Leaf reddening of sweet gum in water imbalance☆
Introduction
Many tree species possess ornamental value because of their particular colour of leaves, flowers and/or other organs at certain developmental stages. The juvenile leaf on the twig tip and the mature leaf before shedding often show red or purplish-red colours (Feild et al., 2001). The red autumn leaves of most tree species are caused by the synthesis of a large quantity of anthocyanins in the epidermal and/or palisade leaf layers. It is still unclear why anthocyanins are synthesised in autumn just before the leaf falls. This synthesis has been considered a warning colour in the context of co-evolutionary interaction with insects (Archetti, 2000), a by-product of physiological function (Matile, 2000) or a means of eliminating toxins (Ford, 1986), among others. Leaf reddening has also been widely recognised as an environmentally driven, species adaptative mechanism with such functions as photoprotection (Feild et al., 2001, Gould et al., 1995, Hughes et al., 2007), protection from low-temperature stress (Close et al., 2002, Pietrini and Massacci, 1998) and osmotic regulation (Chalker-Scott, 1999, Chalker-Scott, 2002). Although the autumn leaf colour of many tree species was considered the result of photoperiod (Howe et al., 1995) or nutrition-induced metabolic change, the leaf colour change of certain tree species was found to be sensitive to water imbalance during growth. For example, under the influence of the east Asian monsoon climate, leaves of the sweet gum tree (Liquidambar styraciflua L.) do not redden every year in Japan (Wang et al., 2009b), especially in years with plentiful rainfall and reduced light irradiation. During the present study, we found that persistent transpiration-cooling failure and water stress induced by leaf severing, coarse sandy soil, overgrowth and the shedding process can trigger leaf reddening of sweet gum trees. Therefore, water stress and transpiration-cooling failure may be the major triggers of leaf reddening in sweet gum.
Section snippets
Materials and methods
The studied sweet gum trees are street trees planted along an arch-like street near a previous riverbed in Yamaguchi, Japan. As indicated in Fig. 1a, the right side of the area has fine gley soil, the left side has coarse lowland soil, and the curve lies at the transition between the two soil types. In all, 86 eight-year-old plants were studied by analysing photographic images of the trees. The photographs were taken on the ground from the southern side with a Canon CCD camera (IXY 6.0) on a
Leaf reddening under different site conditions and vigour states
In modern cities, the planting of trees with coloured leaves in an asymmetric design and of different tree species along the same street is common. Fig. 1 shows an Oct. image of sweet gum trees planted in an asymmetric design along a gradient of site conditions in Yamaguchi, Japan, and the geographic position of the trees (Fig. 1 insets 1 and 2). From Fig. 2a, the G/L values of the crowns of the sweet gum trees were larger on the right side, smaller on the left side and intermediate at the
Discussion
After stress from severe water imbalance, trees usually show a sequence of dissimilar symptoms. First, stress manifests as a decrease in growth rate and biomass as the result of a lower photosynthetic rate. Then, trees often reduce their transpiration surface by defoliation, scorch (Wang and Omasa, 2012) and/or shortened leaves. As the water stress becomes more serious, branch dieback or death of the whole plant occurs. The type and severity of symptoms are dependent on the environmental
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Cited by (2)
Leaf structural reddening in smoke tree and its significance
2015, Urban Forestry and Urban GreeningCitation Excerpt :After vein severance, the high consistency with which we observe a relationship between a water stressed area and persistent/advanced leaf reddening suggests that a persistent water imbalance triggers a protective response in partial areas of leaf lamina. An inspection of transverse sections of red juvenile leaves of smoke trees, staghorn sumacs and purple blow maples under a microscope show that the red layer is formed at the epidermis or palisade tissue to reduce levels of red light absorbance (Wang et al., 2014). In addition, we found that the back surface of leaves in the smoke tree and staghorn sumac redden when they are turned inwards for several days.
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Supported by the National Natural Science Funds of China, ID number 31170671 and the Project of Science and Technology Development in Shandong, China; ID number 2012GNC11107.