Hydraulic limitations in different plant species

• Ano de defesa: 2015
• Autor(a) principal: Martins, Samuel Cordeiro Vitor
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa
• Palavras-chave em Português: Fisiologia vegetal; Plantas - Relação hídrica; Controle estomático; Fisiologia Vegetal
• Link de acesso: http://www.locus.ufv.br/handle/123456789/7530

Hydraulic traits such as leaf hydraulic conductance (Kleaf) and leaf capacitance (Cdyn) define the xylem operational limits at the leaf level. Despite stomata control in ferns and conifers having been proposed to be regulated via a hydropassive model, it remains untested whether hydraulic traits can successfully predict stomata behaviour to changes in vapour pressure deficit (VPD). Additionally, little is known on how hydraulic traits influence leaf water enrichment at the sites of evaporation and the mixing of enriched and unenriched water within a leaf. In a first experiment, we examined the stomata response to stepwise increases in VPD in two ferns and four conifers structurally different covering a large range in Kleaf and Cdyn. Water vapour and liquid fluxes were also measured in order to determine the online leaf water potential (Ψl) and its relationship with stomatal conductance (gs). We found that different KL and Cdyn led to distinct balances between liquid and vapour phase significantly impacting stomata responsiveness as seen by the differences in stomata closure half-times ranging from 48 to 248 seconds. The hydraulic passive model successfully modelled stomata response to VPD in all species studied. Furthermore, considering a changing rather than a fixed Kleaf improved model predictions suggesting VPD-induced changes in Kleaf. In any case, the extents of changes were small and in agreement with recent models predicting a shift from perivascular to peristomatal transpiration in response to increases in VPD. In a second experiment, we studied leaf water enrichment in 18 O during transpiration in four species with contrasting Kleaf and Cdyn. Additional evidence in support for the two pool model in three out of four species was found. Our data also suggest a possible interplay between vein density and associated ground tissues as determinants of the fraction of unenriched water. Moreover, we suggest that a low Kleaf, leading to an increased radial resistance for water transport in ferns, can have a role constraining the mixing of enriched and unenriched leaf water. In a third experiment, leaf hydraulic vulnerability was assessed in two field-grown coffee cultivars under a severe drought to test coffee susceptibility to hydraulic dysfunctions. Coffee leaves were characterized as moderately tolerant to hydraulic dysfunctions; however, the large negative Ψl experienced under drought were sufficient to cause hydraulic failure and leaf loss. Upon rainfall, An and gs recovery were constrained at different extents probably as a result of hydraulic loss given that no evidence for biochemical limitations to An was found. In any case, after two months of rainfall, full recovery of An, but not gs, was observed in leaves expanded under drought in comparison to leaves expanded in the rainy season. Under wet conditions, stomata aperture seems to be regulated to prevent loss of conductivity of reaching levels higher than c. 30% by means of active mechanisms likely ABA-related. However, a high variability in Ψl experienced under drought suggests that differential leaf sensitivity to ABA exists as some leaves cannot reach sufficient stomata closure to avoid damaging Ψl to occur. In conclusion, we showed that leaf anatomy, through changes in Cdyn and/or water content, has a significant effect on the speed of stomata movements in ferns and conifers leading to closure rates as fast as those seen in angiosperms, in addition to affect leaf water enrichment properties. In coffee, further ABA-sensitivity studies are necessary to better elucidate the contribution of passive and active mechanisms controlling coffee stomata. Most importantly, studies to determine hydraulic vulnerability in stems and roots as well in other coffee varieties will be of extreme importance to a proper assessment of the impact climate change will have for the coffee crop.