Environmental and Experimental Botany 55 (2006) 87–96 Leaf orientation, photorespiration and xanthophyll cycle protect young soybean leaves against high irradiance in field Chuang-Dao Jiang a, Hui-Yuan Gao b,∗,QiZoub, Gao-Ming Jiang a, Ling-Hao Li a a Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, PR China b Department of Plant Science, Shandong Agricultural University, Taian 271018, PR China Accepted 6 October 2004 Abstract In order to fully understand the adaptive strategies of young leaves in performing photosynthesis under high irradiance, leaf orientation, chloroplast pigments, gas exchange, as well as chlorophyll a fluorescence kinetics were explored in soybean plants. The chlorophyll content and photosynthesis in young leaves were much lower than that in fully expanded leaves. Both young and fully expanded leaves exhibited down-regulation of the maximum quantum yield (FV/FM) at noon in their natural position, no more serious down-regulation being observed in young leaves. However, when restraining leaf movement and vertically exposing −2 −1 the leaves to 1200 ␮mol m s irradiance, more pronounced down-regulation of FV/FM was observed in young leaves; and the actual photosystem II (PS II) efficiency (ФPSII) drastically decreased with the significant enhancement of non-photochemical −2 −1 quenching (NPQ) and ‘High energy’ quenching (qE) in young leaves. Under irradiance of 1200 ␮mol m s , photorespiration (Pr) in young leaves measured by gas exchange were obviously lower, whereas the ratio of photorespiration/gross photosynthetic rate (Pr/Pg) were higher than that in fully expanded leaves. Compared with fully expanded leaves, young leaves exhibited higher xanthophyll pool and a much higher level of de-epoxidation components when exposure to high irradiance. During leaf development, the petiole angle gradually increased all the way. Especially, the midrib angle decreased with the increasing of irradiance in young leaves; however, no distinct changes were observed in mature leaves. The changes of leaf orientation greatly reduced the irradiance on young leaf surface under natural positions. In this study, we suggested that the co-operation of leaf angle, photorespiration and thermal dissipation depending on xanthophyll cycle could successfully prevent young leaves against high irradiance in field. © 2004 Elsevier B.V. All rights reserved. Keywords: Photosynthetic rate; Chlorophyll a fluorescence; Photorespiration; Xanthophyll cycle; Leaf orientation; Soybean Abbreviations: A, antheraxanthin; Chl, chlorophyll; F0, minimal fluorescence in dark-adapted state; FM, maximum fluorescence in dark- adapted state; FV, maximum variable fluorescence in dark-adapted state (=FM−F0); FV/FM, maximum quantum yield of photosystem II; FS, F F steady-state fluorescence under irradiance; M, maximum fluorescence in ligh-adapted state; V, maximum variable fluorescence in light-adapted F − F state (= M 0); PFD, photon flux density; Pn, net photosynthetic rate; Pg, gross photosynthetic rate; Pr, photorespiration; PSII, photosystem II; ФPSII, the actual PSII efficiency under irradiance; NPQ, non-photochemical quenching; qE, the fast relaxing component of non-photochemical quenching; V, violaxanthin; Z, zeaxanthin ∗ Corresponding author. Tel.: +86 538 8241341; fax: +86 538 8249608. E-mail address: [email protected] (H.-Y. Gao). 0098-8472/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2004.10.003 88 C.-D. Jiang et al. / Environmental and Experimental Botany 55 (2006) 87–96 1. Introduction zeaxanthin (Z) and antheraxanthin (A) (Bjorkman¨ and Demmig, 1987). All organisms that exhibit qE have a During leaf development, the newly initiating leaves xanthophyll cycle (Muller¨ et al., 2001). For the last few are often exposed to full sunlight at the topmost canopy, years, many investigators have paid special attentions indicating that those young leaves have to endure ex- to the role of xanthophyll cycle under conditions of tremely high irradiance. However, young leaves have cold-temperature stress (Verhoeven et al., 1999), water lower photosynthesis activity per unit area compared stress (Munne-Bosch´ and Alegre, 2000), and nutrition with fully developed leaves (Krause et al., 1995; Choin- deficiency (Verhoeven et al., 1997; Jiang et al., 2001; ski et al., 2003). These will inevitably result in more Jiang et al., 2002). Just recently, some people have fur- excessive excited energy in young leaves. It is well ther explored the characteristics of xanthophyll cycle known that too much light can lead to largely increased pigments and excited energy dissipation in senescence production of damaging reactive oxygen as byproducts leaves (Munne-Bosch´ et al., 2001; Lu et al., 2001; Lu of photosynthesis, during which photosynthetic rate is et al., 2003). Specifically, a few physiologists argued depressed (Osmond, 1994; Muller¨ et al., 2001). In ex- that xanthophyll cycle pigments and excited energy dis- treme cases, reactive oxygen can cause pigment bleach- sipation were enhanced in young leaves so that the ing and death. Such is the case well known to anyone photosynthetic apparatus could be protected (Krause who tries to move a houseplant outdoors into full sun- et al., 1995; Yoo et al., 2003). However, such ques- light (Osmond, 1994; Muller¨ et al., 2001). Therefore, it tion is still in debate (Ogren,¨ 1991; Krause et al., 1995; is a great challenge for young leaves to be subjected to Bertamini and Nedunchezhian, EEB1511BIB312003). strong irradiance. Nevertheless, plants have developed In all these studies, we noticed that detached and almost a number of strategies to balance the captured light fully expanded leaves were chosen to explore xantho- energy, thereby protecting photosynthetic apparatus phyll cycle. Under field conditions, can these mecha- against photodamage (Anderson et al., 1997). nisms successfully protect young leaves against high Photorespiration provides an effective electron sink irradiance at the early development stages? when CO2 assimilation is low (Kozaki and Takeba, Some authors also noticed that leaves, especially 1996). It is well documented that photorespiration pro- some leguminous plants, can change their orientation tects leaves against high irradiance through not only by inclining upwards and downwards under higher acting as a sink for reducing equivalents but also pre- irradiance, thereby minimizing the interception of venting over-reduction of the electron carriers between irradiance for avoiding photodestruction (Gamon and PS II and PS I (Kozaki and Takeba, 1996; Osmond Pearcy, 1989; Ogren¨ and Evan, 1992; Bjorkman¨ and and Grace, 1995). However, there were few studies Demmig-Adams, 1995; James and Bell, 2000; Feng focusing on changes of photorespiration during leaf et al., 2002). We wonder whether or not leaf orienta- expansion. tions plays a more important role in newly initiating Thermal dissipation of excess irradiance measured leaves than that in fully developed ones when legumi- as non-photochemical quenching (NPQ) is believed to nous plants are subjected to strong irradiance. be of paramount importance in the protection of the The objective of this study is to explore how young photosynthetic apparatus against the deleterious effects soybean leaves cope with high irradiance under field of excess light. It has been known for many years that at conditions, and whether the co-operation of leaf ori- least two main components of NPQ can be resolved by entation, photorespiration and xanthophyll cycle could analyzing its dark relaxation kinetics, the rapidly relax- effectively protect young leaves against strong sunlight ing component (qE) and the slowly reversible compo- in field. nent (often termed qI). qE, the rapidly relaxing compo- nent of NPQ is considered to be an important photopro- 2. Materials and methods tective mechanism to cope with excessive irradiance (Bjorkman¨ and Demmig, 1987; Muller¨ et al., 2001). 2.1. Plant materials The value of qE is always associated with pH gradient across the thylakoid membrane (Briantais et al., 1980) Soybean (Glycine max L.) plants were grown in ten and the formation of the xanthophyll cycle pigments, plastic pots (22 cm in diameter and 30 cm in height) at C.-D. Jiang et al. / Environmental and Experimental Botany 55 (2006) 87–96 89 the beginning of May. The plants grown in pots were ech, UK). The maximum quantum yield of photo- placed in field subjected to natural solar radiation, with system II (FV/FM) was determined in dark-adapted a daily maximum photosynthetic photon flux density (15 min) samples. After the initial Chl fluorescence −2 −1 (PPFD) of above 1600 ␮mol m s , and the maxi- yield (F0) was determined in low modulated mea- mum air temperature was about 33 ◦C. The soybean suring light, a 0.7-s pulse of saturating white light plants were thinned to one plant per pot 2 weeks after (>3000 ␮mol m−2 s−1) was applied to obtain the max- sowing. Nutrients and water were supplied sufficiently imum Chl fluorescence yield (FM) and the FV/FM throughout, to avoid potential nutrients and drought (FV, the variable Chl fluorescence yield, is defined as stresses. After growing for 5 weeks, newly expanding FM−F0). The steady-state fluorescence level (FS) and F leaves with an area of about 33% of fully expanded the maximum Chl fluorescence level ( M) during expo- leaves (33% A), near fully expanded leaves with an sure to illumination were also measured, respectively. area of about 78% of fully expanded leaves (78% A) The fluorescence transient was induced by continu- and fully expanded leaves (100% A) were studied in ous light (1200 ␮mol m−2 s−1) for 2 h. While the pho- the experiments. Generally, three leaves (33% A, 78% tochemical fluorescence-quenching coefficient (qP) A and 100% A) of each plant were used in the differ- and non-photochemical quenching (NPQ) were dis- ent measurements, and at least three replications were criminated by applying saturating pulses after every made. 30 min of the continuous light treatment. In the experiments carried out in open field, all The actual PS II efficiency (ФPSII) was calcu- F − F leaves were kept in their natural positions.