Photosynthetic Responses to Irradiance by the Grey Mangrove, Avicennia marina, Grown under Different Light Regimes

Abstract

Photosynthetic responses to irradiance by the grey mangrove, Avcennia marina (Forstk.) Vierh var. ustraluia (Walp.) Moldenke, were studied using seedlings grown under natural understory shade and exposed conditions as well as in the laboratory under high and low lght regimes, ie. 100% and 6% sunlight, respectively. Leaves in exposed locations were subjected to dayUght quantum flux densities greater than 1,000 microeinsteins per square meter per second from 0900 to 1700 hours, whereas those in understory shade experienced only 30 to 120 microeinsteins per square meter per second, interrupted for brief periods by sunflecks rangin in quantum flux density from 800 to 1,500 microeinsteins per square meter per second. The low lght regime was similar in lght intensity to that of the understory environment, but lacked sunflecks.Leaves from the understory environment showed several properties of 'shade' leaves; ie. they contained more chlorophyll on both a leaf area and fresh weight basis but had a lower specific weight and greater area than exposed leaves, and were enriched in chlorophyll b relative to a. However, there were no significant differences in either the gas exchange or leaf chlorophyll fluorescence characteristics of the two populations, both being typical of 'sun' leaves. Leaves grown in the laboratory under low and high light regimes had similar properties. However, light saturated assimilation rates in the leaves from the low light treatment were 20% less and became light saturated at a lower quantum flux density than those of leaves grown under the high light regime. The ecological significance of these results is discussed.

Stomatal Behavior and CO2 Exchange Characteristics in Amphistomatous Leaves

“…Both upper and lower conductances increased as vapor pressure differences were decreased, and, as with the light response, the two conductances behaved in parallel leading to a constant conductance ratio over the range of vapor pressure differences examined. Again, average conductance ratios were slightly greater than 1.0 for well-watered plants, but stressing the plant reduced upper conductance more than lower conductance causing conductance ratios to be less than 1 were always in close agreement for a particular set of conditions (Fig. 6), and this was true over a wide range of external CO2 concentrations despite widely differing stomatal conductances between the surfaces in some cases.…”

“…These data, plus consideration of the anisolateral nature of the mesophyll in many C3 dicotyledonous species, raise the possibility that different CO2 exchange characteristics may exist for CO2 entering through one surface or the other, caused by either differences in resistance to CO2 diffusion through the intercellular spaces or differences in photosynthetic characteristics between palisade and spongy mesophyll cells. Although differences in carbon metabolism between these two types of cells have been shown not to exist (7), differences in electron transport reactions are indicated by differences in fluorescence characteristics between upper and lower surfaces of leaves (1).…”

Factors Influencing Biodiversity and Distributional Gradients in Mangroves

“…This implied that A. marina might rely on regular, rapid establishment and growth, in an environment where other species (less favoured by predatory crabs) might become established under closed canopies and be well-advanced before gaps form. However, there is a problem here, since A. marina is incapable of growth in understorey shade (Ball & Critchley, 1982). This suggests a further interplay of influencing factors, involving light gaps as well as crabs, which together influence the distribution of A. marina.…”

Environmental tolerances of rare and common mangroves along light and salinity gradients

“…The experiment had a three-way factorial design, examining the independent and interactive effects of light level and salinity on seedling growth and survival of the four mangrove species. Three daytime light treatments were established, spanning the range of field light levels from open sky to forest understory (Ball and Critchley 1982;Cheeseman et al 1991). Photosynthetic photon flux density maxima, measured under clear sky conditions at midday, were: high light (1200 μmol m −2 s −1 ), medium light (400 μmol m −2 s −1 ), and low light (130 μmol m −2 s −1 ).…”