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Tao Hu, Haiying Yi, Longxing Hu and Jinmin Fu

Plants possess abiotic stress responses that alter photosynthetic metabolism under salinity stress. The objective of this study was to identify the stomatal and metabolic changes associated with photosynthetic responses to NaCl stress in perennial ryegrass (Lolium perenne). Five-week-old seedlings of two perennial ryegrass genotypes, PI 516605 (salt-sensitive) and BARLP 4317 (salt-tolerant), were subjected to 0 and 250 mm NaCl for 8 days. The salt tolerance in perennial ryegrass was significantly associated with leaf relative water content (RWC) and photosynthetic capacity through the maintenance of greater metabolic activities under prolonged salt stress. BARLP 4317 maintained greater turf quality, RWC, and stomatal limitations but a lower level of lipid peroxidation [malondialdehyde (MDA)] and intercellular CO2 concentration than PI 516605 at 8 days after treatment (DAT). Ribulose-1, 5-bisphosphate carboxylase:oxygenase (Rubisco) activity and activation state, transcriptional level of rbcL gene, and expression level of Rubisco large subunit (LSU) declined in stressed perennial ryegrass but were higher in salt-tolerant genotype at 8 DAT. Furthermore, photosynthetic rate increased linearly with increasing Rubisco activity, Rubisco activation state, and RWC in both genotypes. The same linear relationship was found between RWC and Rubisco activity. However, MDA content decreased linearly with increasing RWC in both genotypes. Salinity-induced inhibition of photosynthesis in perennial ryegrass was mainly the result of stomatal limitation during early salt stress and metabolic limitation associated with the inhibition of RWC, activity of Rubisco, expression level of rbcL gene, and LSU under a prolonged period of severe salinity.

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Longxing Hu, Tao Hu, Xunzhong Zhang, Huancheng Pang and Jinmin Fu

Salinity stress may involve the accumulation of glycine betaine (GB). The objective of this study was to examine whether exogenous GB would ameliorate the detrimental effect of salinity stress on perennial ryegrass (Lolium perenne). The grass was subjected to two salinity levels (0 and 250 mm NaCl) and three GB levels (0, 20, and 50 mm). Salinity resulted in a remarkable decrease in vertical shoot growth rate (VSGR), shoot and root fresh weight, relative water content (RWC), relative transpiration rate (Tr), and chlorophyll (Chl) content, superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) activities. Plants subjected to salt exhibited an increase in leaf electrolyte leakage (EL), lipid peroxidation (MDA), and proline content. Application of GB reduced EL, MDA, and proline content in salt-stressed plants. Perennial ryegrass subjected to salt stress plus GB had a greater level of VSGR, RWC, relative Tr, Chl content, and activities of SOD, CAT, and APX when compared with salt-stressed without GB. Salt stress increased Na+ and decreased K+ content, which resulted in a higher Na+/K+ ratio in perennial ryegrass. Application of 20 mm GB suppressed Na+ accumulation, whereas the K+ content was significantly increased in shoot, which led to a higher K+/Na+ ratio under saline conditions. These results suggested that GB-enhanced salt tolerance in perennial ryegrass was mainly related to the elevated SOD, CAT, and APX activity and alleviation of cell membrane damage by reducing oxidation of membrane lipid and improving the ion homeostasis under salt stress.

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Huiying Li, Hongji Luo, Deying Li, Tao Hu and Jinmin Fu

Lead pollution is an important issue in the world. Perennial ryegrass (Lolium perenne), as one of the widely used turfgrass and forage species, has a potential for bioremediation. The objective of this study was to investigate how antioxidant enzymes and their gene transcripts respond to Pb stress in perennial ryegrass. Ryegrass seedlings were subjected to 0, 0.5, and 3.2 mm of Pb(NO3)2 for 7 days in a hydroponic system maintained in a greenhouse. Both root and shoot growths were inhibited by Pb compared with the control. However, contents of chlorophyll (Chl) a and total Chl were unaffected by Pb treatment. Results from this study showed a substantial increase of malondialdehyde (MDA) content in leaf tissues when perennial ryegrass was exposed to Pb at 3.2 mm. The MDA content from plants in the 0.5 mm Pb treatment was lower than the control, indicating that an effective defense mechanism existed. Circumstantial evidence came also from the content of soluble protein in 0.5 mm Pb treatment, which was not different from the control. Furthermore, the activity of catalase (CAT) increased at 0.5 mm Pb compared with the control, indicating that CAT might play an important role in scavenging reactive oxygen species (ROS). The expression profiles of eight genes encoding antioxidative enzymes were upregulated within 24 hours of Pb treatment. In conclusion, antioxidant enzymes responded to Pb at an early stage of exposure and their gene expression profiles provided more details in time courses of the activation of those systems.

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Zhuangjun Zhao, Margaret Mukami Gitau, Tao Hu, Yan Xie, Longxing Hu and Jinmin Fu

Plants growing in salt-affected soils may have retarded growth and inhibited or altered metabolic processes. This study aims at investigating the impact of subsurface soil salinity on root growth and metabolic processes in perennial ryegrass (Lolium perenne). The seeds of perennial ryegrass (cv. Quick Start II) were planted in polyvinyl chloride (PVC) tubes (10 cm diameter × 42 cm long) for 2 months. The experiment consisted of three treatments: 1) control, 40 cm filled with sand–peat mixture (7 sand : 3 peat wt/wt); 2) T20, a 20-cm-deep layer of saline soil covered with a 20-cm-deep layer of sand–peat mixture; and 3) T30, a 30-cm-deep layer of saline soil covered with a 10-cm-deep layer of sand–peat mixture. Our study showed that soil salinity at the subsurface inhibited the growth of perennial ryegrass roots. Compared with the control, the root activity in saline soil layer decreased, whereas it remained high in the mixture-soil zone. The content of amino acids in the roots obtained from the surface soil (0–10 cm) in T30 was greater than that in both the T20 and the control regimes. The content of soluble sugars in the roots went up with the decrease of the depth of sand–peat mixture. The increased root activity and free amino acids content in the roots sampled from the upper soil layers coupled with the increased soluble sugars in the roots subjected to soil salinity stress in the bottom soil layer represents some adaptive responses and regulative mechanisms in perennial ryegrass.