Genotypic Response to Assimilate Demand in a Variegated Leaf Mutant of Nerium Oleander

Summary

A naturally occurring variegated leaf biotype of Nerium oleander L. was tentatively identified to be a plastome coded mutant. The yellow sectors leaves in the mutant had less than 1–2% of the total quantity of chlorophyll, and 5–8% of the carotenoids, normally found in the wild type; and were totally lacking in light-dependent 02 evolution as well as CO₂ fixation. However, the green sectors of the variegated leaves in the mutant compensated for the thinning effect imposed by the heterotrophic yellow tissue, since they had high rates of C02 assimilation. We looked at the basis of the superior photosynthetic efficiency of the green sectors of the mutant leaves, which was two times greater than that in the wild type leaves. The chloroplasts from the green sectors showed greater capacity for light utilisation with increased amounts of electron transport complexes (PQ and Cyt t) and ATP synthase per unit chlorophyll in their thylakoid membranes as compared to those in the wild type, thus resembling the chloroplasts from plants adapted to high irradiance. However, at the gross morphological level the variegated leaves mimicked shade leaves with greater surface areas and greater intercellular air space volume than the wild type leaves. We propose that these contrasting strategies at the organelle and organ levels help the green sectors to achieve greater photosynthetic rates and overcome the heterotrophic stress imposed by the non-green tissue. Also, these responses appeared to involve a co-ordinated gene expression in nucleo-cytoplasmic and plastid compartments.