Environmental and Experimental Botany 58 (2006) 261–268 Increased photosynthetic activities and thermostability of photosystem II with leaf development of elm seedlings (Ulmus pumila) probed by the fast fluorescence rise OJIP Chuang-Dao Jiang a,∗ , Gao-Ming Jiang a,∗ , Xianzhong Wang b , Ling-Hao Li a , D.K. Biswas a , Yong-Geng Li a a Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, PR China b Department of Biology, Indiana University–Purdue University Indianapolis, 723 West Michigan St., Indianapolis, IN 46202-5132, USA Received 31 March 2005; received in revised form 13 September 2005; accepted 30 September 2005 Abstract Experiments were conducted to investigate the photosynthetic activity and thermostability of photosystem II (PSII) in elm seedling (Ulmus pumila) leaves from initiation to full expansion. During leaf development, photosynthesis, measured as CO 2 fixation, increased gradually and reached a maximum value when leaves were fully developed. In parallel with the increase of carbon assimilation, chlorophyll content increased. The chlorophyll a fluorescence measurements showed that the maximum quantum yield of PSII primary photochemistry (ϕ po ), the efficiency with which the energy of trapped excitons is converted into the electron transport beyond Q A (Ψ o ) and the quantum yield of electron transport beyond Q A (ϕ Eo ) increased gradually. The low light experiments confirmed these results independently. When subjected to heat stress, young leaves exhibited progressively lower ϕ po and maximal fluorescence (F m ) values with considerably higher minimal fluorescence (F o ) than mature leaves, demonstrating that PSII in newly initiating leaves is more sensitive to heat stress. Further analysis revealed that PSII structure in newly initiating leaves showed a robust alteration under heat stress, which was reflected by the clear K phase in the OJIP curves. Therefore, we suggest that the enhanced thermostability of PSII in the case of leaf growth might be associated with an improvement of the stability of the oxygen-evolving complex (OEC) to heat stress during leaf development. © 2006 Elsevier B.V. All rights reserved. Keywords: Elm; Leaf growth; JIP-test; Thermostability of photosystem II 1. Introduction During leaf development, the formation of chloroplast ultrastructure, chlorophyll accumulation and the synthesis of the major components of the photosynthetic apparatus proceed almost in parallel and often result in a proportional increase of net photosynthesis ( ˇ Sest´ ak, 1985). Photosystem II (PSII) is one of the major protein complexes of the pho- tosynthetic apparatus of higher plants. During chloroplast maturation, Guenther and Melis (1990) observed different developmental states of the PSII complex. Recently, Choinski ∗ Corresponding authors. E-mail addresses: jcdao@ibcas.ac.cn (C.-D. Jiang), jgm@ht.rol.cn.net (G.-M. Jiang). et al. (2003) noticed that the maximum quantum yield of PSII primary photochemistry increases along with leaf growth. Here, attention has been paid to the development of PSII com- plexes under natural conditions. In contrast, most previous investigations concentrated on plants growing under artifi- cial conditions with suboptimal irradiance levels (Guenther and Melis, 1990; Lebkuecher et al., 1999; Srivastava et al., 1999). Therefore, the development of photosystem II during leaf growth under field conditions is not completely under- stood. In this study, we grew plants under natural irradiance to explore the development of the PSII during the process of leaf growth. Photosystem II is very sensitive to environmental stresses (Verhoeven et al., 1999; Jiang et al., 2002). Photosystem II complexes in newly initiating leaves are not yet fully devel- 0098-8472/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2005.09.007