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.