Is acclimation required for success in high light environments? A case study using Mycelis muralis (L.) Dumort (Asteraceae)

Abstract

In the field significant differences in maximum photosynthetic O.j-exchange ratf (P,J were found between leaves oi Mycelis muralis (L.) Dumon (Asteraceae) collected from woodland and exposed habitats, with tht-highest values in the exposed sites. However, there were no differences in the P^^ of leaves collected from plants growing in grikes (fissures in the limestone pavement), or exposed limestone pavement, despite a greater than four-fold difference in the integrated daily irradiance. Lea\es of plants from the open pavement had lower photon yields (0,) and higher dark respiration rates and light compensation points, in comparison to shaded plants. Under controlled enviranmcntal conditions the highest P^, of leaves from plants subjected to variations in irradiance were found at the intermediate (8-6 mol photon m"M ') growth light level used. At the highest growth irradiance 17-3 mol photon m ^ d"') used in tht-laboratory both P,^^ and <p, were reduced, although the largest plani biomass was found at this irradiance. No changes were found in the chlorophyll u ;t ratio over the same range o/irradiances. Exammation of plant populations of M. muralis, collected from open or shaded habitats and exposed to growth irradiances that covered the range over which increases in photosynthesis were observed in the laboratory-{0-86-8-6mol photon m" d"'), resulted in changes in leaf structure and pigment composition. The chlorophyll u: ft ratio was low and largely independent of irradiance or the origin of the plant population. Differences in total chlorophyll content were small with the lowest values in the Durrow woodland populations at both irradiances. No variations were found in a number of chloroplast thylakoid structural features. In' particular, the ratio of apprcssed to non-appressed membranes was unchanged by growth at the two irradiances, consistent with an invariant chlorophyll a : h ratio. Bastd on peaks in the difference spectra the woodland populations had an enhanced in vivo absorption at A = 650 and 706 nm when grown at low irradiance. These peaks were absent from the population collected from the open limestone pavement. The significance of tht-enhanced absorption at low irradiance and the possibility that these peaks represent long-wavelength forms of chlorophyll a [A = 706) and b (A = 650} is discussed. A particular feature of plants grown at high irradiance was an enhanced anthocyanin content in comparison to those grown at low irradiance. This was associated with an increase in absorptance, particularly in the green region {A = 551) nm) of the visible spectrum. Overall these results suggest that complete acclimation of photosynthesis and an ability to modulate light-harvesting is not a prerequisite for success in a high light environment.

Leaf optical properties of two liana species Canavalia parviflora Benth. and Gouania virgata Reissk in different light conditions

“…This increase was not followed by reductions in the chlorophyll a/b ratio, indicating similar structure of thylakoid membranes and showing that although reductions on chlorophyll a/b were a common response for a great number species, it is not universal (Murchie & Horton 1997). Similarly, Mycelis muralis (L.) Dumort (Osborne et al 1994), a herbaceous and shade-tolerant species showed constant values of chlorophyll a/b ratio under contrasting light conditions. These results indicate the inability of some species to acclimate all components of the photossynthetic apparatus to light conditions.…”

“…However carotenoids were significantly higher in 10% and 1.5% PFD, indicating also their role as an accessory pigment. One of the most common responses to light conditions is a change in the chlorophyll a/b ratio (Osborne et al 1994). Typically, the chlorophyll a/b ratio decreases and the total amount of chlorophyll associated with the light harvest proteins increases in response to low irradiance, leading to a greater efficiency of light utilization (Anderson et al 1988).…”

Crescimento inicial de Euterpe edulis Mart. em diferentes regimes de luz

“…A densidade de estômatos mostrou uma leve tendência ao aumento em plantas transferidas para maior quantidade de luz. O aumento da densidade de estômatos com aumento de irradiância é uma resposta encontrada em plântulas de muitas espécies (Holmes & Cowling 1993) (Osborne et al 1994, Poorter 1999, Valladares et al 2000. A posição sucessional das espécies tem sido utilizada por alguns autores para explicar a maior ou menor plasticidade das plantas em responder à variação de luz, com espécies de estágios sucessionais mais tardios tendo menor plasticidade que espécies de estágios sucessionais mais iniciais (Huante & Rincón 1998).…”

“…O menor desempenho das plantas em condições de plena luz ou forte sombreamento pode limitar a capacidade competitiva da espécie em grandes clareiras ou sob dossel fechado (Oberbauer & Strain 1985, Chazdon 1992. Embora E. edulis não tenha mostrado se adaptar ao aumento da quantidade de luz acima de 20-30% da luz solar plena, isto não significa que a espécie não possa explorar ambientes com altas irradiâncias, como já verificado para Mycelis muralis (Osborne et al 1994) e Platycyamus regnellii (Scalon & Alvarenga 1993), que se mostraram capazes de crescer em irradiâncias bastante altas sem apresentarem alterações morfológicas ou fisiológicas.…”

Shade responses of five grasses native to southwestern U.S. <i>Pinus ponderosa</i> forests

“…However, exceptions to these patterns exist. Some shade species can achieve photosynthetic rates similar to those of sun species (Chow et al 1988;Murchie and Horton 1997), and they can be successful in high-light environments despite low acclimation of their photosynthetic apparatus (Adamson et al 1991;Osborne et al 1994). A study of 22 species from different shade environments indicated that the mechanism of acclimation differed among species (Murchie and Horton, 1997): sun species responded at the leaf morphological level by increasing specific leaf area (SLA), while shade species responded at the chloroplast level.…”