Photosynthesis and photoinhibition in canopy leaves of four Chinese dipterocarp species

April 25, 2006 by

J.L. Zhang1,2, L.Z Meng1,2, K.F. Cao1,3 & Z.F. Xu1

1Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan, China 666303

2GraduateSchool of Chinese Academy of Sciences, 19A Yuquan Road, Beijing, China 100039

3Author to whom correspondence should be addressed (caokf@xtbg.ac.cn)


Abstract


Photosynthetic light responses, diurnal gas exchange and chlorophyll fluorescence were measured from canopy leaves of four Chinese dipterocarp species, Dipterocarpus retusus, Hopea hainanensis, Parashorea chinensis and Vatica xishuangbannaensis. The trees studied were 14 to 22 m tall and grown in a plantation in a northern tropical site (21°41¢N, 101°25¢E, 570 m a.s.l.) in southern Yunnan, China. Results showed that maximal photosynthetic rates (Amax) per unit leaf area, mass and nitrogen differed largely across species. D. retusus possessed highest Amax on area and nitrogen bases (18 μmol m–2 s–1 and 95 μmol mol–1 s–1, respectively), while V. xishuangbannaensishad lowest Amax either on area or mass or nitrogen bases (8 μmol m–2 s–1, 89 nmol g–1 s–1 and 62 μmol mol–1 s–1, respectively). In the diurnal courses, all species exhibited strong decrease in nett photosynthetic rate (An) from the mid-morning onwards，especially on midday. The statistical path analysis revealed that An was mainly controlled byleaf-to-air vapour pressure (LAVPD) via regulation of stomata, thus the high LAVPD during the midday can be seen as a main cause of photosynthetic suppression. However, this did not explain the photosynthetic suppressionin the afternoon, and we speculated that dynamic vessel embolism caused by strong transpiration and path-length effect of canopy leaves could also be contributing factors. Chronic photoinhibition occurred in D. retusus and V. xishuangbannaensis, as evidenced by the predawn Fv/Fm (maximal quantum yield of photosystem II, PSII) lower than 0.8. Moreover, all species suffered from strong midday photoinhibition, as indicated by significant decrease in quantum yield of PSII (ΦPSII), which was mainly induced by high photosynthetic ally photon flux density in all species and accelerated by high leaf temperature in D. retusus and H. hainanensis. Non-photochemical quenching in D. retusus, H. hainanensis and V. xishuangbannaensis while photorespiration in P. chinensis contributed significantly to mitigate photoinhibitory damage. The study concluded that the carbon gain of the canopy leaves of these four dipterocarp species are constrained by high LAVPD, dynamic vessel embolism and chronic and dynamic photoinhibition.


Keywords:canopy leaves, chlorophyll fluorescence, Dipterocarpaceae, hydraulic resistance, leaf to air vapor pressure deficit, path analysis, photoinhibition, photosynthesis.




Introduction


The canopy leaves are the main organs of forest ecosystems to fix CO2. The canopy leaves of tropical rain forests are exposed to high irradiance and air temperature, relatively low air humidity and thus high leaf-to-air vapor pressure deficit (LAVPD), especially on midday (Ishida et al. 1999, Brodribb & Holbrook 2004), and can be also constrained by hydraulic resistance due to path length effect and dynamic vessel embolism induced by strong transpiration (Brodribb & Holbrook 2004, Koch et al. 2004). Both the high LAVPD and hydraulic resistance can induce stomatal limitation of photosynthesis (Ishida et al. 1999, Brodribb & Holbrook 2004, Iio et al. 2004), and high irradiance and air temperature can induce non-stomatal limitation of photosynthesis through the effect of photoinhibition and inhibition of the activities of photosynthetic enzymes as explained below. During a day, CO2 exchanges of the canopy leaves of tropical rain forest trees usually exhibit a two-peak fluctuation pattern (Brodribb & Holbrook 2004). The first peak appears in the mid-morning when the photosynthetic photon flux density (PPFD) is near light saturation point and at that time the temperature is optimal to photosynthesis. The second peak is usually smaller and occurs in the afternoon, when LAVPD is decreasing and the embolism of vessels is refilling (Brodribb & Holbrook 2004).


When photosynthesis of canopy leaves is suppressed on midday, the light energy loading to them remains high, resulting in access light energy in chloroplasts. This can lead to photoinhibition and even photoxidation (Powles 1984). High temperature can suppress activity of Rubisco and adversely affect the stability of membrane, and consequently accelerate photoinhibition as well. If photoinhibition does not recover overnight, the photoinhibitory damage to photosystem (PS) II accumulates and leads to irreversible damage of PSII reaction centers (i.e. chronic photoinhibition). This can causes reduction of photosynthesis, too.


Heat dissipation of excess light energy absorbed by PSII through non-photochemical quenching (NPQ) is one of the major mechanisms to protect photosynthetic apparatuses against photoinhibitory damage (Niyogi 1999). Several recent studies have showed that photorespiration plays a role in photoprotection of canopy leaves as well (Muraoka et al. 2000, Franco & Lüttge 2002, Iio et al. 2004).


In facing of the elevation of CO2 in the atmosphere and consequently global warming, there is a great concern on the carbon sequestration of forests. However, our knowledge on the photosynthetic processes of forest canopy leaves is very limited (Ishida et al. 1996, Ishida et al. 1999, Kenzo et al. 2003, 2004). Diperocarpaceae is the most important tree family in both ecology and economy in tropical Asia, many of which are canopy or emergent trees of the forests (Whitmore, 1990). Nevertheless, the present knowledge on photosynthesis of dipterocarps is mainly limited to the seedlings or saplings (Lee et al. 1997, Scholes et al. 1997, Zipperlen & Press 1997, Cao 2000, Cao & Booth 2001,Leakeyet al. 2003).


In the present study, we characterized the gas exchange and chlorophyll fluorescence in canopy leaves of four Chinese dipterocarp species that are grown in a plantation forest in a northern tropical site. The following questions are addressed: (1) how do the four species differ in photosynthesis and diurnal changes in photosynthesis and chlorophyll fluorescence in their canopy leaves? (2) How do the ambient environmental factors affect these diurnal processes? …

Sources: 8th Round-Table Conference on Dipterocarps

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