Papaya plant leaves collection
The papaya plants were obtained from a village near Partur, Dist. Jalna, India (19.8410°N 75.8864°E). Atmosphere of the place for the month of Sept. 2020 was found rainy and moist. The average daily temperature ranges 20° to 27°C. A one-hectare infected plant farm was selected for the study. The leaves of the plants were collected from the study area.
Papaya plant external symptoms
The papaya plants were found with severe viral or fungal infections. A typical curling and thickening were observed (Fig 1). These included chlorotic spots (Fig 1), ringspots and line pattern (Fig 1), leaf distortion, mosaic, dieback and curling.
In plants showing distortion symptoms, greasy streaks and grey spotting were usually found on stems. Whether in the field, infected plants initially developed vein banding symptoms, accompanied in some cultivars by mosaic, distortion and occasional stunting. Ringspot symptoms sometimes appeared on the fruits, but virus could not be recovered from such fruits. All the symptoms suggested an infection of the ringspot virus in the papaya plants
(Allan, 1980).
Application of BamO on papaya plants
BamO solution was prepared using 4.5 kg BamO powder mixed with 9 litres of water. The stock solution was used to prepare dilutions of the BamO working solutions. BamO formulation was diluted to 1:250, 1:500 and 1:1000. More than 400 plants were used for treatment. The BamO was sprayed on plants as per schedule given in Table 1.
Analysis of secondary metabolites from leaves
Metabolites extraction
The leaves of the BamO Sprayed papaya plants and Untreated plants were collected from the site. The metabolites of treated samples were extracted using a hot methanolic extraction method. Hot methanolic extraction: 10 mL methanol:water (1:1) was mixed with 1 gram of crushed seeds/seedlings and kept at 70°C for 15 min. Then, the mixture was incubated for room temperature and used for the preparation of esters.
Methyl ester preparation
250 µg or 50 µL of lipid samples were added to 1 mL of 1% methanolic NaOH (freshly prepared). The samples were heated at 55°C for 15 min. In hot tubes, 2 ml of 5% methanolic HCl (freshly prepared) was added and the mixture was again heated for 15 min at 55°C. The fatty acid methyl esters (FAME) were eluted by adding 1 mL of hexane to the above mixture. FAME were then stored at 8-10°C.
GCMS
The methyl esters obtained from the samples (as described in previous subsection) were used for GCMS. GCMS analysis (Agilent 5975 C gas chromatography system) was performed following the users’ guide to calibrate method with FAME standards, available on Fiehn GC-MS Metabolomics library-2008 (Agilent Chem Station, Agilent Technologies Inc., Wilmington, USA), with slight modifications, using retention time locking method. HP-5MS capillary column (30 m length, 10 m Duraguard, 0.25 mm diameter; narrow bore and 0.25 µm film) manufactured by Agilent JandW GC columns, USA, was used for the analyses. Following oven temperature program was maintained: 60°C (1 m), followed by 325°C at 10°C/min as final hold for 10 min before cool-down. Run time was 37.5 min. Injection temperature was set at 250°C, MSD transfer line at 290°C and ion source at 230°C. Helium was used as carrier gas (constant flow rate of 0.723 mL/min; carrier linear velocity 31.141 cm/s). Sample (1 µL) was injected onto the column via split mode (split ratio was 1:5). Chromatograms were analysed using Automated Mass Spectral Deconvolution and Identification System (AMDIS). Metabolite identification was performed by comparing the retention times (Rt), retention indices (RI) and mass spectral fragmentation pattern of compounds using the references present in Agilent Fiehn Metabolomics library. Many of the metabolites were also identified by comparing chromatographic and spectral properties with that of standard compounds.
Analysis of the growth and yield of papaya fruit
Leaf colour, size, shape and number were observed from BamO treated and untreated control plants. The fruits size, shape, weight and colour were recorded. The plant height and size were recorded.