The ¹⁸O‐signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms

Abstract

The 18O signature of atmospheric water vapour (δ18OV) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18O‐signal transfer differs among plant species and growth forms. We performed a 9‐hr greenhouse fog experiment (relative humidity ≥ 98%) with 18O‐depleted water vapour (−106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18O‐signal transfer by calculating the mean residence time of O in leaf water (MRTLW) and sugars (MRTSugars) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRTSugars was shorter in C3 and C4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRTSugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ18OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18OV variations, which is important for the interpretation of δ18O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

Improving the extraction and purification of leaf and phloem sugars for oxygen isotope analyses

“…In addition, myo-inositol, which has also been found in phloem samples, shows only minor isotopic variations during a growing season. 49,50 Such compounds do not exchange their oxygen atoms as they lack the carbonyl group needed for it, 51,52 but can be 18 O-depleted compared with hexoses and sucrose. 12 An increase in their concentration may thus additionally cause an 18 O-depletion in the bulk phloem sugar fraction.…”

Leaf water δ

“…Precipitation and source water inputs are usually depleted in 18 O by around -5 to -10‰ relative to the Vienna standard mean ocean water (VSMOW) mass spectrometric standard but varies on a seasonal basis (Cernusak et al 2016). For C 3 plants, leaf water is normally evaporatively enriched in 18 O during transpiration, and then subject to an additional biochemical enrichment of some +27‰ when transferred into organic material (Sternberg et al 2006;Cernusak et al 2016;Lehmann et al 2020).…”

“…The d 18 O signal in leaf water and organic material in bromeliads reflects the relative inputs from contrasting water sources such as precipitation, tank water (which may become evaporatively enriched between rain events) and atmospheric water vapour (Farquhar and Cernusak 2005;Seibt et al 2008;Cernusak et al 2016;Lehmann et al 2020;Benettin et al 2021).…”

“…The net efflux of water vapour during gas exchange normally leads to evaporative enrichment of 18 O in leaf water (d 18 O lw ) (Harwood et al 1998), whereas water vapour ingress under high ambient humidity can lead to more depleted d 18 O lw (Farquhar and Cernusak 2005), allowing niche specialisation to be defined for C 3 and CAM epiphytic bromeliads (Helliker and Griffiths 2007;Reyes-García et al 2008;Helliker 2011;Lehmann et al 2020). Provided that the interplay between water sources, water use and degree of steady-state equilibration for isotopic signals in both C 3 and CAM tissues can be well defined, 18 O signals in both leaf water (d 18 O lw ) and organic material (d 18 O OM ) of epiphytes can be used to model climatic conditions along altitudinal and latitudinal gradients for the epiphytic bromeliad Tillandsia usneoides Sw. (Helliker 2014).…”

“…We expected that CAM species would be more prevalent in the drier lowlands, with a higher abundance of C 3 species in higher elevation forest systems. The contrasting climatic conditions and water source inputs along the altitudinal gradient allowed us to test the hypothesis that net water vapour uptake under high humidity would re-set the d 18 O lw for epiphytes (Farquhar and Cernusak 2005;Helliker and Griffiths 2007;Reyes-García et al 2008;Seibt et al 2008;Lehmann et al 2020). For C 3 bromeliads, we hypothesised that that d 18 O lw would show evaporative enrichment in lowland habitats, but would be more depleted at high altitude cloudforest habitats due to net water vapour inputs.…”