The responses of isoprene emission rate to temperature are characterized by complex time-dependent behaviors that are currently not entirely understood. To gain insight into the temperature dependencies of isoprene emission, we studied steadystate and transient responses of isoprene emission from hybrid aspen (Populus tremula 3 Populus tremuloides) leaves using a fast-response gas-exchange system coupled to a proton-transfer reaction mass spectrometer. A method based on postillumination isoprene release after rapid temperature transients was developed to determine the rate constant of isoprene synthase (IspS), the pool size of its substrate dimethylallyldiphosphate (DMADP), and to separate the component processes of the temperature dependence of isoprene emission. Temperature transients indicated that over the temperature range 25°C to 45°C, IspS was thermally stable and operated in the linear range of its substrate DMADP concentration. The in vivo rate constant of IspS obeyed the Arrhenius law, with an activation energy of 42.8 kJ mol 21 . In contrast, steady-state isoprene emission had a significantly lower temperature optimum than IspS and higher activation energy. The reversible temperature-dependent decrease in the rate of isoprene emission between 35°C and 44°C was caused by decreases in DMADP concentration, possibly reflecting reduced pools of energetic metabolites generated in photosynthesis, particularly of ATP. Strong control of isoprene temperature responses by the DMADP pool implies that transient temperature responses under fluctuating conditions in the field are driven by initial DMADP pool size as well as temperature-dependent modifications in DMADP pool size during temperature transients. These results have important implications for the development of process-based models of isoprene emission.

Photosynthesis rate (An) becomes unstable above a threshold temperature, and the recovery upon return to low temperature varies because of reasons not fully understood. We investigated responses of An, dark respiration and chlorophyll fluorescence to supraoptimal temperatures of varying duration and kinetics in Phaseolus vulgaris asking whether the instability of photosynthesis under severe heat stress is associated with cellular damage. Cellular damage was assessed by Evans blue penetration (enhanced membrane permeability) and by H2O2 generation [3,3Ј-diaminobenzidine 4HCl (DAB)-staining]. Critical temperature for dark fluorescence (F0) rise (TF) was at 46-48°C, and a burst of respiration was observed near TF. However, An was strongly inhibited already before TF was reached. Membrane permeability increased with temperature according to a switch-type response, with enhanced permeability observed above 48°C. Experiments with varying heat pulse lengths and intensities underscored the threshold-type loss of photosynthetic function, and indicated that the degree of photosynthetic deterioration and cellular damage depended on accumulated heat-dose. Beyond the 'point of no return', propagation of cellular damage and reduction of photosynthesis continued upon transfer to lower temperatures and photosynthetic recovery was slow or absent. We conclude that instability of photosynthesis under severe heat stress is associated with timedependent propagation of cellular lesions.

In growing leaves, lack of isoprene synthase (IspS) is considered responsible for delayed isoprene emission, but competition for dimethylallyl diphosphate (DMADP), the substrate for both isoprene synthesis and prenyltransferase reactions in photosynthetic pigment and phytohormone synthesis, can also play a role. We used a kinetic approach based on post-illumination isoprene decay and modelling DMADP consumption to estimate in vivo kinetic characteristics of IspS and prenyltransferase reactions, and to determine the share of DMADP use by different processes through leaf development in Populus tremula. Pigment synthesis rate was also estimated from pigment accumulation data and distribution of DMADP use from isoprene emission changes due to alendronate, a selective inhibitor of prenyltransferases. Development of photosynthetic activity and pigment synthesis occurred with the greatest rate in 1-to 5-day-old leaves when isoprene emission was absent. Isoprene emission commenced on days 5 and 6 and increased simultaneously with slowing down of pigment synthesis. In vivo Michaelis-Menten constant (Km) values obtained were 265 nmol m −2 (20 μM) for DMADP-consuming prenyltransferase reactions and 2560 nmol m −2 (190 μM) for IspS. Thus, despite decelerating pigment synthesis reactions in maturing leaves, isoprene emission in young leaves was limited by both IspS activity and competition for DMADP by prenyltransferase reactions.

In water-stressed leaves, accumulation of neutral osmotica enhances the heat tolerance of photosynthetic electron transport. There are large diurnal and day-to-day changes in leaf sugar content because of variations in net photosynthetic production, respiration and retranslocation. To test the hypothesis that diurnal and day-to-day variations in leaf sugar content and osmotic potential significantly modify the responses to temperature of photosynthetic electron transport rate, we studied chlorophyll fluorescence rise temperatures (i.e. critical temperatures at break-points in fluorescence versus temperature response curves, corresponding to enhanced damage of PSII centers and detachment of pigment-binding complexes) in the dark at a background of weak far-red light ( T FR ) and under actinic light ( T L ), and responses of foliar photosynthetic electron transport rate to temperature using gas-exchange and chlorophyll fluorescence techniques in the temperate tree Populus tremula L. Sucrose and sorbitol feeding experiments demonstrated strong increases of fluorescence rise temperatures T FR and T L with decreasing leaf osmotic potential and increasing internal sugar concentration. Similar T FR and T L changes were observed in response to natural variation in leaf sugar concentration throughout the day. Increases in leaf sugar concentration led to an overall down-regulation of the rate of photosynthetic electron transport ( J ), but increases in the optimum temperature ( T opt ) of J . For the entire dataset, T opt varied from 33.8 °°°° C to 43 °°°° C due to natural variation in sugars and from 33.8 °°°° C to 52.6 °°°° C in the sugar feeding experiments, underscoring the importance of sugars in modifying the response of J to temperature. However, the correlations between the sugar concentration and fluorescence rise temperature varied between the days. This variation in fluorescence rise temperature was best explained by the average temperature of the preceding 5 or 6 days. In addition, there was a significant year-to-year variation in heat sensitivity of photosynthetic electron transport that was associated with year-to-year differences in endogenous sugar content. Our data demonstrate a diurnal variation in leaf heat tolerance due to changes in sugar concentration, but they also show that this short-term modification in heat tolerance is superimposed by long-term changes in heat resistance driven by average temperature of preceding days.Key-words : day-to-day variation, diurnal variability, heat stress, optimum temperature, osmotic potential, photosynthetic electron transport, sugar concentration.Abbreviations : F 0 , basal fluorescence yield; F m , maximum fluorescence yield; F m ′ , light-adapted maximum fluorescence yield; F S , steady-state light-adapted fluorescence; J , photosynthetic electron transport rate; PSI, photo-system I; PSII, photosystem II; Q , photosynthetic quantum flux density; T , temperature; T D , fluorescence rise temperature (break point) in dark without far-red light; T FR , fluo...