Physiological and transcriptomic modifications induced by atmospheric and soil water deficits in robusta coffee

Water deficit is a wide-spread limiting factor in coffee production, but the development of adapted cultivars is hampered by a limited understanding of the physiological tolerance mechanism and the narrow genetic base of plant material used in breeding. Therefore, the objective of this study was to identify potential genetic and physiological markers for drought tolerance. Atmospheric water vapour (humidity) is an important environmental element that influences plant growth by conditioning directly transpiration and other gas exchange.The effect of atmospheric water deficit on plants has mostly been neglected in climate impact studies. A sudden change in relative humidity (RH) may cause significant damage to leaves. Here, we explore alterations in physiology and gene expression in leaves of Coffea canephora caused by a sharp drop in relative humidity (RH) at three different temperatures. Stomatal conductance (gs) and CO2 assimilation (A) fell quickly at all temperatures after transfer to low RH. Reductions in leaf relative water content (RWC) confirmed the leaves experienced substantial water losses, and that stomatal closure and/or water re-supply was not fast enough to avoid significant evaporative losses. At 27°C and 35°C, upper leaves showed significant decreases in Fv/Fm compared to lower leaves, suggesting a stronger impact on the upper leaves. At 42°C, both upper and lower leaves showed similar strong decreases in Fv/Fm. Quantitative expression analysis of genes for transcription factors linked with dehydration stress, ABA signalling, heat stress, and senescence showed that temperature-dependant, transcriptional changes were associated with the ‘HuS’ treatments.Whilst there has been a little focus on atmospheric water deficits and elevated temperatures, more is known about the impacts of soil water deficits. To compare the physiological and gene expression responses induced by a ‘HuS’ with those induced by a soil water deficit, we subjected Robusta coffee plants to a classical soil water deficit experiment in a growth chamber. The results obtained from this comparative experiment indicate a number of similar responses to atmospheric and soil water deficits. We also presented the results obtained from an RNA-Seq analysis of gene expression during the soil water deficit experiment. Finally, the data obtained was discussed in relation to similarities and differences in the physiological and gene expression responses of Robusta coffee to atmospheric and soil water deficits.Overall, the data shows that Robusta plants subjected to a water deficit exhibit major temperature-dependant alterations in leaf physiology and important changes in the expression of genes associated with abiotic stress. The results presented suggest more detailed studies on the combined effects of water deficit and high temperature are warranted.