Site, Soil and Season of the Experiment: The experiment was conducted at the Horticultural Research Farm, BSMRAU, Gazipur, Bangladesh during winter season of 2010-11 and 2011-2012. The location of the experimental site was at the center of Madhupur Tract (24.090N latitude and 90.260 longitudes) at 8.5m above the sea level. The soil of the experimental field contained pH 6.4. This area is moderately drought prone, and face drought both winter and late winter season. The experimental site is situated in a sub- tropical climate zone and characterized by no rainfall during December to March. Planting Materials and Date of Planting: Out of five selected potato genotypes, three were relatively tolerant (CIP 393371.58, CIP 396031.119 and CIP 396244.12), one was highly susceptible (CIP 391004.18) and another was high yielding in non-stress condition but susceptible to water stress condition (Asterix) were used in this experiment. Apparently diseases free, uniform (28-40 mm) sized well sprouted potato were selected as planting material for the experiment. Seed tubers were planted on 25th November, 2010 and 17th November, 2011.Crop Management: Plant spacing was 60 x 25 cm. Fertilizers were applied @ 160-44-132-15 kg/ha of N, P, K and S, respectively. Full amount of P, K and S and 50% of N were applied as basal and the remaining amount of urea was top dressed at 30 days after planting. Intercultural operations such as weeding, earthing up were done manually. Furadan 5G was applied @10 kg/ha as basal during land preparation and Malathion (0.2%) sprayed in two installments at 45 and 60 DAP control insects. The crops were also sprayed with Dithane-M 45 (0.2%) and Secure (0.1%)) alternately five times (at 30, 40, 50, 60, 70 DAP) to prevent the late blight infection of potato. Design and Treatments of the Experiment: The experiment was laid out in the field following strip plot design with three replications. Genotypes were considered as horizontal factor and drought treatment were considered as vertical factor. Three drought treatments were applied (T1= severe drought, T2=moderate drought and T3= well-irrigated control). In severe drought plots, one irrigation was applied at 30 days after planting for good crop stand, but no irrigation was applied thereafter until harvest. In moderate drought plot, irrigations were applied twice at 30 and 45 days after planting, but no water was applied thereafter till harvest. The control plot was irrigated four times at 30, 45, 60 and 75 days after planting. Climatological Data: Air temperature and humidity, precipitation, evaporation, soil temperature and ground water table were recorded throughout the crop period Data Collection Data were collected on a) plant canopy and b) yield attributes. Under plant canopy parameters data were collected on, i) plant height at 60 days after planting; ii) numbers of above ground shoots per plant at 60 DAP. For yield attributes data were collected on i) tuber number per plant; ii) tuber yield (ton/ha). Sequential Harvest (for Growth Analysis); Two plants were harvested from each of plot at each day of 40, 50, 60, 70, 80 and 90. Biomass was divided into leaves, stems, and tubers. Roots were not considered. For each biomass group, total fresh weight was determined, and their dry matter was estimated using fraction samples of 150 g fresh weight (dried in an oven for 48 to 72 hours at 75°C until constant weight was achieved). Leaf Area Index (LAI) was calculated by using the following formula-
LAI = Leaf area / Land area
Crop growth rate (CGR) was calculated by using the following formula-
CGR = [1 x (WT2- WT1)] / LA x (T2-T1) g m-2 day-1
Where,
WT1= Dry weight of tuber at time T1
WT2= Dry weight of tuber at time T2
LA = Land area
Tuber growth rate (TGR) was calculated by using the following formula-
TGR = [1 x (WT2- WT1)] / LA x (T2-T1) g m-2 day-1