Nocturnal water relations and predawn water potential disequilibrium in temperate deciduous tree species
Failid
Kuupäev
2020-07-13
Autorid
Ajakirja pealkiri
Ajakirja ISSN
Köite pealkiri
Kirjastaja
Abstrakt
Tavapäraselt uuritakse taimi päevasel ajal, kui toimub fotosüntees ning intensiivne veevahetus taime ja keskkonna vahel. Kuid paaril viimasel kümnendil on uurijate hulgas olulist tähelepanu pälvinud ka öine ja eriti koidueelne taimede veevahetus. Senini ei ole selgelt teada, millised faktorid ja kuidas mõjutavad öist ksüleemivoolu ehk vee liikumist vartes, lehtedel asuvate õhulõhede avatust öösel ja enne koitu ning lehe koidueelset veepotentsiaali, mida sageli kasutatakse mulla veestaatuse hinnanguna.
Kasvuhoones üheksa parasvöötme puuliigi peal läbi viidud katse näitas, et mulla piisava veevarustuse korral on kiiremakasvuliste pioneerliikide öine õhulõhede juhtivus kõrgem kui aeglasekasvulistel liikidel. See reegel kehtib liikide puhul, mis asustavad parasvöötme niiskemaid ja jahedamaid piirkondi: harilik haab (Populus tremula), arukask (Betula pendula) ja sanglepp (Alnus glutinosa). Põuatingimustes liikide vahel erinevused õhulõhede juhtivuses kaovad. Eestis läbi viidud eksperimendi käigus kogutud andmed demonstreerisid hübriidhaaval (Populus tremula × P. tremuloides) erinevust lehe ja mulla veepotentsiaalide vahel ja öise ksüleemivoolu esinemist. Mõlema tunnuse positiivne sõltuvus atmosfääri kuivusest viitab selgelt asjaolule, et sel liigil on õhulõhed öösel avatud. Sellest tulemusest järeldub, et kuivemate ja soojemate ööde korral ei saa koidueelset lehe veepotentsiaali kasutada mulla veestaatuse hinnanguna. Lisaks tõestasid meie eksperimendid, et hübriidhaab on võimeline omastama kastevett lehtede pinnalt, mis võib koidueelset veepotentsiaali omakorda vastupidiselt mõjutada. Kasvukambris läbi viidud katses näitasid kõrge päevase õhuniiskuse tingimustes kasvanud hübriidhaavad, et nende öine ksüleemivool suureneb, kui järsult õhuniiskust vähendada. Mõõduka õhuniiskuse käes kasvanud puudel sellist reaktsiooni ei täheldatud. See viitab kõrgema õhuniiskusega keskkonnas kasvanud puude nõrgenenud õhulõhede kontrollile. Välikatsetes sellist efekti ei tuvastatud. Võttes arvesse nii regionaalseid kui globaalseid kliimamuutuste trende, on väga tõenäoline, et suureneb nii puude öise transpiratsiooni osakaal kogu ööpäevases veekaos kui ka vajadus arvestada öise transpiratsiooniga ökosüsteemide veebilansi arvutustes.
Plants are usually studied during daytime when photosynthesis and intensive water exchange between the plant and the environment takes place. However, nocturnal and predawn water relations of plants have also gained considerable attention during recent decades. Nevertheless, there are still uncertainties what factors and how may affect characteristics like nocturnal sap flow, nocturnal and predawn stomatal conductance, and predawn leaf water potential – a characteristic commonly used for estimating soil water status. The aim of this thesis was to compare nocturnal stomatal conductance and predawn water potential disequilibrium between leaf and soil (PDD) in temperate deciduous tree species differing in ecological demands and life strategies; to elucidate the environmental factors that govern nocturnal transpiration and PDD; and to test whether increasing daytime atmospheric humidity – a climate trend predicted for high northern latitudes – affects nocturnal water use in broadleaved trees. An experiment in nine temperate tree species in a greenhouse demonstrated that under ample soil water availability, fast-growing pioneer species exhibited higher night-time stomatal conductance compared to slow-growing late-successional species. That was true about the species originating from more humid and cooler regions (e.g. Populus tremula, Betula pendula, Alnus glutinosa). In drought conditions the differences in stomatal conductance between the species disappeared. Field studies carried out in Estonia proved the occurrence of significant PDD and nocturnal sap flow in hybrid aspen (Populus tremula × P. tremuloides). Positive effect of nocturnal atmospheric evaporative demand on both characteristics is a solid proof of open stomata at night. This suggests that under drier and warmer atmospheric conditions predawn leaf water status cannot be used as a reliable estimate of soil water status. Our experiments proved that hybrid aspen trees are able to absorb dew water from leaf surface, exhibiting foliar water uptake, which might also bias predawn leaf water status. In a growth chamber experiment, hybrid aspen saplings grown under elevated air humidity responded with increased sap flux density to a rise in atmospheric evaporative demand at night, which evidences weakened stomatal control over water loss, although the field studies did not support that finding. Taking into account the regional and global climate trends, it is very likely that the proportion of nocturnal water losses in total daily transpiration of trees as well as the necessity for considering nocturnal transpiration in calculations of landscape or ecosystem water balance will increase in the future.
Plants are usually studied during daytime when photosynthesis and intensive water exchange between the plant and the environment takes place. However, nocturnal and predawn water relations of plants have also gained considerable attention during recent decades. Nevertheless, there are still uncertainties what factors and how may affect characteristics like nocturnal sap flow, nocturnal and predawn stomatal conductance, and predawn leaf water potential – a characteristic commonly used for estimating soil water status. The aim of this thesis was to compare nocturnal stomatal conductance and predawn water potential disequilibrium between leaf and soil (PDD) in temperate deciduous tree species differing in ecological demands and life strategies; to elucidate the environmental factors that govern nocturnal transpiration and PDD; and to test whether increasing daytime atmospheric humidity – a climate trend predicted for high northern latitudes – affects nocturnal water use in broadleaved trees. An experiment in nine temperate tree species in a greenhouse demonstrated that under ample soil water availability, fast-growing pioneer species exhibited higher night-time stomatal conductance compared to slow-growing late-successional species. That was true about the species originating from more humid and cooler regions (e.g. Populus tremula, Betula pendula, Alnus glutinosa). In drought conditions the differences in stomatal conductance between the species disappeared. Field studies carried out in Estonia proved the occurrence of significant PDD and nocturnal sap flow in hybrid aspen (Populus tremula × P. tremuloides). Positive effect of nocturnal atmospheric evaporative demand on both characteristics is a solid proof of open stomata at night. This suggests that under drier and warmer atmospheric conditions predawn leaf water status cannot be used as a reliable estimate of soil water status. Our experiments proved that hybrid aspen trees are able to absorb dew water from leaf surface, exhibiting foliar water uptake, which might also bias predawn leaf water status. In a growth chamber experiment, hybrid aspen saplings grown under elevated air humidity responded with increased sap flux density to a rise in atmospheric evaporative demand at night, which evidences weakened stomatal control over water loss, although the field studies did not support that finding. Taking into account the regional and global climate trends, it is very likely that the proportion of nocturnal water losses in total daily transpiration of trees as well as the necessity for considering nocturnal transpiration in calculations of landscape or ecosystem water balance will increase in the future.
Kirjeldus
Väitekirja elektrooniline versioon ei sisalda publikatsioone
Märksõnad
water exchange, temperate regions, deciduous trees