ISHS Acta Horticulturae 175: Symposium on Physiology of Productivity of Subtropical and Tropical Tree Fruits
CITRUS LEAF FLUORESCENCE : WATER AND SALT EFFECTS |
| Authors: | J. Lloyd, P. Kriedemann, L. Prior, A. Grieve |
| Abstract:
Solar radiation absorbed by citrus leaves results in transpiration, photosynthesis, a reduction on leaf moisture status and an increase in sensible heat. Less than 10% of energy absorbed is "gainfully employed", the remainder is dissipated, and of that greater proportion, a tiny fraction is accountable as chlorophyll fluorescence. Energy lost in this form from the leaf's photosynthetic apparatus has a characteristic emission spectrum and is readily detected in vivo. Signal strength can thus offer a useful probe for photosynthetic responses by following fluorescence induction kinetics on dark adapted leaves. Changes in variable fluorescence (Fv) then provide an index of leaf response to physical inputs and especially the cumulative effects of environmental stress. Diurnal patterns of microclimate and leaf response were followed on mature leaves in irrigated orchards at Dareton (N.S.W. Dept. of Agric.) and Loxton (S.A. Dept. of Agric.). These plantations of valencia orange form part of long term salinity trials where irrigation water ranges up to 30 mM Cl-1. Leaf data collected from three field surveys during Feb.-Mar. 1985 included chloride levels, stomatal conductance, water potential and in vivo chlorophyll fluorescence. Orchard level observations were complemented by laboratory studies of fluorescence responses to cumulative radiation using leaf tissue from greenhouse trees. Citrus foliage showed a progressive loss in Fv under strong irradiance, and especially on salinised trees. Adaxial (upper) surfaces were especially vulnerable to this apparent photoinhibitory damage, whereas lower (shaded) surfaces remained virtually unaltered. A ratio between these two sets of variables, together with an index of fluorescence quenching, showed a systematic diurnal pattern according to salinisation treatment. Interrelated changes in stomatal conductance and leaf moisture status over this same measurement period appeared correlative rather than causal. | |
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