High susceptibility to photoinhibition of young leaves of tropical forest trees

Abstract

Photoinhibition of photosynthesis was studied in young (but almost fully expanded) and mature canopy sun leaves of several tropical forest tree species, both under controlled conditions (exposure of detached leaves to about 1.8 mmol photons.m-2.s -1) and in the field. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence emission (Fv/FM) and also by gas-exchange measurements. For investigations in situ, young and mature leaves with similar exposure to sunlight were compared. The results show a consistently higher degree of photoinhibition in the young leaves. In low light, fast recovery was observed in both types of leaves in situ, as well as in the laboratory. The fluorescence parameter 1-Fs/F~ (where Fs=stationary fluorescence and Fh = maximum fluorescence during illumination) was followed in situ during the course of the day in order to test its suitability as a measure of the photosynthetic yield of photosystem II (PSII). Electron-transport rates were calculated from these fluorescence signals and compared with rates of net CO2 assimilation. Measurements of diurnal changes in PSII 'yield' confirmed the increased susceptibility of young leaves to photoinhibition. Calculated electron transport qualitatively reflected net CO2 uptake in situ during the course of the day. Photosynthetic pigments were analyzed in darkened and illuminated leaves. Young and mature leaves showed the same Chl a/b ratio, but young leaves contained about 50% less Chl a + b per unit leaf area. The capacity of photosynthetic O2 evolution per unit leaf area was decreased to a similar extent in young leaves. On a Chl basis, young leaves contained more a-carotene, more xanthophyll cycle pigments and, Abbreviations: Chl = chlorophyll; Fo = initial chlorophyll fluorescence ; FM = maximum total fluorescence; Fv = maximum variable fluorescence ( = FM --Fo); JF = rate of PSII-driven electron transport qbvs n = photosynthetic yield of PSII; PFD = photon flux density (400-700 nm); PS = photosystem Correspondence to: G.H. Krause; FAX: 49 (211) 311 3706 under strong illumination, more zeaxanthin than mature leaves. The high degree of reversible photoinhibition observed in these young sun leaves probably represents a dynamic regulatory process protecting the photosynthetic apparatus from severe damage by excess light.

Do mature shade leaves of tropical tree seedlings acclimate to high sunlight and UV radiation?

“…A slower recovery phase (requiring several hours) appears to be associated with turnover of the Dl protein in the PSII reaction centre (Leitsch et al 1994;Thiele et al 1996Thiele et al , 1998. Similar diurnal responses of PSII have also been observed in outer canopy sun leaves of tall trees (Krause et al 1995). In contrast, when mature shade leaves of tropical plants were exposed for the first time to fiill solar radiation, strong photoinhibition of PSII occurred, which required extremely long recovery periods (up to 2 weeks) in low light (Mulkey and Pearcy 1992;Krause et al 1999a) indicating sustained damage.…”

Cryptic coloration of Macaranga bancana seedlings: A unique strategy for a pioneer species

“…Excessive bright light condition could reduce photosynthesis efficiency and could possibly lead permanent damage to the chloroplasts, cells and tissues. 17 However, Krause et al 18 argued that for tropical plants (young leaves), the main component against photoinhibition are xanthophylls, a-carotene, and zeaxanthin. This argument is supported by Dominy et al, 19 in stating that if anthocyanins are involved in photoprotection, there should be more tropical trees with red leaves at the canopy level, compared to the understory.…”

Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees

“…The abrupt drop in photosynthetic rate for P. mariana and P. glauca during the spring transition was likely an artefact of leaf-level sampling. Budburst near branch tips for spruce trees made accurate sampling of mature needles difficult, leaving only young leaves to be sampled, which are less productive than fully mature leaves [40].…”

“…Maximum F S values seen in the early spring for L. laricina, P. mariana, and P. glauca, and seen later in the season for P. banksiana, coincided with the emergence of new leaf tissues. As young leaves are less productive, with lower chlorophyll concentrations than mature leaves [40], they have more excess absorbed light to dissipate [46], and require greater photoprotection [47]. This excess light may result in greater dissipation through fluorescence.…”