This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life‐history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.

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Abstract. Herbaceous C3 plants grown in elevated CO2 show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO2‐enriched herbaceous plants and decrease for CO2‐enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO2 show no alteration in the whole‐plant partitioning of biomass. Little information is available for interactions between temperature and CO2. Cold‐adapted plants show little response to elevated levels of CO2, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO2‐enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.

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We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ' push ' hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ' pull ' hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ' shared control ' hypothesis) -that acquisition of C by roots is controlled by a range of variables distributed between root and shoot -accords with both experiment and theory. Top-down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short-term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.

What determines the way in which roots provide carbon to and interact with other components of the soil? Roots lose metabolites and signal molecules to the soil at rates of significance to soil organisms, and we need to know if the mechanisms of passive diffusion identified in hydroponics apply in soil, and whether other, active mechanisms complement them. New insights from biosensors into the heterogeneity and localization of exudation are transforming our understanding of root–microorganism relations. We need to know more about compounds that are exuded at subnutritional rates in soil and may act as signal molecules modifying the biology of soil organisms. Insights into one suite of such compounds is coming from studies of border cells. These cells are lost from the root cap at a rate regulated by the root and secrete compounds that alter the environment of and gene expression in soil microorganisms and fauna. The amount of root places an upper limit on the effect roots can have; carbon flow to the rhizosphere is a function of root growth. Top‐down metabolic control analysis shows that the control over the rate at which roots grow is shared between root and shoot, with most control being in the shoot. Corresponding Editor: D. A. Phillips. For reprints of this Special Feature, see footnote 1, p. 815

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO 2 (C a ) is increased. Recent reviews suggest that doubling the present C a will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated C a and appears to be specific to C 3 species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochromec-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient C a would represent 3 Gt of carbon per annum in the global carbon budget.Key-words: acclimation to rising CO 2 ; dark respiration; global carbon cycle; rising CO 2 .Abbreviations: ATP, adenosine triphosphate; K m , MichaelisMenton coefficient; C a , concentration of CO 2 in the air (µmol mol -1 ); NAD, oxidized nicotin adenine dinucleotide; NADH, reduced nicotin adenine dinucleotide; NADP, oxidized nicotin adenine phosphate dinucleotide; NADPH, reduced nicotine adenine phosphate dinucleotide; R, rate of respiration per unit DW [µmol g DW -1 ], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; V c , max , maximum in vivo rate of carboxylation at Rubisco (µmol m -2 s -1 ).

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