Document: ADR-3-99-106

Spatial patterning of pigmentation in evergreen leaves in response to freezing stress.

NICOTRA, A.B.*, M.HOFMANN, A.D.GILMORE and M.C.BALL

Australian National University, Canberra, ACT Australia 1

Abstract:
Evergreen leaves of temperate climate plants are often subject to frosts that affect photosynthesis. Changes in carbon gain patterns arise from freezing related tissue damage, and from interactions between light and temperature stress. We examined relationships between spatial patterns in freezing and concentrations of chlorophyll and anthocyanin, a protective pigment. Patterns of freezing in intact, attached leaves of the tree Eucalyptus pauciflora (the "snow gum" of alpine southeast Australia) were studied with infra-red thermal imaging. As leaves were cooled at 2oC per hour, freezing was observed to initiate at the midvein area near the petiole and spread throughout the leaf - the rate depending on the shape, thickness and water content of the leaf. The combined effects produced spatial gradients in freezing temperatures within leaves that would be expected to influence spatial patterns in freezing damage. Spatial patterns in pigmentation in leaves that had been exposed or protected from naturally occurring frosts were determined by conventional extraction techniques combined with high resolution hyperspectral imaging of reflectance from intact leaves. Predictive indices (r2 ranging from 0.6 to 0.9) were developed to relate reflectance to chlorophyll content and chlorophyll a/b ratios within intact leaves. Leaves exposed to frosts had lower chlorophyll contents and more variable a/b ratios than protected leaves. In frost affected leaves, chlorophyll content was highest near leaf centers and decreased toward leaf tips and edges. Decline in chlorophyll content was associated with shifts in chlorophyll a/b ratios and increases in red pigmentation due to anthocyanin, with effects being greater on leaf sides exposed directly to the sun. These altered patterns in pigmentation were consistent with patterns in freezing. Our results show the spatial scale of leaf response to freezing, and raise important questions about impacts of freezing on photosynthetic function in overwintering evergreens.

Keywords: Eucalyptus pauciflora

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This abstract is being presented at: 4:30 PM in session:
Oral Session #10: Light Relations in Plants.