Optical properties of etiolated plant tissues ( internal reflectance / light - guiding / acceptance angle / photomorphogenesis / phototropism )

Etiolated tissues of several plants are multiple bundles of fiber optics capable of coherent transfer of light over at least 20 mm. The acceptance angles (the angles at which light can be intercepted and then internally reflected longitudinally) for mung beans, oats, and corn are 47, 590, and 52"-542, respectively. The shapes of the curves that describe the acceptance angles are the same for various tissues of the same plant but differ between species. The pattern of light transmitted longitudinally through a tissue is dependent on the angle at which the light intercepts the side ofthe tissue and is strongly influenced by the tissue geometry. When 0.5mm of the tip is irradiated, the amount oflight traveling down the "shaded" side of the coleoptile is equal to or 2to 3-fold greater than the amount traveling down the "lighted" side. Internal reflectance of light by etiolated tissues has been discussed cursorily (e.g., refs. 1 and 2) but, to our knowledge, until recently no hard evidence for the phenomenon has been published. The present study was spurred by the discovery of the extreme photosensitivity of etiolated oats: even brief exposure to conventional "safe" light can induce oat photomorphogenesis (3). Even modest light-guiding by the seedlings could allow effective light transmission from the coleoptilar tip to potential sites of photoperception located near the node (4) and thus induce photomorphogenesis well before most of the seedling had emerged from the soil. The logarithm of percentage axial light transmission (at 635 nm) of both oat mesocotyl and leaf tissues is a linear function of tissue length (4). This paper (i) shows that axial light transmission is a property ofmany etiolated plant tissues, (ii) details some physical parameters that describe internal reflectance in these organs, and (iii) discusses the possible significance ofthese physical parameters to seedling photobiology. MATERIALS AND METHODS Growth of Plants. All seeds were imbibed and all plants were grown in absolute darkness for 2-5 days at 26 ± 10C. Oat, corn, and mung bean seedlings [Avena sativa-L., cv Lodi, lot 0170B, and Zea mays L., WF9 X 38, from Dakota Seed and Grain (Watertown, SD); Phaseolus aureus L., purchased from a local grocery] were either used immediately or stored at 40C until used. All manipulations prior to measurement of light-guiding were performed in absolute darkness. Measurement of Axial Light Transmission by Whole Tissues. Axial light transmission through tissues was quantified by applying light to one end ofa deliberately curved tissue segment and measuring the light output at the other end in arbitrary units with a photomultiplier as described (4). However, the light-emitting diode used previously was replaced by a 0.5-mW He-Ne laser (model 155, 0.25 W/cm2, Spectra-Physics, Mountain View, CA). This laser produces about a 1-mm-diameter monochromatic beam at 632.8 nm. All measurements of axial light transmission were made in dim green light (3). No visible greening occurred in these tissues. Measurement of the Pattern of Axial Light Transmission Across the Cut End ofthe Tissue. The patterns of light (or light gradients) across the cut ends ofthe tissues when narrow beams of laser light were applied to the tissues were sampled by an array of optical fibers (Fig. 1).