Soil biological parameters
Cultivated cotton species (CCS) recorded significantly (P < 0.05) higher soil basal respiration rate (SBR) than wild cotton species (WCS) (Fig. 1a). Among the CCS, higher SBR was observed in G. herbaceum (53.81 mg CO2 kg−1 soil day−1) and G. hirsutum (53.63 mg) followed by G. barbadense (53.08 mg) and G. arboreum (51.21 mg). The percent decrease in SBR compared to G. hirsutum was 1% and 4.4% respectively, for G. barbadense and G. arboreum. Among the WCS, higher SBR was recorded in G. raimondii (48.58 mg), followed by G. barbosanum (48.31 mg), and the lowest SBR was observed in G. anomalum (44.46 mg). The percent decrease in SBR compared to G. hirsutum was 9.4%, 9.9%, and 17%, respectively, for G. raimondii, G. barbosanum, and G. anomalum. There was not much variation in soil microbial biomass C (MBC) content between CCS and WCS compared to SBR (Fig. 1b). G. arboreum (959 mg kg−1) followed by G. herbaceum (916 mg kg−1) recorded higher MBC among the CCS. While, among the WCS, higher MBC was recorded in G. australe (959 mg kg−1) followed by G. aridum (926 mg kg−1), and the lowest MBC was recorded in G. somalense (845 mg kg−1). A 10% increase in MBC was recorded in G. arboreum compared to G. hirsutum in CCS. While, G. australe and G. aridum recorded 10% and 6.3% increase in MBC compared with G. hirsutum.
The microbial metabolic quotient (qCO2) (respiration-to-biomass ratio) an indicator of soil organic matter conversion and heterotrophic respiration was higher in CCS compared to WCS (Fig. 1c). G. hirsutum (0.062) and G. barbadense (0.061) recorded higher qCO2 in cultivated species. In WCS, G. thurberi (0.058) followed by G. capitis-virides (0.056) recorded higher qCO2. The lowest qCO2 was observed in G. anomalum and G. australe (0.48). The percent decrease in qCO2 compared to G. hirsutum was 21.8%, 21.7 and 13%, respectively, for G. austral, G. anomalum, and G. arboreum. The easily-extractable glomalin related soil proteins (EE-GRSP) (indicator of mycorrhizal colonization in plants) was significantly (P < 0.05) higher in WCS compared to CCS (Fig. 1d). Among WCS, higher EE-GRSP was recorded in G. thurberi (4.83 mg g−1) and G. aridum (4.80 mg g−1), and the lowest in G. barbosanum (2.30 mg g−1). Among the CCS, G. arboreum recorded the highest EE-GRSP (4.44 mg g−1), while G. barbadense recorded lowest EE-GRSP (1.33 mg g−1). The percent increase in EE-GRSP compared to G. hirsutum was 262%, 260%, and 233%, respectively, for G. thurberi, G. aridum and G. arboreum.
WCS recorded significantly (P < 0.05) higher soil polysaccharide (SPS) content compared with CCS (Fig. 1e). Among the WCS, higher SPS was observed in G. aridum (19.1 mg g−1 soil) followed by G. anomalum (18.4 mg), and the lowest SPS was recorded in G. barbosanum (13.6 mg). The percent increase in SPS compared to G. hirsutum was 32% and 27%, respectively, for G. aridum and G. anomalum. Among the CCS, higher SPS was recorded in G. herbaceum (15.7 mg), followed by G. hirsutum (14.4 mg), and the lowest SPS was observed in G. barbadense (10.4 mg). WCS recorded significantly (P < 0.05) higher soil protein (SoP) content compared with CCS (Fig. 1f). Among the WCS, higher SoP was observed in G. aridum (2.49 mg g−1 soil) followed by G. stocksii (1.88 mg), and the lowest SoP was recorded in G. capitis-virides (1.54 mg). The percent increase in SoP compared with G. hirsutum was 57% and 18%, respectively, for G. aridum and G. stocksii. Among the CCS, higher SoP was recorded in G. herbaceum (1.99 mg), followed by G. barbadense (1.66 mg), and the lowest SoP was observed in G. hirsutum (1.58 mg).
Soil enzyme activities
WCS recorded significantly (P < 0.05) higher acid phosphatase (AcP) activity compared to CCS (Fig. 2a). Among the WCS, higher AcP was observed in G. aridum (328.5 µg PNP g soil−1 h−1) followed by G. anomalum (285.3 µg), and the lowest AcP was recorded in G. barbosanum (170 µg). The percent increase in AcP compared to G. hirsutum was 133% and 103%, respectively, for G. aridum and G. anomalum. Among the CCS, higher AcP was recorded in G. herbaceum (164 µg), followed by G. hirsutum (141 µg), and the lowest AcP was observed in G. barbadense (112 µg). WCS recorded significantly (P < 0.05) higher alkaline phosphatase (AlkP) activity compared to CCS (Fig. 2b). Among the WCS, higher AlkP was observed in G. anomalum (800 µg PNP g soil−1 h−1) followed by G. stocksii (682 µg), and the lowest AlkP was recorded in G. barbosanum (346 µg). The percent increase in AlkP compared to G. hirsutum was 24% and 6% respectively for G. anomalum and G. stocksii. Among the CCS, higher AlkP was recorded in G. hirsutum (644 µg), followed by G. arboreum (612 µg), and the lowest AlkP was observed in G. barbadense (336 µg).
The β-glucosidase (BG) activity as a measure of organic matter degradation and C recycling was significantly (P < 0.05) higher in WCS compared to CCS (Fig. 2c). Among the WCS, higher BG was observed in G. anomalum (419 µg PNP g soil−1 h−1) followed by G. australe (406 µg), and the lowest BG was recorded in G. barbosanum (182 µg). The percent increase in BG compared with G. hirsutum was 38% and 34%, respectively, for G. anomalum and G. australe. Among the CCS, higher BG was recorded in G. arboreum (398 µg), followed by G. hirsutum (304 µg), and the lowest BG was observed in G. barbadense (202 µg). The dehydrogenase activity (DHA) as an indicator of microbial activity was significantly higher (P < 0.05) in the CCS compared to WCS (Fig. 2d). G. hirsutum (14.4 µg TPF g−1 h−1) followed by G. arboreum (13.1 µg) recorded higher DHA in CCS, and the lowest DHA was observed in G. barbadense (11.9 µg). In WCS, G. anomalum (14.1 µg) followed G. raimondii (13.6 µg) recorded higher DHA, and the lowest DHA was observed in G. barbosanum (8.73 µg). The percent decrease in DHA compared with G. hirsutum was 39.5%, 38.7, and 38.3%, respectively, for G. barbosanum, G. somalense, and G. capitis-virides.
Plant carbohydrate and protein
Significant differences (P < 0.05) in the carbohydrate concentration were recorded in root and shoot tissues of WCS and CCS, however, shoot samples exhibited higher carbohydrate concentrations (Fig. 3a). Between wild and cultivated species, WCS recorded significantly (P < 0.05) higher carbohydrate concentration in shoot (SC) and roots (RC). Among the WCS, higher SC was observed in G. anomalum (97.2 mg g−1) followed by G. australe (86.3 mg), and the lowest SC was recorded in G. somalense (33.7 mg). In cultivated species, higher SC was recorded in G. hirsutum (45.9 mg) followed by G. arboreum (38.7 mg), and the lowest in G. barbadense (32 mg). The percent increase in SC compared with G. hirsutum was 112% and 88%, respectively, for G. anomalum and G. australe. Among the WCS, higher root carbohydrate (RC) was recorded in G. australe (47.4 mg), followed by G. aridum (46.1 mg), and the lowest RC was observed in G. thurberi (20.2 mg). In cultivated species, G. herbaceum (35.4 mg) recorded higher RC, and the lowest was recorded in G. barbadense (13.3 mg). The percent increase in RC compared to G. hirsutum was 152%, 146%, and 89%, respectively, for G. australe, G. aridum, and G. herbaceum.
Significant differences (P < 0.05) in the protein concentration were recorded in root and shoot tissues of WCS and CCS, however, shoot samples exhibited higher protein concentration in the wild and cultivated species (Fig. 3b). Cultivated species recorded significantly (P < 0.05) higher protein content concentration in shoot (SP) and roots (RP) compared to WCS. Among the CCS, higher SP was observed in G. herbaceum (74.2 mg g−1) followed by G. arboreum (66.8 mg), and the lowest SP was recorded in G. hirsutum (58 mg). In wild species, higher SP was recorded in G. australe (73.3 mg) followed by G. anomalum (66 mg), and the lowest in G. capitis-virides (35 mg). The percent increase in SP compared to G. hirsutum was 28%, 26%, and 15%, respectively, for G. herbaceum, G. australe, and G. arboreum. Among the CCS, higher RP was observed in G. barbadense (25.9 mg g−1) followed by G. herbaceum (22.9 mg), and the lowest RP was recorded in G. hirsutum (20.8 mg). In wild species, higher RP was recorded in G. anomalum followed by G. stocksii (23.8 mg) and G. raimondii (23.8 mg). The lowest RP was recorded in G. capitis-virides (13.9 mg). The percent increase in RP compared with G. hirsutum was 25%, 10%, 17.5, and 14.7%, respectively, for G. barbadense, G. herbaceum, G. anomalum, and G. stocksii/ G. raimondii.
Plant antioxidant/defense enzymes
WCS recorded significantly (P < 0.05) higher peroxidase activity (POD) in shoots and root tissues compared to CCS (Fig. 4a). Among the WCS, higher shoot POD was observed in G. aridum (7.54 units (U) g−1 protein min−1) followed by G. somalense (4.37 U), and the lowest in G. australe (3.26 U). Among the CCS, higher shoot POD was observed in G. arboreum (3.80 U) followed by G. hirsutum (3.0 U), and the lowest in G. barbadense (2.34 U). In wild species, G. aridum (10.7 U) followed by G. somalense (8.2 U) recorded higher root POD, while, lowest activity was recorded in G. barbosanum (4.64 U). Among the CCS, higher root POD was observed in G. arboreum (6.93 U) followed by G. hirsutum (4.5 U), and the lowest activity in G. barbadense (3.53 U). The percent increase in POD in G. aridum and G. arboreum compared with G. hirsutum was 144% & 139% and 23% & 54%, respectively, for shoot and root tissues.
Significantly (P < 0.05) higher polyphenol oxidase activity (PPO) was recorded in WCS compared to CCS in the shoot and root tissues (Fig. 4b). G. thurberi (0.489 U g−1 protein min−1) followed by G. australe (0.419 U) recorded higher PPO in shoot and root samples among the WCS, while, the lowest PPO in shoot and root sample was recorded in G. capitis-virides (0.237 U) and G. barbosanum (0.145 U), respectively. Among the CCS, higher shoot and root PPO were observed in G. arboreum (0.487 & 0.229 U) followed by G. hirsutum (0.319 & 0.210 U), respectively, and the lowest PPO was observed in G. barbadense (0.252 U) and G. herbaceum (0.185 U) respectively for the shoot and root samples. The percent increase in PPO in G. thurberi, G. australe, and G. arboreum compared with G. hirsutum was 54% & 31%, 55% & 38%, 53% & 9%, respectively, for shoot and root tissues.
WCS recorded significantly (P < 0.05) higher phenylalanine ammonia lyase activity (PAL) in the shoot as well as in root tissues compared to CCS (Fig. 4c). G. stocksii (28.9 moles t-cinnamic acid mg−1 protein h−1) followed by G. thurberi (28 moles) recorded higher PAL in shoot sample, while, in root samples, G. stocksii (23 moles) followed G. somalense (19 moles) recorded higher PAL. Among the WCS, G. barbosanum recorded lesser PAL in both shoot (12.4 moles) and roots (9.9 moles). In CCS, G. arboreum (19.6 & 19 moles) followed by G. hirsutum (18.9 & 17.0 moles) recorded higher PAL, respectively, for shoot and root samples, while, G. barbadense recorded lower PAL in shoot (18.4 moles) and root (13.6 moles) samples. The percent increase in the PAL in G. stocksii and G. arboreum compared to G. hirsutum was 53% & 34% and 4% & 11%, respectively, for shoot and root tissues.
Significantly (P < 0.05) higher catalase activity (CAT) was recorded in WCS compared to CCS in the shoot as well as root tissues (Fig. 4d). G. aridum (0.589 & 0.468 U g−1 protein min−1) followed by G. somalense (0.463 & 0.422 U) recorded higher CAT in shoot and root samples, respectively, while, G. stocksii recorded lowest CAT in shoot (0.257 U) and roots (0.212 U). Among the CCS, higher shoot and root CAT were observed in G. arboreum (0.287 & 0.262 U), and the lowest CAT was observed in G. barbadense (0.141 & 0.127 U) respectively for the shoot and root samples. The percent increase in CAT in G. aridum and G. arboreum compared with G. hirsutum was 138% & 112% and 16% & 18%, respectively, for shoot and root tissues.
Both shoot and roots showed significant differences (P < 0.05) in the phenolic content, however, the shoot samples exhibited much higher values in the WCS and CCS (Fig. 4e). Among the WCS, the shoot phenolic concentrations were significantly higher in G. barbosanum (4.57 mg caffeic acid equivalent (CAE) g−1 fresh wt.) followed by G. capitis-virides (4.50 mg) and G. anomalum (4.41 mg), and the lowest phenol content was recorded in G. davidsonii (2.30 mg). In CCS, G. herbaceum (3.91 mg) showed the highest shoot phenolic concentration followed G. arboreum (3.74 mg), and lesser phenols were recorded in G. hirsutum (2.97 mg). There were no much differences in mean root phenolic concentrations between WCS and CCS. However, the root phenolic concentrations varied significantly (P < 0.05) among the species. Among the WCS, higher phenols were recorded in G. anomalum (2.40 mg) followed by G. aridum (2.08 mg), and lowest in G. stocksii (1.20 mg). G. herbaceum (2.19 mg) recorded higher root phenols in CCS, and the lowest was recorded in G. arboreum (1.41 mg). The percent increase in shoot phenol content in G. barbosanum and G. herbaceum compared with G. hirsutum was 54% & 32%. Similarly, the percent increase in root phenol content in G. anomalum and G. herbaceum compared with G. hirsutum was 65% & 51%.