Role of the Cotyledons in the Phototropic Response of Lavatera cretica Seedlings

Abstract

Young seedlgs of Lavatera cretwa L. exhibit positive phototropism.The hypocotyl perceives unilateral illumination with blue light and curves towards the light source by unequal growth. In addition, the cotyledonary laminas perceive the vectorial component of unilateral illumination with blue light and reorient normal to the beam by creating a turgor differential in their pulvini. Excision of one cotyledon resulted in negative organotropic curvature of the hypocotyl, away from the remaining cotyledon. Illumination of the cotyledonary lamina did not participate in the phototropic curvature of the hypocotyl, so long as the lamina was free to reorient to face the beam. When the lamina was continuously exposed to vectorial photoexcitation, elongation of the hypocotyl on the side carrying the cotyledon could be enhanced, or inhibited, depending on the direction of the beam striking its lamina. Mericarps of L. cretica L. were pretreated in 70% (v/v) H2SO4 for I h and then thoroughly washed in tap water and planted either in drained plastic cups (5 cm diameter) or in drained plastic flats containing a 1:1 (v/v) mix of basalt gravel and Vermiculite. The plants were irrigated every 1-2 days with nutrient solution. The plants were grown at 22 ± I C in daily cycles of 16 h light/8 h dark. Light was provided from above, by a bank of cool-white WS Gro-Lux fluorescent tubes (Sylvania), providing a photon fluence rate of 150 ,uE m 2 s-1 (400-700 nm) at plant level.Phototropic illumination was generally provided from a 500-w Quartzline lamp (General Electric) installed in a model No. 10 reflector housing fitted with a hot mirror in front and a cold mirror in the back (Dicrolite Co., Everett, Washington). Photon fluence rate was 150 ME m-2 s-1 (400-700 nm

Phytochrome-Mediated Phototropism in De-Etiolated Seedlings

“…6) is consistent with results obtained in other dicotyledonous seedlings irradiated with B light or WL (16,17,28,31) and with the hypothesis that it is the R:FR gradient established across the hypocotyl that elicits the bending response. This observation is very important with respect to the ecological significance of stem phototropism.…”

Photosynthetic patterning: a mechanism for sun tracking

“…An experimental investigation of Lupinus arizonicus by Wainwright (1977), where lanthanum was used as an ion-pump inhibitor, suggested the same role for K+ ion fluxes in the turgor adjustments of sun tracking as has been demonstrated for guard cells (Dayanandan and Kaufman 1975) and for nyctinasty (Satter and Galston 1981). Two studies by Schwartz andKoller (1978, 1980), using L. cretica, have shown that the capacity to detect the direction of light is associated with the major vein or veins most nearly parallel to the incident rays. In these experiments, when either the distal or proximal Can.…”

“…The various traces from each vein become widely separated and are redeployed to specific radial sectors of the pulvinus. Within the pulvinus itself, the vasculature forms a cylinder of parallel traces surrounded by cortical motor cells (Fisher and Fisher 1982;Schwartz and Koller 1978). At the lower end of the pulvinar zone, the vasculature redivides, otherwise without further rearrangement, into six alternately large and small bundles which continue parallel down the length of the petiole.…”

“…Well before sunrise and in complete darkness, "trained" plants turn their leaves at least part of the way towards the point where the sun will emerge an hour or so later. This capability may be repeatedly induced by 2 or more full days of "training" exposure to clear or fairly clear solar movement (Yin 1938;Schwartz and Koller 1978). A trained leaf that has been exposed for 2 days to nondirectional light, such as the diffuse light of an overcast sky, does not continue anticipatory movements unless retrained but remains in a neutral or "rest" position more or less transverse to the orientation of its own petiole (D. Koller, personal communication).…”

“…One such sun-tracking species is Lavatera cretica, the tree mallow of southern Europe, in which asymmetrical turgor adjustments in response to light from almost any quarter can produce a continuous range of lamina tilting of almost 180 degrees about the axis of the petiole (Schwartz and Koller 1978). Under certain circumstances, pulvinar motor-cell adjustments may be augmented by simultaneous bending responses of the whole petiole, particularly when leaves have not yet reached full maturity.…”