Investigating The Role Of Volatile Signalling In Plant Responses To Drought

Abstract

Volatiles released by plants are becoming important to understand how plants may exchange information. With a wide chemical variety, plant derived volatiles have been shown to be used by plants for pollination, and defence against biotic stress. In a drought stress situation, where stomata play a central role in tolerance, plants have been observed to have their volatile emission decreased, associated with stomatal closure, as well as increased emission of other volatiles. However, the specific functions of the volatiles remain obscure. In many studies on plant responses to drought and rehydration, but without volatile analysis, a particular phenomenon has been observed where the well-watered plants displayed a drought-like response similar to the water-stressed plants when co-located in the same environment. Indeed, while a reduction of stomatal conductance (gs) of plants under water deficit is an expected response, it is not for plants with continuous adequate watering. Thus, the main hypothesis to be tested by this thesis is that volatiles are released by water-stressed plants that induce stomatal closure in nearby well-watered plants. Supposedly, the waterstressed plants would emit volatiles triggering a closure of stomata of the nearby plants in order to preserve water in the likely event of further reduced water availability. To test the hypothesis, Vitis vinifera and Arabidopsis thaliana potted plants were examined in three configurations of drought/rehydration experiments: i) having well-watered (WW) and water-stressed (WS) treatments together, ii) separating the treatments with custom-made individual plastic chambers and, iii) having both treatments together and a separate growth cabinet for controls. For each configuration, volatiles were extracted with solid-phase micro-extraction (SPME) using DVB/CAR/PDMS coated fibres which were desorbed and analysed on a gas chromatogram combined with a mass spectrometer (GC-MS). All results combined tend to support the hypothesis of the gs of WW plants not being stable during the severe stress phase applied on the WS group, and supported by multilinear regression analysis showing a stronger effect of WS gs on WW gs than light or VPD. When WS grapevines were enclosed in chambers, this interaction was not evident. The volatile samples revealed a change in the emission profile during the drought stress phase and some volatiles showed strong significant correlations with WW and WS gs. Especially, 1,2,3- trimethylbenzene was significantly negatively correlated with gs for both WW and WS plants. The last part of this study was to develop a method to test the effect of volatile(s) on single leaf stomatal regulations avoiding the use of leaf-attached chambers common for leaf gas exchange analysis that are restrictive with monitoring released or applied volatiles. By connecting the petiole of a detached leaf to a sensitive liquid flow meter, responses to volatiles could be determined while simultaneously monitoring some alcohols in real-time with gas sensors. The placement of two sensors one close and one further from the leaf surface, allowed detection of changes in concentration of externally applied volatile alcohols as well as those released from the leaf. Results showed similar responses to normal conditions over time, light-to-dark transitions and revealed a strong effect of volatile methanol that induced a rapid closure of stomata. The measurements of flow (Q) into the leaf were also compared with transpiration (E) from the leaf using an attached infra-red gas-analyser (IRGA). This revealed a potential problem with measuring gas exchange in Arabidopsis due to restriction of the petiole xylem by the seals on the IRGA chamber that was not evident with measurements on Vitis vinifera leaves. For the latter the E and Q were not always well correlated also indicative of a capacitance in the water pathway to the stomata. Despite these interesting effects, the technique may be developed further to enable routine testing of potential volatile signalling molecules that impact stomatal regulation.