest. Yu and Tuinstra (2001) have reported that sorghum hybrids are generally more vigorous than that of the inbred parental lines, justifying the need for developing sorghum hybrids with enhanced early-stage chilling tolerance. In the US, all sorghum grain production is with hybrids, which further adds to the relevance of developing early-stage chilling tolerant hybrids. An additional challenge posed with improving chilling tolerance in sorghum is related to the tight linkage between tannins and chilling tolerance (Rooney et al., 2004). Tannins are known to reduce the protein digestibility of the grain, thereby lowering its value for human or animal feed (Wu et al., 2012; Proietti et al., 2015). Only a few studies, have extended their objectives to connect early-stage chilling stress with yield (León-Velasco et al., 2009; Maulana and Tesso, 2013; Chiluwal et al., 2018), whereas none of them have determined the impact on grain quality. In general, abiotic stress exposure during different growth stages in crops is shown to impact yield and also grain protein and starch compositional dynamics (Halford et al., 2014; Lawas et al., 2019). Hence, ascertaining the same in early-stage chilling tolerant genotypes is essential to ensure that the grain quality from the newly developed sorghum hybrids do not negatively impact the feedstock or biofuel industry.

Extensive screening of recently developed germplasm, both under controlled environments and field conditions, allowed us to identify tannin free inbred lines that are early-stage chilling tolerant (Chiluwal et al., 2018). This progress presented a unique opportunity for developing chilling tolerant tannin-free grain sorghum hybrids. Although significant efforts have been invested in the past to capture early-stage chilling tolerance in different inbred lines (Franks et al., 2006; Kapanigowda et al., 2013; Maulana and Tesso, 2013), diversity panels or mapping populations (Knoll et al., 2008; Knoll and Ejeta, 2008; Chopra et al., 2017; Ortiz et al., 2017; Moghimi et al., 2019), there has been no attempt to develop chilling tolerant hybrids by utilizing the diversity captured. Keeping these knowledge gaps in mind the objectives of this study were to (i) Identify sorghum hybrids with enhanced early-stage chilling stress tolerance with stable agronomic performance; (ii) Establish a robust phenotyping approach to better relate controlled environment chamber findings to field conditions and (iii) Quantify the degree of early-stage chilling stress tolerance of newly developed grain sorghum hybrids and the impact on flowering time, yield and grain quality characteristics.