SummaryIn Arabidopsis leaves, high light stress induces rapid expression of a gene encoding a cytosolic ascorbate peroxidase (APX2), whose expression is restricted to bundle sheath cells of the vascular tissue. Imaging of chlorophyll fluorescence and the production of reactive oxygen species (ROS) indicated that APX2 expression followed a localised increase in hydrogen peroxide (H 2 O 2 ) resulting from photosynthetic electron transport in the bundle sheath cells. Furthermore, leaf transpiration rate also increased prior to APX2 expression, suggesting that water status may also be involved in the signalling pathway. Abscisic acid stimulated APX2 expression. Exposure of ABA-insensitive mutants (abi1-1, abi2-1) to excess light resulted in reduced levels of APX2 expression and confirmed a role for ABA in the signalling pathway. ABA appears to augment the role of H 2 O 2 in initiating APX2 expression. This regulation of APX2 may reflect a functional organisation of the leaf to resolve two conflicting physiological requirements of protecting the sites of primary photosynthesis from ROS and, at the same time, stimulating ROS accumulation to signal responses to changes in the light environment.

The effects of herbivores on plant production and fitness may not relate directly to the quantity of biomass removed because folivory may alter photosynthetic rates at a considerable distance from the damaged tissue [ ]. An impediment to understanding the effects of leaf damage on photosynthesis has been an inability to map photosynthetic function within a single leaf. We developed an instrument for imaging chlorophyll fluorescence and used it to map the effects of caterpillar feeding on whole-leaf photosynthesis in wild parsnip. The adverse effects of caterpillar feeding on photosynthesis were found to extend well beyond the areas of the leaflet in which caterpillars removed tissue. These ''indirectly'' affected areas remained impaired for at least 3 days after the caterpillars were removed and were six times as large as the area directly damaged by the caterpillars. Although photosynthesis in indirectly affected areas was reduced and not eliminated, these areas accounted for three times as much of the overall reduction in photosynthesis as the area removed by the caterpillars. The size of the indirect effects was positively correlated with defense-related synthesis of furanocoumarins, suggesting that costs of chemical defense may be one factor that accounts for the indirect effects of herbivory on plants.I n average years, net primary productivity removed by herbivores ranges from 2 to 15% in forests and from 4 to 24% in oldfields and grasslands (1). However, impacts on individual plants and plant communities are not readily predicted by the amounts of tissue taken by herbivores. Such unpredictability results from a variety of phenomena, including distribution of the damage within a plant (concentrated in one area or dispersed; ref.2), specific location (e.g., proximity to a vein; ref.3), induced production of allelochemicals that can be costly to produce (4), and the potential for compensatory increases of photosynthetic rates in intact leaf tissue (5).Many of the phenomena that mediate the effects of damage on photosynthesis have been undetectable to researchers because measurement of small areas of leaves by conventional gas exchange methods has been technically impossible. We developed an instrument that spatially maps chlorophyll fluorescence of intact leaves and extends the capabilities of previous instruments by providing a complete fluorescence-quenching analysis (6), imaging of larger areas of leaves (4-6 cm 2 ), and simultaneous gas-exchange measurements. In this study, we produced high-resolution images of the quantum efficiency of photosystem II (FЈ q ͞FЈ m ). The pixel values are directly related to the rate of electron transfer through the photosystems (7) and are highly correlated with the rate of carbon dioxide assimilation (8). We used this instrument to examine the effects of caterpillar damage in wild parsnip (Pastinaca sativa), a weedy species of European origin that occurs widely in the northeastern United States. When foliage of this species is damaged, furanocoumarins, compounds with bio...

Phytoplankton primary productivity is most commonly measured by 14 C assimilation although less direct methods, such as O 2 exchange, have also been employed. These methods are invasive, requiring bottle incubation for up to 24 h. As an alternative, Fast Repetition Rate fluorometry (FRRf) has been used, on wide temporal and spatial scales within aquatic systems, to estimate photosystem II (PSII) electron flux per unit volume (JV PSII ), which generally correlates well with photosynthetic O 2 evolution. A major limitation of using FRRf arises from the need to employ an independent method to determine the concentration of functional photosystem II reaction centers ([RCII]); a requirement that has prevented FRR fluorometers being used, as stand-alone instruments, for the estimation of electron transport. Within this study, we have taken a new approach to the analysis of FRRf data, based on a simple hypothesis; that under a given set of environmental conditions, the ratio of rate constants for RCII fluorescence emission and photochemistry falls within a narrow range, for all groups of phytoplankton. We present a simple equation, derived from the established FRRf algorithm, for determining [RCII] from dark FRRf data alone. We also describe an entirely new algorithm for estimating JV PSII , which does not require determination of [RCII] and is valid for a heterogeneous model of connectivity among RCIIs. Empirical supporting evidence is presented. These data are derived from FRR measurements across a diverse range of microalgae, in parallel with independent measurements of [RCII]. Possible sources of error, particularly under nutrient stress conditions, are discussed. *Corresponding author: E-mail: koxborough@chelsea.co.uk AcknowledgmentsWe would like to thank Tania Cresswell-Maynard (University of Essex) for providing many of the cultures used within this study and Kimberley Walrond (University of Manchester) for helpful discussions. Contributions of DJS and RJG were supported by the EU project PRO-TOOL (EU-226880). The contribution of CMM was supported by the Natural Environment Research Council, UK (NE/G009155/1). The authors owe extreme thanks to Hugh MacIntyre (Dalhousie University, Halifax, Canada) and Marie-Hélène Forget (Bedford Institute of Oceanography, Dartmouth, Canada) for their assistance and contributions in facilitating the Mk I FASTtracka data sets evaluated here.