2. Materials and methods2.1. Plant materials and treatmentsShoots of bamboo (Phyllostachys violascens) were harvested at Lin’an (Zhejiang Province, China) in April 2008 and immediately transported to our laboratory, where they were selected for uniformity of shape and size. The shoots were washed in tap water and hand-peeled. About 5 cm was removed from the cut-end of the shoots with a sharp knife and the remainder was washed again with 0.04% sodium hypochlorite solution. A total of 720 bamboo shoots were divided into two equal groups, comprising three replicates of 120 in each group. In a preliminary study, the shoots were stored at 10 °C for 12 days after treatment with sodium nitroprusside (SNP, NO donor) at concentrations of between 0.1 and 1.0 mM. It was found that treatment with SNP at 0.5 mM resulted in better flesh quality and this concentration therefore was used. Shoots of one group were dipped in 0.5 mM SNP aqueous solutions for 1 h at 25 °C, and the other group was treated with water as the control. All shoots were then surface-dried with cold air. Each stack of 40 bamboo shoots were packed in 0.01 mm thick polyethylene bags (long 90 cm, wide 45 cm) with several holes for ventilation and stored at 10 ± 0.5 °C, RH 90%. Shoot firmness, browning index and ethylene production were assessed every 2 days. Tissue samples (about 100 g from each replicate) were frozen in liquid nitrogen and stored at −80 °C until used for the determination of total phenols, lignin and cellulose contents and activities of PPO, POD and PAL.2.2. Browning indexSurface browning was assessed by measuring the extent of the total browned area on each shoot of the 10 in each replicate according to a 5 grade scale: 0, none; 1, browning area <10%; 2, browning area 10–25%; 3, browning area 25–50%; 4, browning area >50%. The browning index was calculated as Σ(browning level × number of shoots with that browning level)/(total number of shoots).2.3. Determination of ethylene productionA sample of 3 shoots from each replicate (3 × 3 per treatment) was sealed in a 1 L jar for 1 h at 20 °C and then a 1 mL gas sample was collected from the jar with a syringe. The ethylene concentrations were determined using a SP-6890 gas chromatograph (Lunan Chemical Industrial Instruments Co., Shandong, China) with a flame ionization detector and a GDX-502 column held at 90 °C. Rates of ethylene production were expressed as μL ethylene kg−1 h−1 FW.2.4. Texture measurementTexture was measured on 10 individual shoots per treatment, comprising 3, 3, and 4 shoots from each of the three replicates. Firmness (N) was tested at the middle of the shoot using a TA-XT2i texture analyzer (Stable Micro Systems, England) fitted with a 5 mm diameter probe. The penetration rate was 1 mm/s with a final penetration depth of 10 mm and data were expressed in N.2.5. Determination of total phenolsTotal phenols were extracted by homogenizing 3 g of frozen tissue powder with 12 mL of 80% ethanol and centrifuging the homogenate at 12,000 × g for 20 min. Total phenols were estimated on the supernatant by a colorimetric assay based on procedures described by Singleton and Rossi (1965). Absorbance at 725 nm was measured using gallic acid as a standard. The phenol contents were expressed as gallic acid equivalents in milligrams on a fresh weight (FW) basis.
2.6. Determination of lignin and cellulose contents
About 5 g of frozen samples were used for lignin analysis. Lignin was gravimetrically determined according the method of Luo et al. (2008a) and the results expressed as g lignin per 100 g fresh weight.
Cellulose was extracted and measured by the method of Oomena et al. (2004) with modifications. For the isolation of cell wall material (CWM), 10 g of frozen tissue powder was extracted in a 50 mM Tris:HCl, pH 7.2 solution containing 1% SDS for 3 h at room temperature with continuous shaking. The extract was centrifuged at 16,000 × g for 20 min. The residue was washed with water, ethanol, and acetone and air-dried. Fifty milligrams of CWM was incubated for 90 min at 120 °C in 5 mL 2 M trifluoroacetic acid. The remaining cellulose was pelleted and washed with water and ethanol. The pellet was solubilized in 67% (v/v) H2SO4 at 37 °C for 60 min and diluted appropriately for the determination of cellulose content colorimetrically at 620 nm using anthrone as coloring agent and glucose as the quantification standard.
2.7. Assays of enzyme activities
Phenylalamine ammonia lyase (PAL) was extracted from 5 g frozen tissue with 0.2 M sodium borate buffer at pH 8.8 containing 40 g/L polyvinylpyrrolidone, 2 mM EDTA and 5 mM of β-mercaptoethanol. Five grams of frozen tissue was ground with 25 mL of 0.2 M sodium phosphate buffer (pH 6.5) containing 1% polyvinylpyrrolidone (PVP) for polyphenol oxidase (PPO), or with 50 mM sodium borate buffer at pH 8.7 for peroxidase (POD). All extract procedures were conducted at 4 °C. The extracts were then homogenised and centrifuged at 12,000 × g for 30 min at 4 °C. The supernatants were used for the enzyme assays.
PAL was assayed by the method described by Zucker (1968). One unit of PAL activity was defined as the amount of enzyme that causes the increase in absorbance of 0.01 at 290 nm in 1 h under the specified conditions. PPO activity was assayed following the method of Murr and Morris (1974). One unit of PPO activity was defined as the amount of enzyme that causes the increase in absorbance of 0.01 at 410 nm in 1 min under the specified conditions. POD activity was assayed using guaiacol as a donor and H2O2 as a substrate according to the method of Kochba et al. (1977). One unit of POD activity was defined as an increase of 0.01 in absorbance per minute at 460 nm under the assay conditions.
Protein content was determined according to the method of Bradford (1976), with bovine serum as the standard.
2.8. Statistical analysis
Each experiment was repeated three times and the data were processed by analysis of variance (ANOVA). Treatments were compared at P = 0.05 according to least significant difference (LSD) test.
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