Gas Exchange

The leaves of some plants display an optical patchiness on their upper side, displaying light- and dark-green areas
with high and low reflectance, respectively. In this investigation, we studied the fine structure of the corresponding
sectors and we asked whether the lost reflected light entails a photosynthetic cost to these leaves. Four species, i.e.
Arum italicum, Ranunculus ficaria, Cyclamen hederifolium and Cyclamen persicum were investigated. Scanning electron
microscope examination revealed that epidermal cells of light-green sectors of all species are more bulgy than
corresponding cells of neighboring dark-green leaf sectors. The comparative anatomical study revealed that (i)
epidermis thickness of the light-green areas and the number of mesophyll cell layers does not differ from those of the
adjacent dark-green leaf sectors and (ii) palisade cells of light-green sectors are slightly larger and more loosely
arranged, allowing a much higher percentage of intercellular air spaces. The latter histological feature seems to provide
the structural basis for the different optical properties between the two leaf sectors. Contrary to expectations, net
photosynthetic rates (expressed on a leaf area basis) were similar in the light-green and the dark-green areas of the two
cyclamen species. Yet, in C. persicum net photosynthesis was higher in the light-green areas, if expressed on a dry mass
basis. The small size of the light-green spots in the rest of the test plants precluded CO2 assimilation measurements, yet
maximum linear photosynthetic electron transport rates displayed no differences between the two sectors in all plants.
Thus, the assumption of a photosynthetic cost in the light-green areas was not confirmed. On the contrary, a higher
construction cost was evident in the dark-green areas of three species, displaying a significantly higher specific leaf
mass, without any photosynthetic benefit. The results on net photosynthesis were compatible with leaf optical
properties and pigment levels. Thus, in spite of the considerably higher reflectance of the light-green areas and their
lower (yet normal for a green leaf) chlorophyll levels, corresponding differences in absorptance were slight. In addition,
dry mass-based pigment contents in dark-green areas were higher, while chlorophyll a/b (in two species) and
carotenoid/chlorophyll ratios (in three species) were lower, pointing to a shade adaptation in these sectors. We
conclude that in variegated leaves of this kind, dark-green areas are more costly to build and probably less
photosynthetically active. We argue that the high pigment contents of dark-green areas establish steep light gradients
in the corresponding mesophyll, rendering deeper chloroplast layers more shade adapted.