Effect of Water Deficit Stress and Nitrogen on Yield and some Physiological Parameters of Oilseed Pumpkin (Cucurbita pepo L.)



Oilseed pumpkin is an introduced medicinal plant, the seeds as well as some other organs of which are being utilized in treating an array of human diseases in Iran. Considering the plant's significance in the health products' industry on the one hand, and the countrywide drought stresses and a lack of information on mineral nutrition of the oilseed pumpkin, the present study was aimed to study the impact of irrigation regime as well as N application on chlorophyll (Chl) content, gas exchange properties, proline content, yield and yield components of this useful medicinal plant. A 5-replicate split plot, randomized complete block experiment was conducted at the Lavark Research Farm, College of Agriculture, Isfahan University of Technology-Isfahan, Iran in 2009. Three irrigation regimes based on evaporation from a Class-A Standard Evaporation Pan including irrigation after 65 mm (I1=control), 95 mm (I2¬= moderate water deficit) and 130 mm (I3= severe water deficit) were assigned as the main plots. Three nitrogen (N) application rates including zero N (N1= control), 120 N (N2) and 220 N (N3) kg ha-1 of N as urea were chosen as subplots. Leaf Area Index (LAI), gas exchange attributes, Chl and proline content, fruits/plant, seeds/fruit, 1000-seed weight, fruit and seed yields, oil content as well as yield were evaluated. I1¬ and I3 irrigation levels with 2.11 and 1.46 and N3 and N1 nitrogen levels with 2.50 and 1.16 produced the highest and lowest LAI, respectively. Water deficit stress and lowered N application led to significant decreases in leaf Chl content. Water deficit led to decreases in the net photosynthetic rate (A) and leaf stomatal conductance (gs) but it led to increases in internal CO2 concentrations (Ci). While A and gs increased with N application rate, Ci decreased with an increase in N application. Proline content increased with both increases in water deficit stress and N application rate. I3 irrigation level led to 23, 33, 5, 51, 52 and 55 percent decreases in the traits of: fruits/plant, seeds/fruit, 1000-seed weight, seed and fruit yield as well as oil yield, respectively, as compared with the I1 level. N3 level of nitrogen application resulted in 28, 22, 27, 9 and 33 percent decreases in fruits/plant, fruit and seed yield, oil percent as well as oil yield, respectively, as compared with the N1 level. It could be concluded that the negative impact of water deficit stress on yield and its components in oilseed pumpkin is mainly through the decreases in the photosynthetic surfaces, photosynthetic rate and as well as Chl content. Furthermore, application of 220 kg ha-1 of N may lead to increases in LAI, Chl and A, though the latter increases, seemingly, enhance the growth of vegetative organs rather than the reproductive ones and grain yield components.