Press release Lime-tolerant rhododendrons: a win-win for growers, consumers and climate

Rhododendron, the popular evergreen garden plant, has a difficult time as an acid-loving shrub in our often lime-rich gardens. A screening method developed by a researcher at ILVO-UGent allows rapid, objective detection of lime tolerance in a large number of seedlings simultaneously, offering important perspectives for the development of adapted cultivars. The researcher and his colleagues also mapped tens of thousands of wild rhododendron species from China based on ancient herbarium specimens. By linking the location of those rhododendron species with a database of soil data, he was able to select 76 types that grow well in calcareous (lime) soils. The first pot trials with crosses of those more lime-tolerant rhododendrons at ILVO are already promising.

Rhododendron grows best in moderately acidic soil, a pH of 4 to 6 is suitable. In soils with higher pH, the plant does not grow well because essential building blocks such as iron are much harder to absorb there.

Leen Leus (ILVO): “Often soils naturally have a neutral to slightly lower pHbut in modern gardens the pH is often higher due to paving with dolomite, cement products in stabilizers under patios and pathways, or concrete copings. Frequent liming of lawns also increases soil pH. This makes our gardens less suitable for rhododendrons.”

Growing on peat-free substrate

Arborists are realizing that they need to adjust the assortment if they want to grow and sell rhododendrons in the future. For cultivation itself, it is also good to develop pH tolerant cultivars so that they can grow on less acidic substrate with peat replacements such as compost. The use of peat in substrate is under pressure because it is extracted from peatlands, thus contributing to global CO2 emissions.

Method for rapid screening

However, the development of new cultivars - so-called breeding - takes many years. That process can now be faster, thanks to the screening method that researcher Shusheng Wang developed with his colleagues at ILVO. The key to this new method is early screening of the chlorophyll fluorescence of germinating seeds.

Chlorophyll is the green dye in the chloroplasts in the leaves that gives plants their green color, but also enables the plant to engage in photosynthesis and thus grow. Chlorophyll fluorescence proved to be a good indicator of stress levels when the newly germinated plantlets were raised on a nutrient medium with high pH (Figure 1). Greenhouse experiments with the outgrown plants confirm this. Plants that experienced stress as a result of higher acidity in the potting soil showed lesser root and shoot development (Figure 2).

Leen Leus (ILVO): “The great advantage of the new screening method is that breeders can do this for many seedlings simultaneously ánd automatically, making it fast and the assessment also objective. Moreover, it can be done at a very early stage with a technique that reveals stress differences between plants that cannot be seen with the naked eye. In some seedlings we could detect stress as early as 2 days after germination on a nutrient medium with high pH.”

Successful crossing with wild rhododendrons

Since 1884, 90,000 rhododendrons have been collected from the wild in China and recorded in herbaria. Based on the digitized version of this Chinese herbarium, Shusheng Wang and his colleagues at the Lushan Botanical Garden selected 31,146 different rhododendrons whose locations could be linked to appropriate GPS coordinates and soil characteristics. This allowed the researcher to identify 76 rhododendron types that occur in nature on soils with high pH and are therefore promising for introducing calcium tolerance into cultivated rhododendrons via breeding.

Seedlings of three of these promising species (R. chihsinianum, R. fortunei and R. vernicosum) were tested in Shusheng's doctoral trial. Crosses of these species with our local rhododendron cultivars were also tested for their calcium tolerance in lab and greenhouse trials.

Leen Leus (ILVO): “Especially offspring of R. fortunei did well in less acidic potting soil. This shows that there is potential in a targeted breeding program toward more lime-tolerant rhododendron for the European market.”

Susheng Wang successfully defended his doctorate "Increased tolerance in Rhododendron to calcareous soils: insights from geographic distribution analyses, in vitro experiments and serrestudies" in March 2024 at UGent. His promoters were Leen Leus for ILVO and Prof. Emmy Dhooghe and Prof. Marie-Christine Van Labeke, both UGent.