The herbivore assemblage, intensity of herbivory and factors determining herbivory levels on the mangrove Kandelia obovata (previously K. candel, Rhizophoraceae) were studied over a 13-month period at two forests with contrasting growing conditions in Hong Kong. Mai Po was part of an eutrophic embayment in the Pearl River estuary and generally offered more favourable conditions for mangrove growth, whereas Ting Kok had a rocky substratum and oceanic salinity. Twenty-four insect herbivore species were recorded on K. obovata, with lepidopteran larvae that consume leaf lamina being the dominant species. While leaf litter production was similar at the two forests, herbivory level at Mai Po (mean = 3.9% in terms of leaf area loss) was more severe than that at Ting Kok (mean = 2.3%). Peak herbivory levels were found in summer at both locations (6.5% for Mai Po and 3.8% for Ting Kok). Young leaves of K. obovata at both locations were generally preferred by the herbivores from the period of late spring to summer. Concentrations of most feeding deterrents (ash, crude fibre, and total soluble tannins) were significantly higher in both young and mature leaves at Ting Kok, whereas leaf nutrients (total nitrogen and water) were the same at the two sites. Young leaves at Ting Kok contained about 30% more tannins than their counterparts at Mai Po. Significant differences in leaf chemistry also existed between young and mature leaves at either site. The differences were concomitant with the observed patterns of leaf herbivory on K. obovata, and suggest a potential relationship between environmental quality and plant defence against herbivory.

The effects of defoliation on leaf and propagule production, and leaf chemistry of the mangrove Kandelia candel (L.) Druce were evaluated in a manipulative experiment in Hong Kong. Artificial defoliation of leaf lamina at 50% of the length of midrib resulted in significant reduction in leaf, twig and propagule production, and size of the latter. Through the negative effects on propagule number and size, severe herbivory may influence fitness of the mangrove, and, thus, community structure. In contrast, no apparent adverse effects on growth and production were observable at 25% defoliation. Defoliation also significantly affected leaf chemistry of the trees, particularly those suffering 50% defoliation. Concentrations of soluble tannins and carbohydrates in leaves were significantly lower at 50% defoliation compared with the control. Total nitrogen also decreased significantly with increased per cent defoliation. Plants surviving in stressful habitats, such as mangroves, are probably more affected by loss of leaf biomass than those surviving in favourable environments.

The gall wasp, Leptocybe invasa (Hymenoptera; Eulophidae), is a devastating pest of eucalypt plantations in the Middle East, the Mediterranean basin, Africa, India, South-East Asia, and China. Heavy galling causes the leaves to warp and in extreme cases it may stunt the growth of the trees of Eucalyptus camaldulensis. However, the physiological mechanisms underlying how L. invasa inhibits the growth of plants of E. camaldulensis are unclear. Because the growth rate of plants is mainly dependent on photosynthesis that is largely correlated with hydraulic architecture, we speculate that galling of L. invasa depresses hydraulic conductance of stem and leaf. In the present study, we examined the effects of L. invasa galling on hydraulic architecture and photosynthetic parameters in E. camaldulensis plants. We found that galling of L. invasa significantly decreased stem hydraulic conductance (Kstem), midday leaf water potential (Ψmd), minor vein density, and stomatal density (SD). Furthermore, the stomatal conductance (gs), chlorophyll content, CO2 assimilation rate (An) and photosynthetic electron flow were reduced in infected plants. Therefore, the galling of L. invasa not only declined the water supply from stem to leaves, but also restricted water transport within leaf. As a result, galled plants of E. camaldulensis reduced leaf number, leaf area, SD and gs to balance water supply and transpirational demand. Furthermore, galled plants had lower leaf nitrogen content, leading to decreases in chlorophyll content, CO2 assimilation rate and photosynthetic electron flow. These results indicate that the change in hydraulic architecture is responsible for the inhibition of growth rate in galled plants.

No abstract

BACKGROUND: The wasp Leptocybe invasaFisher & LaSalle (Hymenoptera: Eulophidae), a Eucalyptus (Myrtaceae) pest native to Australia, has caused economic and ecologic losses in China. It is a serious pest in southern provinces. Because climate is a limiting factor in insect distribution, we used the model CLIMEX to predict the effect of climate change on potential current and future distributions of L. invasa in China. Data were collected on the current locations of this wasp, along with the damage incurred to Eucalyptus. These data were used to create a forecast model to predict potential current and future distribution maps of L. invasa in China. RESULTS:The verification results showed that 99.5% of the distribution samples formulated by the model are highly reliable and accurate. The result predicted that the potential current distribution of L. invasa will concentrate south of the Yellow River basin. The future distribution maps predicted a small-scale potential expansion north-northwest of Guangxi and more areas within China will provide increasingly suitable habitats for colonization by L. invasa. CONCLUSION: These distribution predications will be useful in determining where preventive and control measures should be implemented against this pest wasp in Eucalyptus throughout China.

It is believed that high levels of mesophyll conductance (gm) largely contribute to the high rates of photosynthesis in herbaceous C3 plants. However, some sclerophyllous C3 plants that display low levels of gm have high rates of photosynthesis, and the underlying mechanisms responsible for high photosynthetic rates in sclerophyllous C3 plants are unclear. In the present study, we examined photosynthetic characteristics in two high-photosynthesis plants (the sclerophyllous Eucalyptus camaldulensis and the herbaceous Nicotiana tabacum) using measurements of gas exchange and chlorophyll fluorescence. Under saturating light intensities, both species had similar rates of CO2 assimilation at 400 μmol mol−1 CO2 (A400). However, E. camaldulensis exhibited significantly lower gm and chloroplast CO2 concentration (Cc) than N. tabacum. A quantitative analysis revealed that, in E. camaldulensis, the gm limitation was the most constraining factor for photosynthesis. By comparison, in N. tabacum, the biochemical limitation was the strongest, followed by gm and gs limitations. In conjunction with a lower Cc, E. camaldulensis up-regulated the capacities of photorespiratory pathway and alternative electron flow. Furthermore, the rate of alternative electron flow was positively correlated with the rates of photorespiration and ATP supply from other flexible mechanisms, suggesting the important roles of photorespiratory pathway, and alternative electron flow in sustaining high rate of photosynthesis in E. camaldulensis. These results highlight the different mechanisms used to maintain high rates of photosynthesis in the sclerophyllous E. camaldulensis and the herbaceous N. tabacum.