Quantifying nitrogen effects on castor bean (Ricinus communis L.) development, growth, and photosynthesis

Functional relationships between leaf nitrogen (N) and crop growth processes are not available in many crops including castor bean plant that is considered as a potential bioenergy crop. An out-door pot-culture experiment was conducted to determine N deficiency effects on castor bean plant growth and physiology. Castor bean, cv. ‘Hale’, was seeded in 12-L pots filled with fine sand and irrigated with full-strength Hoagland's nutrient solution from emergence. After 34 days of sowing (DAS), the treatments imposed were full-strength Hoagland's nutrient solution (control, 100N), reduced N to 20% of the control (20N) and no N (0N) until final harvest, 66 DAS. Growth (plant height, leaf development and leaf area), photosynthesis and leaf N were measured twice weekly and plant components biomass was measured, 66 DAS. Maximum growth and developmental rates were achieved at 7.0 g N kg−1, much higher than many other crops grown under similar nutrient conditions. Even though all growth rates declined with lower leaf N, leaf area expansion was more sensitive to leaf N followed by rates of stem elongation, node addition and photosynthesis. Critical leaf N levels (90% of maximum) varied for various processes; 55.3 g N kg−1 for stem elongation, 63 g N kg−1 for node addition, 65.4 g N kg−1 for leaf area expansion, and 60.3 g N kg−1 for photosynthesis. Among the plant components, leaf dry weight had the greatest decrease while root/shoot ratio increased under N deficiency. The functional algorithms and critical leaf N levels for various growth processes will be useful for modeling, leaf N assessment and managing castor bean crop the in the field.