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Notes: Isoprene emission from plants is highly temperature sensitive and is common in forest canopy species that experience rapid leaf temperature fluctuations. Isoprene emission declines with temperature above 35 °C but the temperature at which the decline begins varies between 35 and 44 °C. This variability is caused by the rate at which leaf temperature is increased during measurement with lower temperatures associated with longer measurement cycles. To investigate this we exposed leaves of red oak (Quercus rubra L.) to temperature regimes of 35–45 °C for periods of 20–60 min. Isoprene emission increased during the first 10 min of high temperature exposure and then decreased over the next 10 min until it reached steady state. This phenomenon was common at temperatures above 35 °C but was not noticeable at temperatures below that. The response was reversible within 30 min by lowering leaf temperature to 30 °C. Because there is no storage of isoprene inside the leaf, this behaviour indicates regulation of isoprene synthesis in the leaf. We demonstrated that the variability in isoprene decline results from regulation and explains the variability in the temperature response. This is consistent with our theory that isoprene protects leaves from damage caused by rapid temperature fluctuations.

Notes: The capacity for photosynthesis is often affected when plants are grown in air with elevated CO2 partial pressure. We grew Phaseolus vulgaris L. in 35 and 65 Pa CO2 and measured photosynthetic parameters. When assayed at the growth CO2 level, photosynthesis was equal in the two CO2 treatments. The maximum rate of ribulose-1,5-bisphosphate (RuBP) consumption was lower in plants grown at 65 Pa, but the CO2 partial pressure at which the maximum occurred was higher in the high-CO2-grown plants, indicating acclimation to high CO2. The acclimation of RuBP consumption to CO2 involved a reduction of the activity of RuBP carboxylase which resulted from reduced carbamylation, not a loss of protein. The rate of RuBP consumption declined with CO2 when the CO2 partial pressure was above 50Pa in plants grown under both CO2 levels. This was caused by feedback inhibition as judged by a lack of response to removing O2 from the air stream. The rate of photosynthesis at high CO2 was lower in the high-CO2-grown plants and this was correlated with reduced activity of sucrose-phosphate synthase. This is only the second report of O2-insensitive photosynthesis under growth conditions for plants grown in high CO2.

Notes: Abstract. Isoprene (2-methyl 1, 3-butadiene) is emitted from many plants, especially trees. We tested the effect of growth at high CO2 partial pressure and sun versus shade conditions on the capacity of Quercus rubra L. (red oak) and Populus tremuloides Michx. (quaking aspen) leaves to make isoprene. Oak leaves grown at high CO2 partial pressure (65 Pa) had twice the rate of isoprene emission as leaves grown at 40Pa CO2. However, aspen leaves behaved oppositely, with high CO2-grown leaves having just 60-70% the rate of isoprene emission as leaves grown in 40 Pa CO2. Similar responses were observed from 25 to 35 °C leaf temperature during assay. The stimulation of isoprene emission by growth at high CO2 and the stimulation in high temperature resulted in isoprene emission consuming over 15% of the carbon fixed during photosynthesis in high-CO2 grown oak leaves assayed at 35 °C. Leaves from the south (sunny) sides of trees growing in natural conditions had rates of isoprene emission double those of leaves growing in shaded locations on the same trees. This effect was similar in both aspen and oak. The leaves used for these experiments had significantly different chlorophyll a/b ratios indicating they were functionally sun (from the sunny locations) or shade leaves (from the protected locations). Because the metabolic pathway of isoprene synthesis is unknown, we are unable to speculate about how or why these effects occur. However, these effects are more consistent with metabolic control of isoprene release rather than a metabolic leak of isoprene from metabolism. The results are also important for large scale modelling of isoprene emission and for predicting the effect of future increases in atmospheric CO2 level on isoprene emission from vegetation.

Notes: We investigated the genetic control of cytosolic fructose-1,6-bisphosphatase (cytFBPase) activity, and the relationships between sucrose synthesis capacity and photosynthesis, growth, flowering and whole-plant carbon partitioning in Flaveria linearis Lag. F1; F2, and selfed lines generated from plants with low or high cytFBPase activity were used. CytFBPase activity was controlled by one gene and inherited co-dominantly, giving three classes of activity (low, intermediate and high). Reversed O2 sensitivity of photosynthesis, which indicates an end-product limitation on photosynthesis, was controlled by one gene and co-segregated with low cytFBPase activity. A low activity of cytFBPase decreased the growth rate. A recessive day-neutral flowering trait in Flaveria linearis did not co-segregate with cytFBPase activity. Plants with low cytFBPase activity had an increased shoot-to-root ratio, and flowering caused an additional shift in carbon partitioning to shoots only in plants with low cytFBPase activity. These data indicate that altering sucrose synthesis can affect photosynthesis and plant growth and development.

Notes: Isoprene (2-methyl 1,3-butadiene) is emitted from many plants, but the signals regulating isoprene emission are unknown. Mounting leaves in a gas exchange chamber or taking small leaf punches for biochemical analysis was found to reduce the rate of isoprene emission (Loreto & Sharkey 1993). This phenomenon was investigated by putting terminal leaflets of velvet bean (Mucuna deeringeniana L.) and kudzu [Pueraria lobaia (Willd) Ohwi.] into a gas exchange chamber and monitoring isoprene emission and photosynthesis. Lateral leaflets or remote leaves were then wounded or mechanically stimulated. The rate of isoprene emission was reduced after 1 min by up to 75% by burning a lateral leaflet with a match. Even a 7 ms−1 (25km h−1) wind imposed on a lateral leaflet reduced isoprene emission from the terminal leaflet by 18%. Photosynthesis rates were either unaffected by these treatments or reduced more slowly than isoprene emission rates, indicating that the effect of isoprene emission rates was not a consequence of changes in photosynthetic activity. Isoprene emission from a terminal leaflet was reduced by burning leaves above and below the monitored leaflet when on the same stem. The effect was much reduced if the burned leaf (all three leaflets) was on a different stem from the monitored leaflet. Reduction of the rate of isoprene emission was observed even when the burned leaf was 52 cm distant from the measured leaflet. Increasing the distance between the stressed leaf and the monitored leaf caused the effect to be slower and smaller. It is speculated that a signal is generated by wounding which propagates through the plant at 1.3 mm s−1. This velocity was consistent throughout the measurements and is similar to the rate of propagation of electrical signals such as action potentials and variation potentials. The effect of the environmental stress, and particularly the wind effect, can be frequent in nature and should be considered when estimating local and regional emission of isoprene for modelling atmospheric chemistry. If leaf samples used for isoprene determination are exposed to the type of stress we investigated, isoprene emission inventories based on leaf level measurements will be underestimated.

Notes: Isoprene is emitted from the leaves of many plants in a light-dependent and temperature-sensitive manner. Plants lose a large fraction of photo-assimilated carbon as isoprene but may benefit from improved recovery of photosynthesis following high-temperature episodes. The capacity for isoprene emission of plants in natural conditions (assessed as the rate of isoprene emission under standard conditions) varies with weather. Temperature-controlled greenhouses were used to study the role of temperature and light in influencing the capacity of oak leaves for isoprene synthesis. A comparison was made between the capacity for isoprene emission and the accumulation of other compounds suggested to increase thermotolerance of photosynthesis under two growth temperatures and two growth light intensities. It was found that the capacity for isoprene emission was increased by high temperature or high light. Xanthophyll cycle intermediates increased in high light, but not in high temperature, and the chloroplast small heat-shock protein was not expressed in any of the growth conditions. Thus, of the three thermotolerance-enhancing compounds studied, isoprene was the only one induced by the temperature used in this study.

Notes: Isoprene emission from plants accounts for nearly half of all non-methane hydrocarbons entering the atmosphere. Light and temperature regulate the instantaneous rate of isoprene emission but there is increasing evidence that they also affect the capacity for isoprene emission (i.e. the rate measured under standard conditions). We tested the rate of acclimation of the capacity for isoprene emission following step changes in growth conditions. Acclimation to new growth temperatures was very rapid, with most of the change occurring within a few hours and complete adjustment occurring within a day. Acclimation to new light levels was more complicated. Following a switch from low-light growth conditions to standard assay conditions (30 °C and 1000 µmol photons m−2 s−1), there was a rapid (5–10 min) and a slightly slower (10–50 min) acclimation of the capacity for isoprene emission. After accounting for these short-term changes, there was also a small, long-term (4–6 d) acclimation of the isoprene emission capacity to the light level of growth conditions. We found no effect of growth conditions on the coefficients used to describe the instantaneous light and temperature response of isoprene emission. Therefore, current models of isoprene emission will only need to be altered to account for changes in the capacity for isoprene emission.

Notes: Isoprene (C5H8) is emitted from many plants and has a substantial effect on atmospheric chemistry. There are several models to estimate the rate of isoprene emission used to calculate the impact of isoprene on atmospheric processes. The rate of isoprene synthesis will depend either on the activity of isoprene synthase or the availability of its substrate dimethylallyl pyrophosphate (DMAPP). To investigate long-term regulation of isoprene synthesis, the isoprene emission rate of 15 kudzu leaves was measured. The chloroplast DMAPP level of the five leaves with the highest emission rates and the five leaves with the lowest rates were determined by non-aqueous fractionation of the bulked leaf samples. Leaves with high basal emission rates had low levels of DMAPP whereas leaves with low basal emission rates had high DMAPP levels in their chloroplasts indicating that the activity of isoprene synthase exerts primary control over the basal emission rate. To investigate short-term regulation, isoprene precursors were fed to leaves. Feeding dideuterated deoxyxylulose (DOX-d2) to Eucalyptus leaves resulted in the emission of dideuterated isoprene. Results from DOX-d2 feeding experiments indicated that control of isoprene emission rate was shared between reactions upstream and downstream of the DOX entry into isoprene metabolism. In CO2-free air DOX always increased isoprene emission indicating that carbon availability was an important control factor. In N2, isoprene emission stopped and could not be recovered by adding DOX-d2. Taken together, these results indicate that the regulation of isoprene emission is shared among several steps and the relative importance of the different steps in controlling isoprene emission varies with conditions.

Notes: 
D, deuterium
δD(NMR), chemical shift axis in a deuterium NMR spectrum
F6P, fructose-6-phosphate
G6P, glucose-6-phosphate
IRMS, isotope ratio mass spectrometry
NMR, nuclear magnetic resonance
PGI, phosphoglucose isomerase

Intramolecular deuterium distributions of the carbon-bound hydrogens of glucose were measured using deuterium nuclear magnetic resonance. Glucose isolated from leaf starch of common bean (Phaseolus vulgaris cv. Linden) or spinach (Spinacia oleracea cv. Giant nobel) was depleted in deuterium in the C(2) position, compared with glucose isolated from leaf sucrose or bean endosperm starch. In beans, the depletion of C(2) was independent of the light intensity during growth (150 or 700 μmol photons s–1 m–2). The ratio of glucose-6-phosphate to fructose-6-phosphate ([G6P]/[F6P]) in bean chloroplasts was 0·9 in high light, indicating that the phosphoglucose isomerase reaction was not in equilibrium ([G6P]/[F6P]) ≈ 3). This implies that the kinetic isotope effect of phosphoglucose isomerase depleted deuterium in the C(2) position of G6P. Because the depletion was the same, the chloroplastic ([G6P]/[F6P]) ratio was in disequilibrium irrespective of the light intensity. If the ([G6P]/[F6P]) ratio was in equilibrium, a large chloroplastic pool of G6P would be unavailable for regeneration of ribulose-1,5-bisphospate. We argue that chloroplast phosphoglucose isomerase activity is regulated to avoid this. The deuterium depletion of C(2) explains the known low overall deuterium abundance of leaf starch. This example shows that measurements of intramolecular deuterium distributions can be essential to understand overall deuterium abundances of plant material.