Photosynthetic Responses to the Environmental Factors and Leaf Nitrogen Contents in the Mexican Sunflower (Tithonia diversifolia)

Abstract

Currently, the Mexican sunflower (Tithonia diversifolia) is an unexplored plant in Okinawa. Prior to establishing a manual for field cultivation, we examined the photosynthetic performance of plants grown at different nitrogen levels. Results showed that the photosynthetic ability of the plant was characterized by a higher light saturation point (1,200–1,800 μmol m^-2 s^-1), a higher initial slope of the A/Ci curve (0.17 mol m^-2 s^-1), a wide optimum temperature range (20°C–40°C), less photorespiration (14.9%–21.2%), a low correlation between the photosynthetic rate and stomatal conductance (r = 0.602, P < 0.01), and a polynomial relation of photosynthesis with specific leaf nitrogen (R² = 0.786, P < 0.001). These results were consistent with the facts observed in the original habitat. Consequently, the present study provides the following information in cultivating the plant. First, the plant requires high levels of radiation and nitrogen, which impacts planting density and fertilizer application. Second, the plant exhibited lower stomatal responsiveness to changes in environmental factors, indicating that it required careful water management, which could represent a potential burden for its cultivation management.