Effects of grass leaf anatomy, development and light/dark alternation on the triple oxygen isotope signature of leaf water and phytoliths: insights for a new proxy of continental atmospheric humidity

Abstract

<p><strong>Abstract.</strong> Continental relative humidity (RH) is a key-climate parameter. However, there is a lack of quantitative RH proxies suitable for climate model-data comparisons. Recently, a combination of climate chamber and natural transect calibrations laid the groundwork for examining the robustness of the triple oxygen isotope composition (δ<sup>18</sup>O, δ<sup>17</sup>O) of phytoliths as a new proxy for past changes in RH. However, it was recommended that besides RH, additional factors that may impact δ<sup>18</sup>O and δ<sup>17</sup>O of plant water and phytoliths be examined. Here, the effects of leaf anatomy, leaf development stage and day/night alternations are addressed from the growth of the grass species <i>F. arundinacea</i> in climate chambers. Plant water and phytoliths are analyzed in δ<sup>18</sup>O and δ<sup>17</sup>O. Silicification patterns are examined using light and scanning electron observation of phytoliths. The isotope data show the increasing contribution of evaporated epidermal water to the bulk leaf water, from sheath to proximal and apical leaf blade. However, despite this isotope heterogeneity, δ<sup>18</sup>O and δ<sup>17</sup>O of the bulk leaf water can be predicted by the Craig and Gordon model, in the given experimental conditions (high RH). Regarding phytoliths, their forming water (mainly epidermal) is, as expected, more impacted by evaporation than the bulk leaf water. This discrepancy increases from sheath to proximal and apical blade and can be explained by the steepening of the radial concentration gradient of evaporated water along the leaf. However, we show that because most of silica polymerizes in epidermal long cells of the apical blade of the leaves, the δ<sup>18</sup>O and δ<sup>17</sup>O of bulk grass phytoliths should not be impacted by the diversity in grass anatomy. The data additionally show that most of silica polymerizes at the end of the leaf elongation stage and at the transition towards leaf senescence. Thus, climate conditions at that time should be considered when interpreting δ<sup>18</sup>O and δ<sup>17</sup>O of phytoliths from the natural environment. At least, no light/dark effect was detected on the δ<sup>18</sup>O and δ<sup>17</sup>O signature of plant water and phytoliths of <i>F. arundinacea</i>. However, when day/night alternations are characterized by significant changes in RH, the lowest RH conditions favoring evaporation and silica polymerization should be considered when calibrating the phytolith proxy. This study contributes to the identification of the parameters driving the δ<sup>18</sup>O and δ<sup>17</sup>O of bulk grass phytoliths. It additionally brings elements to further understand and model the δ<sup>18</sup>O and δ<sup>17</sup>O of grass leaf water, which influences the isotope signal of several processes at the soil/plant/atmosphere interface.</p>

Silicon fertilization increases gas-exchange and biomass by silicophytolith deposition in the leaves of contrasting drought-tolerant sugarcane cultivars under well-watered conditions

“…On the other hand, the plants in this study also demonstrated the highest silicophytolith contents with silicate fertilization, in agreement with studies on other plants (Faisal et al 2012;Hartley et al 2015;Henriet et al 2006;Jones and Handreck, 1967;Ma and Takahashi 2002), including sugarcane (Bokhtiar et al 2012;Ramouthar et al 2016;De Tombeur et al 2020). Although the silicophytolith content was determined at 42 DAT in the TVD leaves, Si deposition frequently started at the first developmental stage and continued during the remainder of the plant's life cycle (Alexandre et al 2019;Fernández Honaine et al 2013De Tombeur et al 2020). Therefore, the silicophytolith contents and their morphotypes could be used to explain the gas exchange response results, which were based on the same type of leaves (TVD).…”

“…However, more studies are needed in order to understand the effect of the silica deposition in stomata in physiological processes. Furthermore, silicification is a passive process that is a consequence of transpiration or senescence in several cells (Alexandre et al 2019;Kumar and Elbaum 2018;Kumar et al 2017a, b). However, the silicification of bilobates (short cells) commonly occurs by metabolic control, and there has not yet been any report describing the specific role of bilobates in plants (Strömberg et al 2016).…”