Leaf margin variation and the effect of asexual propagation on the stability of leaf thorn

Abstract:【Objective】Ananas comosus is an important tropical fruit tree whose leaves have spines or no spines. The solid leaf spines are inconvenient for field management and are regarded as an important index to evaluate the economic characters of pineapple cultivars. In this paper, the characteristics of spines in pineapple leaves and variations in leaf margins were analyzed and the effect of asexual propa- gation on traits of leaf margins was observed in order to provide a reference for pineapple seedling culti- vation.【Methods】The leaf margin types and leaf spine morphology were inspected in 14 cultivars from 4 species of Bromeliades. The structures of leaf spines of 5 cultivars including‘Shenwan’and‘MD-2’were observed with freehand sections. The stem tips from different leaf margin types of the cultivars‘Shenwan’and‘MD-2’were taken as explants, and young stems were obtained from the axillary buds on MS medium after about 30 days. The young stems were transferred to MS + 2.0 mg·L^-1 6-BA + 0.1 mg·L^-1 NAA solid medium for proliferation, and then transferred to MS + 0.5 mg·L^-1 6-BA + 0.1 mg·L^-1 NAA solid medium. The effects on the stability of leaf edge characters between the sucker and tissue cultured seedlings were compared.【Results】According to the differences in the locations, densities and proportions of the leaf spines, the pineapple leaf margins could be divided into 5 types: ① completely spineless, ② spiny only at tip, ③ occasionally spiny, ④ occasionally spineless and ⑤ spiny. The cultivars were further divided into three categories: spiny, completely spineless and intermediate types. Most cultivars were intermediate type and only very few were completely spiny, but‘Yellow Mauritius’and ‘Shenwan’, which account for nearly 90% cultivated area in China, were both spiny. The observation of the freehand sections showed that pineapple leaf spines were formed by the outward extension of leaf cells, surrounded by a thick stratum corneum with smooth surface, curving upward like a hook with a sharp apex. The size, opening angle, color and density of spines varied with the germplasm materials, the spacing between spines ranged from 1.99 mm in‘MD-2’to 9.72 mm in Ananas bracteatus var. Tri-color; the length of spines ranged from 1.14 mm in‘Shenwan’in vitro plants to 2.46 mm in Ananas bracteatus var. Tricolor, and the opening angle from 22° in‘Shenwan’in vitro plants to 55° in A. brac-teatus var. Tricolor. Compared with red bract (A. bracteatus‘Tricolor’) and‘Yulinglong’, A. comosus had smaller but higher density spines with lower open angles. The leaf margin characters in terms of spine traits were stable among the progenies obtained from suckers or in vitro propagation. The plants that belonged to ①-④ types could be found in the cultivars of the intermediate types, and different types could be found even in the same plant. The inner leaf margin characters of the cultivars were un- stable and greatly affected by the environment. The proportion of spine ranged from 44.74% (‘HD’) to 80.00% (‘Caine’) in the progenies propagated from suckers with spines or not. The ②-④ type plants could also be found even in the offsprings propagated from plants without thorns. After the plant with- out spines of the intermediate type‘MD-2’was multiplied by bud propagation for 8 to10 generations, the average proportion of plants with spines reached 4.3%. If particular selections for the spine charac-ters were not performed, the proportion reached 47.78% after 8 to 10 generations of in vitro propaga- tion, and exhibited no significant difference from those propagated in field. Except for the spines at the leaf margin, no significant abnormalities occurred in other agronomic traits like plant height, leaf num-ber, leaf length and leaf width among the in vitro seedlings.【Conclusion】The Bromeliadum plants have 5 leaf types according proportion of spines and 3 types of varieties according to leaf margin types. The leaf edge characters for the cultivars with or without spines in whole plants are suitable to be propa- gated with suckers or in vitro, while those of the intermediate type were unstable. The density, size, opening angle and color of spines exhibite great variations among different varieties.