Dimorphic epidermal cell chloroplasts in the mesophyll‐less leaves of an extreme–shade tropical fern, Teratophyllum rotundifoliatum (R. Bonap.) Holtt.: a light and electron microscope study

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Abstract
Summary: This light and electron microscope study of the iridescent blue leaves of the fern Teratophyllum rotundifoliatum, reveals several novel anatomical and cytological features. In this extreme shade species [0.4—‐(3)–4 W m−2] of lowland tropical rainforest in peninsular Malaya, the greater part of the lamina between the simple collateral vascular bundles, consists only of epidermal cells. From 4–(6.9)–12 large chloroplasts 9.4–(17.9)–27 μm in diameter are located adjacent to the inner walls of the lens‐shaped upper epidermal cells. Those in the tessellated lower epidermal cells are smaller [5.0–(7.6)–19.5 μm], much more numerous [21–(53)–83] and regularly distributed around the periphery. They are identical in size and ultrastructure to the chloroplasts in the guard cells of the stomata. Whereas the upper epidermal chloroplasts and those in the stelar tissues contain very little starch, numerous starch grains occupy up to Sl‐9% of the volume of the lower epidermal chloroplasts. As in other extreme‐shade plants the epidermal chloroplasts in T. rotundifoliatum contain numerous large grana, with no preferred orientation, and integranal lamellae completely filling the stroma. The numbers of thylakoids per granum in the lower epidermal cells [9–(29.3)–84] are significantly smaller than those in the upper [22–(86)–28O]. The latter appear to be the largest yet recorded in land plants and this probably accounts for the remarkably low chlorophyll a: b ratio (1.8) in T. rotundifoliatum. Record amounts of chlorophyll per unit leaf fresh weight and per unit leaf area in T. rotundifoliatum reflect the high proportion of leaf cells containing chloroplasts. The unusual features of leaf anatomy and cytology in T. rotundifoliatum appear to be associated with maximizing the efficiency of absorption of very low diffuse radiation.