Rapeseed (Brassica napus L.) is the second-most produced oilseed crop in the world after soybean with about 87 million tons seed yield from 36.59 million hectares of land in 2023–2024 [1]. Besides its edible oil, it is also used as livestock feed, biodiesel and in fertilizer industries [2]. It is the leading oilseed crop in Bangladesh based on total cropped area with a production of 0.547 million MT in 2022-23 [3]. As a winter season crop, it is cultivated during the driest months (November to February) of the country, making it vulnerable to drought stress [4]. According to Ahmed [5], about 47% of the country is drought-prone, which is becoming severe due to variation in rainfall pattern, rise of temperature and groundwater depletion [6]. Furthermore, ongoing climatic change creates challenges for rapeseed production in many areas of the world [7]. Therefore, there is a need to deal with this problem by better understanding how this stress affects rapeseed growth to develop rapeseed varieties in drought-prone regions.
Drought is a period when accessible water in the rhizosphere of plants is less than that of required for a satisfactory crop growth and productivity in a rain-fed agricultural system [8]. The severity of drought is strongly influenced by plant age, species, genotype, growth conditions and stages, drought duration, intensity and frequency, and which plant part is exposed [9]. Drought has a greater impact on above-ground organs than it does on below-ground parts [10]. During the growing season, the reproductive stage is more sensitive to drought as compared to the vegetative stage in rapeseed and other crops [11]. However, drought can cause reduced crop establishment, zero yield or complete crop failure if it occurs at the early vegetative stage [12]. Many studies revealed the effects of drought on seed yield, plant growth, photosynthesis, dry matter partitioning, nutrient acquisition, oxidative damage and antioxidant defense mechanisms of rapeseed and other crops [11, 13, 14, 15, 16, 17]. Previous attempts to improve rapeseed for better drought tolerance generally focused on aboveground traits [18, 19]. Evaluation of root system traits in response to drought stress was first reported in several plants such as wheat [20], maize [21], and soybean [22]. In the case of rapeseed, the evaluation is mostly limited to taproot length and root biomass at the early seedling stage [23, 24]. Generally, geneticists and breeders avoid selecting for root characteristics due to low heritability, difficulty in phenotyping large populations and lack of appropriate screening techniques [25, 26]. Besides, there’s a concern that traits assessed on younger plants in the laboratory won't translate into mature root system characteristics in the field. However, breeding for certain root traits may help in surviving water deficiency in dryland cropping areas [27].
One dehydration avoidance strategy for an annual crop like rapeseed to cope with mild water deficiency is to maintain the plant’s water status by promoting deep root growth [28]. Roots are the first organs to react, assess and secure crop yield under water deficit conditions [29]. B. napus has a taproot system shaped like an inverted cone and consisting of a central, dominant main axis root (embryonic), and lateral roots (post-embryonic) that support the fine roots and root hairs (Fig. 1) [30, 31]. Lateral roots make up the major portion of the entire root system, whereas the taproot accounted for just 1% of root length, 2% of surface area, and less than 15% of root volume [32]. Both main and lateral roots grow into deeper soil layers to absorb water, resulting in a much higher root length/weight ratio under stressed conditions [33]. Root hairs are tubular outgrowths of an epidermal cell of a root that significantly boost the surface area of the root for nutrient acquisition, uptake, water absorption and anchoring [34]. When drought gets severe, the crop must concentrate on tolerance mechanisms such as osmotic adjustment and reactive oxygen species (ROS) scavenging to prevent oxidative damage [35]. To scavenge the ROS, plants generate several different types of antioxidants including peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), superoxide dismutase (SOD), glutathione, glutathione reductase (GR), ascorbic acid, tocopherols and carotenoids [36]. Therefore, these biochemical traits should also be used as a selection indicator for drought tolerance as they show high variability and heritability as well as association with yield [37].
Plant root tissues grown in the field are generally inaccessible. Furthermore, the heterogeneity in soil composition makes reliable assessment of root characteristics even more difficult [32]. In that case, hydroponic culture provides an ample opportunity to study the root morphology easily and intensively [38]. Polyethylene glycol (PEG) is the best drought stress simulator in hydroponics because of its larger molecular weight and non-penetrating nature [39]. Therefore, this study involves exposing 25-days-old hydroponically grown plants of 10 rapeseed genotypes to PEG induced drought stress for 15 days to investigate the plasticity of root system traits over the stress period. Relevant shoot and biochemical data were also taken to analyze trait association, variability and heritability.