L’impatto dello stress idrico e delle alte temperature sull’emissione di isoprene, sulla fotosintesi e respirazione
Author(s)
Brilli, Federico
Date Issued
April 23, 2008
Type
Doctoral Thesis
Abstract
Isoprene is the most abundant volatile organic compounds (VOC) released by plants in the atmosphere (44% of the total amount of VOCs) (Gunther et al. 1995; Kesselmeier & Staudt, 1999). Isoprene influences the oxidative capacity of the troposphere (Zimmerman et al. 1978) and is a substrate for ozone production in polluted air (Chameides et al. 1988). Isoprene is mainly synthesized in the chloroplasts (Lichtenthaler 1999) from photosynthetic intermediates in a reaction catalyzed by isoprene synthase (ISPS) (Silver & Fall, 1995; Schnitzler et al. 1996). Incoming Global Change events leading to environmental stress conditions such as drought coupled with rising temperatures (and with high ozone concentration), may:
- strongly influence isoprene biosynthesis with various consequences on its emission;
- have contrasting effects on biogeochemical carbon cycles regarding photosynthetic carbon assimilated (and respired) by forests.
To investigate the impact of the interaction between rising temperatures and water stress on isoprene emission (IE), photosynthesis (A) and respiration (R), one-year-old Populus nigra L. saplings were first grown under two different temperatures (25 C° and 35 C°), and then exposed to progressive limiting soil water content. During progressive drought stress conditions, (IE), (A) and (R) were monitored and referred both to soil water availability (FASW) and to the amount of transpirable soil water (FTSW). Moreover, to investigate the biochemical regulation of (IE), proton-transfer-reaction mass spectrometer (PTR-MS) (Lindinger et al. 1998) was used to record the appearance of individually 13C-labeled atoms in the unfragmented isoprene molecule following exposure to air containing 13CO2.
Experimental results showed that in well-watered non-stressed plants higher growing temperature (35 C°) doubled IE and decreased A by ~ 30%, while R showed a not significant increase of ~ 15%. These results pointed out that both IE and A did not acclimate to a 10 °C higher growing temperature whereas R seemed likely to acclimate (Ow et al. 2008). Drought did increase the (IE) to (A) ratio, because (IE) was inhibited with a slower kinetics than (A), both at 25 °C and at 35 °C. FTSW calculation addressed that the percentage of carbon lost as (IE) dramatically increased earlier at 35 °C than at 25 °C. In unstressed leaves (IE) was temporary stimulated when (A) dropped at progressively stronger levels of drought at both temperatures, before starting to decrease to minimum levels. In plants grown at 25 °C, even when carbon fixation by photosynthesis was compensated by respiratory processes under limiting soil water availability, isoprene was emitted at a 30% rate of the emission observed in irrigated controls. Differently from pre-stress conditions, at
a severe drought stage only 16 – 42% of the (IE) was labeled by 13CO2 suggesting that extrachloroplastic carbon sources may increasingly supply carbon to isoprene when (A) became to be limited (Brilli et al. 2007). In severely drought-stress leaves, photosynthesis independent-(IE) was also independent of temperature, being the rate of emission similar at 25 °C and 35 °C. After re-watering, (IE) recovered as quickly as (A), but even if plants completely have recovered (A) recorded before drought stress, (IE) did not reach the pre-stress levels, particularly in leaves grown at 35 °C. Even though my experimental results supported previous evidences that isoprene emission is resistant to moderate drought stress conditions (Loreto & Sharkey, 1993; Fang et al. 1996, Brilli et al. 2007), for the first time it has highlighted that drought events are able to suppress temperature sensitivity of the process of isoprene biosynthesis, with important ecological consequences in reducing the emission of high emitters (Fortunati et al. 2008).
Furthermore, to assess the antioxidant properties of isoprene emission, one-year-old Populus nigra L. saplings grown at 25 °C were fumigated with high ozone concentration (150 ± 10 ppb) under progressive drought stress conditions. Ozone treatment showed a strong effect decreasing (IE), exacerbated by drought stress occurrence. Even if not in agreement with previous results (Loreto & Velikova, 2001; Fares et al. 2007), reduced (IE) can be mainly explained by serious (A) impairment (Calfapietra et al. 2007), or by direct isoprene reaction with ozone uptaken by leaves (Loreto & Fares, 2007). In addition, ozone exposure increased (IE) and decreased methanol emission in developing young leaves indicating a possible early senescence stage induction (Fares et al, 2007).
Enhanced thermotolerance by isoprene was studied in leaves of 1- or 2- years-old Platanus orientalis L. plants grown under normal and high CO2 levels. In both cases, after a high temperature treatment (38 °C for 4 h), photosynthesis activity was preserved in isoprene emitting leaves compared to leaves in which isoprene was previously inhibited by fosmidomicyn.
In conclusion, my results confirmed isoprene as an important compound in non-enzymatic defence of plants against oxidative stresses (Sharkey, 2005), possibly contributing to scavenge reactive oxygen species (ROS) (Loreto & Velikova, 2001) and to membranes stabilization capacity, leading to increase stress tolerance (Sharkey & Singsaas, 1995). In addition, the same results indicated that drought stress may have important and unforeseen consequences in regulating isoprene-mediated plant interaction with the global changing environment.
Additional information
Dottorato di ricerca in Ecologia forestale
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