Hydroponic cultivation conditions allowing the reproducible investigation of poplar root suberization and water transport

Abstract

Background With increasing joint research cooperation on national and international levels, there is a high need for harmonized and reproducible cultivation conditions and experimental protocols in order to ensure the best comparability and reliability of acquired data. As a result, not only comparisons of findings of different laboratories working with the same species but also of entirely different species would be facilitated. As Populus is becoming an increasingly important genus in modern science and agroforestry, the integration of findings with previously gained knowledge of other crop species is of high significance. Results To ease and ensure the comparability of investigations of root suberization and water transport, on a high degree of methodological reproducibility, we set up a hydroponics-based experimental pipeline. This includes plant cultivation, root histochemistry, analytical investigation, and root water transport measurement. A 5-week-long hydroponic cultivation period including an optional final week of stress application resulted in a highly consistent poplar root development. The poplar roots were of conical geometry and exhibited a typical Casparian band development with subsequent continuously increasing suberization of the endodermis. Poplar root suberin was composed of the most frequently described suberin substance classes, but also high amounts of benzoic acid derivatives could be identified. Root transport physiology experiments revealed that poplar roots in this developmental stage have a two- to tenfold higher hydrostatic than osmotic hydraulic conductivity. Lastly, the hydroponic cultivation allowed the application of gradually defined osmotic stress conditions illustrating the precise adjustability of hydroponic experiments as well as the previously reported sensitivity of poplar plants to water deficits. Conclusions By maintaining a high degree of harmonization, we were able to compare our results to previously published data on root suberization and water transport of barley and other crop species. Regarding hydroponic poplar cultivation, we enabled high reliability, reproducibility, and comparability for future experiments. In contrast to abiotic stress conditions applied during axenic tissue culture cultivation, this experimental pipeline offers great advantages including the growth of roots in the dark, easy access to root systems before, during, and after stress conditions, and the more accurate definition of the developmental stages of the roots.

Populus × canescens root suberization in reaction to osmotic and salt stress is limited to the developing younger root tip region

“…No pronounced differences were observed in the development of endodermal Casparian bands upon salt or osmotic stress treatment (no data shown), which were previously identified to be already fully developed at 10%–20% relative root length after cultivation in control conditions (Grünhofer, Guo, et al, 2021).…”

“…Phenotypic characterization of hydroponically cultivated poplar shoots subjected to different osmotic and salt stress intensities. (A) Photographs taken at the day of harvest, photographs of the polyethylene glycol series (−0.4, −0.6, and − 0.8 MPa) originate from Grünhofer, Guo, et al (2021). (B) Shoot lengths of plants cultivated for 5 weeks, including 1 week of stress treatments (left, osmotic stress; right, salt stress); n = 15–20 shoots.…”

“…For a more detailed description of the methods applied for hydroponic P . × canescens plant cultivation, histochemical, and analytical analyses of apoplastic transport barriers in poplar roots, and means of root transport physiological investigation, the reader is advised to Grünhofer, Guo, et al (2021).…”

“…The investigated root segments were expressed as relative lengths of the whole root, with 0% representing the root tip and 100% the root base according to Kreszies et al (2019). Two functional developmental zones were defined based on the investigation of roots cultivated in control conditions for 5 weeks (Grünhofer, Guo, et al, 2021). Zone A, showing no endodermal suberization, ranged from 0% to 27.5% of the relative total root length.…”

“…Here, J vr denotes the normalized volume flow rate at 0 MPa applied pneumatic pressure, Δπ is the osmotic potential gradient between the nutrient solution and the exuded xylem sap acting as the sole driving force, and σ sr is the reflection coefficient of nutrient ions in the solution. The latter was previously measured to be 0.47 for the same poplar clone and nutrient solution with whole individual adventitious roots using a pressure probe (Grünhofer, Guo, et al, 2021). The surface area of roots was determined by scanning them after staining with 0.03% (w/v) toluidine blue O (Merck, Germany) and multiplication by π to account for their round geometry.…”