Abstract: [Objective] This study aims to clarify the differences in growth and photosynthetic capacity of intraspecific and interspecific hybrids of Catalpa, and to explore the potential relationship between growth and photosynthetic capacity which is regulated by nitrogen use and distribution in leaves, and to provide favorable basis for Catalpa cultivation and genetic improvement.[Method] The experiment was designed with complete random block design, the height of 1-5 years old trees and DBH of 1-6 years old trees of the Catalpa bungei×C. bungei and C. bungei×C. fargesii f. duclouxii hybrids, and leaf nitrogen content, chlorophyll content, light response curve and CO₂ response curve of the Catalpa hybrids in 6 years old were measured. The non-orthogonal hyperbolic model was used to fit the light response curve to calculate the gas exchange parameters, such as the apparent quantum yield (AQY) and the light compensation point (LCP). The FvCB biochemical model (The biochemical photosynthetic model proposed by Farquhar, von Caemmerer and Berry) was used to fit to the CO₂ response curve, and the maximum carboxylation efficiency (V_{c max}), maximum electron transfer rate (J_max) and other photosynthetic biochemical parameters were estimated via the model. Nitrogen allocation ratio in photosynthetic system, include light-harvesting system, carboxylation system and bio-energy component, were calculated.[Result] Analysis of variance showed that tree height and DBH of intraspecific hybrids (C. bungei×C. bungei) were significantly greater than interspecific hybrids (C. bungei×C. fargesii f. duclouxii) after 2 years. There was no significant difference between the two hybrids in the total amount of chlorophyll. But the content of chlorophyll b of interspecific hybrids was significantly higher (15.08%) than intraspecific hybrids. On the other hand, the chlorophyll a/b and carotenoid/chlorophyll a+b of intraspecific hybrids was significantly higher than interspecific hybrids. The photosynthetic parameters showed that the intraspecific hybrid had a greater maximum net photosynthetic rate, stomatal conductance, maximum carboxylation efficiency and dark respiration rate. It showed that they had stronger photosynthetic capacity. There was no significant difference in leaf nitrogen content between the two hybrid types. However, more nitrogen was invested to photosystem for intraspecific hybrid leading to higher photosynthetic nitrogen utilization efficiency (PNUE). And it may be one of the reasons of high photosynthetic efficiency for intraspecific hybrid. Correlation analysis showed that the PNUE of Catalpa hybrids was significantly positively correlated with the nitrogen allocation in the carboxylation system and bioenergetics. In addition, the PNUE of intraspecific hybrid had a greater (R²=0.531) positive linear relationship with the diameter at breast height (DBH).[Conclusion] 1) Intraspecific hybrids have a stronger adaptability to the local environment (The central plains), it was the reason that growth of Catalpa bungei×C. bungei was significantly better than interspecific hybrid (C. bungei×C. fargesii f. duclouxii). 2) Compared with Yunnan-Guizhou plateau, the longer sunshine duration and higher mean temperature in July in the central plains may be the reason for the formation of a response mechanism (high level Chl a/b and Car/Chl a+b) for intraspecific hybrid to adapt to the high photosynthetic radiation environment. 3) Higher N distribution ratio and N use efficiency in photosynthetic system of C. bungei×C. bungei improved its photosynthetic ability. 4) The genetic material transmitted to the offspring by C. fargesii f. duclouxii did not have the regulatory mechanism to adapt to the environment in the central plains, this is the main reason why the growth and photosynthetic physiology of interspecific hybrids are inferior to that of intraspecific hybrids in the central plains.

Key words: Catalpa bungei, Catalpa fargesii f. duclouxii, hybrids, nitrogen allocation, photosynthesis