Screening and evaluation of Rhododendron progenies for alkaline pH tolerance

Rhododendrons are well-known woody ornamental plants. However, they are typical calcifuges and cannot grow well in lime soils with alkaline pH. Screening genotypes tolerant to higher pH is the basis for the breeding of new cultivars with enhanced alkaline tolerance. Plant phenotyping based on chlorophyll fluorescence imaging allows the early detection and non-destructive screening for abiotic stress of small seedlings. In this study, the seeds of four cross combinations of Rhododendron genotypes were germinated in vitro on media with pH 5.4 and 8.3. First, the top 25% of best-performing seedlings at both pH-levels were selected based on the chlorophyll fluorescence images and calculated F_v/F_m values 40 days after sowing the seeds. Subsequently, an experiment was set up to compare the alkaline tolerance in the two selected groups of seedlings. Thereto the selected seedlings were grown for one month on rooting medium at pH 5.4, after which an alkaline shock treatment was applied on 10 randomly selected seedlings by adding 250 mmol L^‑1 NaHCO₃ to the medium. The stress tolerance was examined by chlorophyll fluorescence imaging during the first 10 days after the shock treatment. The results indicated that F_v/F_m and Φ_PSII allow for early detection of the response of Rhododendron seedlings to alkaline stress. The screening method can help to identify and select seedlings with improved tolerance to alkaline stress as was clearly shown in one of the crosses. The evaluation method can non-destructively and effectively detect the alkaline tolerance levels among crosses. The screening and evaluation methods as well as screened seedlings provide additional knowledge and material for breeding.