Differential seedling growth and tolerance indices reflect drought tolerance in cotton

Abstract

Background: Cotton production is adversely effected by drought stress. It is exposed to drought stress at various critical growth stages grown under a water scarcity environment. Roots are the sensors of plants; they detect osmotic stress under drought stress and play an important role in plant drought tolerance mechanisms. The seedling stage is very sensitive to drought stress, and it needed to explore the methods and plant characteristics that contribute to drought tolerance in cotton.

Results: Initially, seedlings of 18 genotypes from three Gossypium species: G. hirsutum, G. barbadense, and G. arboreum, were evaluated for various seedling traits under control (NS) and drought stress (DS). Afterward, six genotypes, including two of each species, one tolerant and one susceptible, were identified based on the cumulative drought sensitivity response index (CDSRI). Finally, growth rates (GR) were examined for shoot and root growth parameters under control and DS in experimental hydroponic conditions. A significant variation of drought stress responses was observed across tested genotypes and species. CDSRI allowed here to identify the drought-sensitive and drought-resistant cultivar of each investigated species. Association among root and shoots growth traits disclosed influential effects of enduring the growth under DS. The traits including root length, volume, and root number were the best indicators with significantly higher differential responses in the tolerant genotypes. These root growth traits, coupled with the accumulation of photosynthates and proline, were also the key indicators of the resistance to drought stress.

Conclusion: Tolerant genotypes have advanced growth rates and the capacity to cop with drought stress by encouraging characteristics, including root differential growth traits coupled with physiological traits such as chlorophyll and proline contents. Tolerant and elite genotypes of G. hirsutum were more tolerant of drought stress than obsolete genotypes of G. barbadense and G. arboreum. Identified genotypes have a strong genetic basis of drought tolerance, which can be used in cotton breeding programs.

Keywords: Cotton; Drought stress response indices; Drought tolerance; Gossypium spp; Root growth; Seedling growth rate; Shoot growth.

Fig. 1

Drought stress responses and variation among a panel of 18 cotton genotypes, including six from each three-cotton species G. hirsutum, G. barbadense, and G. arboreum. a Hierarchical Cluster and Dendrogram, genotypes are distributed in two major (G1 and G2) and four sub-groups (g1, g2, g3, and g4). b Graphs of cumulative drought stress response index (CDSRI) including cumulative drought stress response index of shoot traits (CDSRI-S) and root traits (CDSRI-R) for 18 genotypes of three cotton species. c groups. c & d Biplot (Principal component analysis) of drought stress response indices (DSRI) for morpho-physiological and early root growth traits at the seedling stage. Scatter-plot shows the distribution of identified drought tolerant (purple) and susceptible (orang) cotton genotypes. Here PH, plant height; ChC, chlorophyll contents; LN, leaf number; LA, leaf area; LT leaf temperature; LFW, leaf fresh weight; LDW, leaf dry weight; LDW, leaf turgid weight; RWC, relative water contents; RL, root length; ARA, analyzed region area; ARW analyzed region width; ARH, analyzed region height; SA surface area; LPA, root length per area; AD average diameter; RV, root volume; RH, root heigh; RN root number; RF root forks; RC root crosses; RW root weight and R/S, root to shoot ratio

Fig. 2

Cumulative growth rate (CGR) and cumulative drought stress response indices of growth rate (DSRI-GR) of 24 shoot and root growth traits of six cotton genotypes, including two genotypes of each three species, one drought-tolerant and one drought susceptible genotype grown in hydroponic conditions under non-stress (NS) PEG-induced drought stress (DS) conditions. a CGR comparison of six genotypes; b proportional comparison of DSRI-GR of six cotton genotypes; c Heatmap of CGR and GR for all traits. Mean values of all traits have given in (Supplementary Table 4). Here SL, shoot length; SD, stem diameter; LT, leaf temperature; ChC, chlorophyll contents (SPAD); ChA, chlorophyll A; ChB chlorophyll B; ChT, total chlorophyll; A/B, chlorophyll A/B ratio; BC, β-carotenoids, PC, proline contents; RL, root length; ARA, analyzed region area; ARW analyzed region width; ARH, analyzed region height; SA surface area; LPA, root length per area; AD average diameter; RV, root volume; RH, root heigh; RN, root number; RF, root forks; RC root crosses; RW root weight; R/S, root to shoot ratio; D3, three days after stress; D6, 6 days after stress

Fig. 3

Comparison of the cotton shoot and root growth of three cotton species under control (NS) and PEG-induced drought stress (DS) at early growth stages