ABSTRACT
Ultraviolet (UV)-absorbing phenolic compounds that have been shown to be protective against the damaging: effects of UV-B radiation (Tevini et al., 1991, Photochem. Photobiol. 53, 329-333) were found in the leaf epidermis of tropical mangrove tree species. These UV-absorbing phenolic compounds and leaf succulence function as selective filters, removing short and energetic wavelengths. A field survey showed that the concentration of UV-absorbing compounds varied between species, between sites that would be experiencing similar levels of UV radiation, and between sun and shade leaves. Sun leaves have greater contents of phenolic compounds than shade leaves, and more saline sites have plants with greater levels in their leaves than less saline sites. In addition, increases in leaf nitrogen contents and quantum yields did not correlate with increasing levels of UV-absorbing compounds. It was concluded from these results that although UV-absorbing compounds form a UV-screen in the epidermis of mangrove leaves, UV radiation may not be the only factor influencing the accumulation of phenolic compounds, thus an experiment which altered the level of UV radiation incident on mangrove species was done. Near ambient levels of UVA and UV-B radiation resulted in a greater content of UV-absorbing compounds in Bruguiera parviflora (Roxb.) Wight and Arn. ex Griff., but did not result in increases in B. gymnorrhiza (L.) Lamk or Rhizophora apiculata Blume. Total chlorophyll contents were lower in R. apiculata when it was grown under near-ambient levels of UV radiation than when it was grown under conditions of UV-A and UV-B depletion, but no differences were observed between the UV radiation treatments in the other two species. There was no difference in leaf morphology, carotenoid/chlorophyll ratios, or chlorophyll a/b ratios between UV treatments, although these varied among species; B. parviflora had the highest carotenoid/chlorophyll ratio and R. apiculata had the lowest. Thus it is proposed that differences in species response tu UV radiation may be influenced by their ability to dissipate excess visible solar radiation.
ABSTRACT
In this report, we analyze the photosynthetic capacity and performance of leaves under field conditions with a case study based on the mangroves Bruguiera parviflora and B. gymnorrhiza. Using a tower through a closed canopy at a field sight in North Queensland and portable infra-red gas analyzers, a large data set was collected over a period of 11 days early in the growing season. The set was used to analyze the relationship between net photosynthesis (Pnet) and light, leaf temperature, stomatal conductance and intracellular CO2 (Ci).There are three objectives of this report: (1) to determine photosynthetic potential as indicated by the in situ responses of Pnet to light and stomatal conductance, (2) to determine the extent to which photosynthetic performance may be reduced from that potential, and (3) to explore the basis for and physiological significance of the reduction.The results indicate that even under harsh tropical conditions, the mangrove photosynthetic machinery is capable of operating efficiently at low light and with maximal rates of more than 15 µmol CO2 m(-2) s(-1). Though stomata were more often limiting than light, in any single measurement the average reduction of Pnet from the maximum value predicted by light or conductance responses was 35%. Analysis of single leaf light and CO2 responses indicated that photosynthetic performance was under direct photosynthetic, or non-stomatal, control at all light and conductance levels. Capacity was adjustable rapidly from a maximum value to essentially nil such that Ci varied inversely with Pnet from ca. 150 µL L(-1) at the highest rates of CO2 exchange to ambient at the lowest.
ABSTRACT
Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO2 assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO2 assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO2 assimilation rate resulted in constant and similar intercellular CO2 concentrations for leaves exposed to photon flux densities above 800 µmol·m-2·s-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO2 assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO2 concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.
