Nitrogen (N) is an important component of chlorophyll, protein, nucleic acid, and some hormones in plant, it also controls the growth and development of plant in many ecosystems. N availability has been recognized as a key constraint to plant growth. The N demand of plant and corresponding N application availability is becoming the research focus of plant nutritionists and physiologists. Jatropha curcas L. is an potential woody oil species which wildly distributed in south China. However, the effects of nitrogen supply on growth and photosynthesis of J. curcas are poorly understood. Whether N application on seedling stage can promote the survival and growth of J. curcas at dry and poor conditions have certainly became a key problem for its extensive cultivation. In the present study, the characteristics of growth and leaf photosynthesis of J. curcas seedlings with different N supply levels were investigated.
The experiment was conducted in greenhouse from May 2009 to September 2009. On May 1^st, J. curcas seeds were sowed in plastic pot (diameter 27 cm, height 22 cm). Twenty four such pots were prepared with four different N application rates (Control N₀: 0 kg N/hm²; Low nitrogen N_L: 96 kg N/hm²; Medium nitrogen N_M: 288 kg N/hm²; High nitrogen N_H: 480 kg N/hm²). After two months of treatment, morphological and biomass indices of seedlings, leaf gas exchange, chlorophyll fluorescence and some biochemical parameters were determined.
The results showed that the content of leaf N, soluble protein, plant height, biomass of different components, net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r) and water use efficiency (WUE) increased when N supply increased from 0 kg N/hm² to 288 kg N/hm², and then decreased when N supply reached 480 kg N/hm². The growth vigor of J. curcas with N_M treatment was the best and all measured gas exchange parameters was the highest among all the four treatments. The promoting effect of N application on the above-ground biomass accumulation was much stronger than that on the under-ground one. N applications significantly decreased the root-shoot ratio. In addition, the leaf chlorophyll content, maximum quantum yield of PSⅡ photochemistry (Fv/Fm), effective quantum yield of PSⅡphotochemistry (F'v/F'm), actual photochemistry efficiency of PSⅡ (Φ_PSⅡ), electron transport rate (ETR) and photochemical quenching values (qP) increased with N application rate, while the non-photochemical quenching values (NPQ) exhibited a converse tendency. Results suggest that when J. curcas grows on the site with N deficiency condition, increasing nitrogen supply can promote its growth of root, shoot and leaf, and also enhance its photosynthetic capacity by improving photochemistry quantum efficiency of PSⅡ, G_s and carboxylation efficiency. While the N_H treatment may have exceeded the optimum N level for J. curcas seedlings. In this study, the optimum N application rate for J. curcas seedlings was 288 kg N/hm².