The experiment was conducted at Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur, which is located at the center of Madhupur Tract (24°05' N latitude and 90°25' E longitude) at an elevation of 8.4 m above the sea level. BARI Alu-25 (Asterix), BARI Alu-28 (Lady Rosetta) and BARI Alu-29 (Courage) were used as plant materials and they were harvested at different days after planting [80, 90, and 100 days after planting (DAP)]. Here, three processing potato varieties represent Factor A and harvesting dates of 80, 90, and 100 DAP represent Factor B. The experiment was laid out in a factorial RCBD design with 4 replications.
The processing potato seed tubers were planted in 4m×3m plot using 60 cm×25 cm spacing at the end of November and sixteen seed tubers were placed in every row. The crop was raised as per the recommended cultural practices for the Modhupur Tract region. A fertilizer dose of N, P, K and S was applied as per fertilizer recommendation guide in the form of Urea, Triple Supper Phosphate (TSP), Muriate of potash (MP), Gypsum, respectively along with cow dung. Full doses of cow dung (10 t ha-1), TSP (150 kg ha-1), MP (250 kg ha-1), Gypsum (120 kg ha-1), and half doses of Urea (125 kg ha-1) were applied at final land preparation. The rest half doses of Urea were applied as top dressing at 30 DAP followed by earthing up and light irrigation. Irrigation was applied five times. First one was given just after planting while the rest irrigation was applied at 20, 30, 40, and 50 DAP. Earthing up was done in two times during growing period. First earthing up was done at 30 DAP when the plant attained a height of about 15-20 cm while the last one was done after 20 days of first earthing up. Ridomil Gold was sprayed at the rate of 5g L-1 once after rainfall to prevent the late blight disease of potato. The haulms were cut at 70, 80, and 90 DAP. Tubers were harvested 10 days after haulm cutting. Data were recorded on the yield of different grades tuber, processable tuber yield, total tuber yield, dry matter content, specific gravity and starch content of the test varieties. The chemical analysis was done in the Crop Botany Laboratory of BSMRAU.
Grading of potato tubers
After harvesting potato tubers were classified into four grades according to Karim et al. (2010), estimating diameter at the middle of the potato tubers. The grades were i) Under grade (<28 mm), ii) Grade A (28-40mm), iii) Grade B (41- 55 mm), and iv) Over grade (>55 mm). Among these grades, Grade A and Grade B together was considered as processable grade tuber.
Determination of tuber dry matter (%) and specific gravity
For dry matter content estimation, five tubers were chopped in thin pieces and about 50g fresh sample were oven dried at 80°C till constant weight was achieved. Then the moisture percentage was determined and this was deducted from the total 100% moisture to get the dry matter (%). Specific gravity was determined according to Schippers (1976).
Extraction and determination of starch
Following the procedure of Kang et al. (2009), potato flesh was extracted using 80% ethanol at 80°C for 30 min at first and then extracts were centrifuged at 5000g for 10 min. Same procedure was repeated for two times taking the supernatants. The remaining residue was washed several times with water to remove soluble sugars remaining in the residue. Then it was dried overnight, added with distilled water and heated in a boiling water bath for 30 min while stirring on the super mixture several times to gelatinize the starch released from the sample. Eventually, 52% (v/v) perchloric acid, 6 ml of distilled water was added, and after centrifugation at 4000g for 20 min, the supernatant was collected in a test tube. This acid hydrolysis was repeated once again and the volume of the supernatants was made up to 50 ml with distilled water into a volumetric flask. One ml of aliquot was mixed with distilled water and anthrone reagent, and boiled for 15 min. After cooling for 10 min in the dark, the absorbance was read at 620 nm. Standard solutions of starch were tested in a same way to obtain a calibration curve. A series of standard solution (0, 62.5, 125, 250, 500, 1000 µg µl-1 starch) was made using starch for the preparation of standard curve.
Statistical analysis
The analysis of variance for various parameters of potato was performed following ANOVA Technique. When F was significant at the p≤ 0.05 level, treatment means were separated using DMRT. Data were reported as mean ± standard error (SE).