Nitrogen fertilization and δ¹⁸O of CO₂ have no effect on ¹⁸O‐enrichment of leaf water and cellulose in Cleistogenes squarrosa (C₄) – is VPD the sole control?

Abstract

The oxygen isotope composition of cellulose (δ O ) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ O of CO (δ O ), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ O , O-enrichment of leaf water (Δ O ) and cellulose (Δ O ) relative to source water, and p p , the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C grass (Cleistogenes squarrosa). δ O and N supply, and their interactions with VPD, had no effect on δ O , Δ O , Δ O and p p . Δ O and Δ O increased with VPD, while p p decreased. That VPD-effect on p p was supported by sensitivity tests to variation of Δ O and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not O relations; conversely, VPD had no effect on growth or morphology, but controlled O relations. The work implies that reconstructions of VPD from Δ O would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p p is ignored. Progress in understanding the relationship between Δ O and Δ O will require separate investigations of p and p and of their responses to environmental conditions.

Effects of nitrogen and vapour pressure deficit on phytomer growth and development in a C4 grass

“…The main variation in plant morphology was observed in tiller development and growth, namely, the number of tillers was significantly increased by 23 % by N supply. These results are in line with the findings that whole-plant relative growth rate of C. squarrosa increased following N addition but was not affected by VPD (Liu et al 2016). A positive effect of N supply on the tillering of grasses is a common response (Longnecker et al 1993; Baethgen et al 1995; McKenzie 1998), as N fertilizer promotes the outgrowth of axillary meristems to produce more tillers (McSteen 2009).…”

“…oxygen isotope composition of leaf cellulose reflects mainly source water isotopic signal and evaporative demand (Helliker and Ehleringer 2002). Accordingly, oxygen isotope composition of leaf cellulose of C. squarrosa was shown to reflect VPD of the growth environment (Liu et al 2016). For this type of studies, the predictability of phytomer development is essential for the dating of isotopic signals imprinted in plant material.…”

The δ18O and δ2H of water in the leaf growth‐and‐differentiation zone of grasses is close to source water in both humid and dry atmospheres

“…Concerning the δ 18 O of cellulose, if we use a p x 18O of 0.94 for C. squarrosa (this work), the mean p ex 18O (proportion of oxygen in cellulose that has exchanged with LGDZ water) was 0.44 in the work of Liu et al . (), matching well – on average – the most commonly accepted plausible range of 0.4 to 0.5 (e.g. Cernusak et al ., ; Barbour, ).…”

“…A modified Hoagland nutrient solution, that corresponded to the high nitrogen (N) solution described by Liu et al . (), was supplied to each container, and maintained c . 7 cm deep in each container by adding new nutrient solution every 2 d until day 30; thereafter nutrient solution was added once daily.…”

“…To date, the oxygen and hydrogen isotope composition of water in the leaf growth‐and‐differentiation zone (LGDZ) of grasses (δ 18 O LGDZ and δ 2 H LGDZ ) is unknown (Liu et al ., ). Knowledge of δ 18 O LGDZ and δ 2 H LGDZ is, however, necessary for understanding the biophysical and biochemical mechanisms determining the oxygen isotope composition of leaf cellulose (δ 18 O Cellulose ) and the hydrogen isotope composition of leaf cuticular wax (δ 2 H Wax ), which are thought to store important (paleo)climatological, environmental, geographic and physiological information (Barbour & Farquhar, ; Helliker & Ehleringer, ,b; Kahmen et al ., ).…”