Abstract
To induce new variation within the Escallonia genus, chromosome doubling was performed in E. rubra, E. rosea, and E. illinita, three important species within this genus of mainly evergreen woody ornamental species. Obtained tetraploids and diploid controls were analyzed for rooting capacity, leaf and flower characteristics, and plant architecture using image analysis and cold tolerance. In the present study, a breeders' collection of 23 accessions was characterized cytogenetically and described morphologically. All analyzed species and cultivars were diploid (2n = 2x = 24), with exception of E. pendula, a tetraploid. Today, breeding in Escallonia is limited to lucky finds in seedling populations and few efforts in interspecific hybridization. Three selected Escallonia species underwent an in vitro chromosome doubling with both oryzalin and trifluralin applied as either a continuous or shock treatment. The treatments successfully induced polyploids in all three species. Image analysis revealed that tetraploid E. rosea had decreased shoot length (from 3.8 to 1.3 cm), higher circularity and more dense growth habit compared to diploids. No significant changes in cold tolerance were seen. Tetraploid E. illinita did not differ in shoot length, but an increased outgrowth of axillary buds on the main axis led to denser plants. For tetraploid E. rubra, an increase in plant height (from 4.9 to 5.5 cm) was observed together with a large decrease in circularity and density due to a more polar outgrowth of branches on the main axis. E. rubra tetraploids bore larger flowers than diploids and had an increased cold tolerance (from −7.7 to −11.8°C). Leaf width and area of tetraploids increased for both E. illinita and E. rubra, while a decrease was seen in E. rosea genotypes. For all three species, the rooting capacity of the tetraploids did not differ from the diploids. We conclude that the effect of polyploidization on Escallonia was highly variable and species dependent.
Highlights
Polyploidization is a breeding tool that creates variation in phenotype and physiology (Horn, 2002; Dhooghe et al, 2011; Sattler et al, 2016)
E. pendula was a tetraploid with 48 chromosomes (Table 1)
The phylogenetic tree resulting from the AFLP analysis (Figure 2) shows that the two E. illinita genotypes and the two E. alpina genotypes differed
Summary
Polyploidization is a breeding tool that creates variation in phenotype and physiology (Horn, 2002; Dhooghe et al, 2011; Sattler et al, 2016). Increased cell size does not implicate increased plant size as the number of cell divisions can be reduced in polyploids and result in more compact growing genotypes (Horn, 2002; Sattler et al, 2016; Hias et al, 2017). Physiological changes, e.g., stress resistance and flowering period, are reported for polyploid plants (Levin, 2002; Van Laere et al, 2010; Regalado et al, 2017) due to an increase in genome flexibility (Levin, 2002). In tetraploid Lonicera, an increase in both heat tolerance and drought was observed (Li et al, 2009, 2011)
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