Effects of drought stress on root growth characteristics of peanut during mid-to-late growth stages

Peanut (Arachis hypogaea L.) is an important economic and oil crop with high drought tolerance. Long-term rainlessness or seasonal drought has not only been a limiting factor of peanut production but also the main driving factor of aflatoxin infection before harvest. Root is the main organ for plant water uptake. Changes in environment could change root morphological, physiological and biochemical characteristics. Plant root configuration is significantly different under different soil moisture conditions, which affects root ability to absorb nutrients and water. It is therefore important to study the relationship between root morphological development and drought tolerance for better understanding peanut water absorption, transport, utilization and loss and for breeding drought-tolerant peanut varieties. The aim of the present experiment was to (1) clarify root morphology during the mid and late growth stages of two peanut varieties with different drought tolerance; and (2) determine peanut root response to drought stress. Thus drought-resistant variety "Huayu 22" and drought-sensitive variety "Huayu 23" were planted in anti-canopy tanks in soil columns with different soil water conditions. The soil water conditions included a well-watered (80% 85% field capacity) and medium drought (45% 50% field capacity). Roots were sampled at flower-pegging, pod-setting and pod-filling stages. Root length, surface area and volume were determined using a scanner and analyzed using WinRhizo Pro Vision 5.0a software. Pod yield was recorded at harvest and drought coefficient (DC) calculated as the ratio of yield under water stress treatment to that under well-watered condition. The results showed that "Huayu 22" had higher yield and drought coefficient than "Huayu 23", which had poorer adaptability to drought stress. Root biomass, total root length and total root surface area of "Huayu 22" were higher, as "Huayu 22" had a more developed root system than "Huayu 23". Total root length, total root surface area and total root volume of the two peanut varieties at flower-pegging stage were smaller under drought stress treatment than under well-watered condition, while root traits were not significantly different at pod-setting and pod-filling stages. Drought stress increased root length density distribution ratio, root surface area and volume of two peanut varieties in soil layer below 40 cm. The increase in root traits of "Huayu 23" was less than that of "Huayu 22". Root surface area and root volume in 20 40 cm soil layer and in layers below 40 cm were significantly positively correlated with total root length, total root surface area and root volume under drought stress. Also root surface area and root volume in 0 20 cm soil layer were significantly correlated with total root length, total root surface area and root volume under well-watered condition. In conclusion, the main root morphology of drought-resistant peanut variety was characterized as lager root system and high root distributions in deeper soil layers. Under water-deficit condition, peanut efficiently utilized water by increased root length, root surface area, root volume and other morphological characteristics in deeper soil layers.