Natural foliar variegation is common in the forest understory. Two main mechanisms of variegation, pigmental and structural, have been reported. Blastus cochinchinensis (Melastomataceae) is common in East Asia, with two types of seedlings, variegated seedlings and normal green seedlings (non-variegated). The variegated leaves display novel strong variegation patterns consisting of series of white spots or chains on a green leaf. The aims of this study are to reveal the mechanism of this remarkable variegation and its effects on ecophysiology to gain understanding of its adaptive significance.
White and green areas, vw and vg of variegated leaves, and non-variegated leaves of seedlings were compared structurally with microtechniques, and characterized for chlorophyll content and fluorescence. Twice repeated field surveys of 10 transects in two forests in central Taiwan were conducted to obtain variegation traits and comparative features of photosynthesis and herbivore damage between the two types of seedlings.
Results/Conclusions
Variegation in Blastus results from physical and pigmental mechanisms, derived from epidermal, air space, upper mesophyll, chloroplast and crystal traits. Vw areas are distinguished by flatter adaxial epidermal cells containing crystals, and the presence of air spaces between the adaxial epidermis and a colorless spongy-like upper mesophyll containing smaller and fewer chloroplasts. The vw area has fewer stomata, but has the largest spongy-tissue chloroplasts and has adaxial trichomes containing crystals. Both leaf types have similar total chlorophyll content, similar Fv/Fm, but differ in a/b ratio.
The net photosynthetic rates of variegated leaves were significantly lower than those of green leaves. Moreover, under low light (300 PAR), the PSII quantum yield and the electron transport rate of the variegated leaves were lower than the normal green leaves, but the non-photochemical quenching of the variegated leaves was higher than the normal green leaves suggesting a photoprotective effect.
The field surveys found approximately equal numbers of variegated and nonvariegated seedlings on the transects. Variegated leaves showed three distinct variegation patterns: punctate, chain-like and lumpy, with vw covering 1.5-24.8% of the leaf area. Herbivory and variegation showed no significant spatial structure, but were strongly negatively associated suggesting that variegation provides protection from herbivory. In conclusion, the slightly weaker photosynthetic performance of variegated leaves seems compensated by better photoprotection, and significantly reduced herbivore damage. Variegation is found only on the first seven pairs of leaves of the variegated seedling, suggesting that variegation is only adaptive in the fragile juvenile plant.