Short Term Phytochrome Control of Oat Coleoptile and Pea Epicotyl Growth

Abstract

Knowledge of the latency period between R4 irradiation of plants and expression of physiological or developmental changes is important in studying the mechanism of phytochrome action. Responses such as changes in membrane electrical potential may have latencies on the order of seconds (18), while several hours may be required for the appearance of certain inducible enzymes (17). Although growth is a complex process, possibly involving many limiting factors in different situations, there is much interest in understanding the mechanism by which phytochrome controls the growth of etiolated seedlings. There have been a few attempts to characterize the kinetics of such responses. The R-induced inhibition of intact pea stem elongation began about 6 h after irradiation (9), and a lag period of 4 h was reported for pea stem segments (3). The lag period for the R-induced growth promotion of apical oat coleoptile segments was less than 3 h (12). Recent work with optical and electronic transducers showed that auxin can promote the growth of pea stem and oat coleoptile segments within 15 min of application (7,24 mounted on a microscope mechanical stage, so that the transducer arm could be adjusted to the horizontal at the beginning of each experiment. The transducer was connected to an Omni-Scribe recorder (Houston Instruments). All manipulations were carried out under green safelights (18) in a room maintained at 23 ± 2 C. Irradiations were given from the side, with a mirror behind the plant, using a 300-w projector and the following filters: red, Rohm and Haas Plexiglas 2423 and Corning filter 3-66 (intensity 3.0 mw/cm2); far red, Corning filters 7-69 and 3-66 (1.4 mw/cm2) (18); blue, Wratten filters 2A and 47B (0.8 mw/cm2).Experiments with Oats. Seedlings ofAvena sativa L. cv. Victory were grown as described previously (19). Our initial experiments were conducted using segments in an Evans and Ray-type apparatus (7). Ten 1-cm apical segments were cut, the apical 1 mm was removed, and the segments strung on nylon line. A glass capillary tube was placed on top ofthe segment column, with the transducer arm resting on the flared-out top of the capillary. The growth chamber was filled with 1 mm K-phosphate (pH 6.2) containing 1.5% sucrose (12). The solution was circulated by air bubbled into the chamber.In most of the experiments, a single 2-cm apical segment (tip intact) was used. A snug fitting plastic sleeve was placed around the middle of the segment, and the sleeve and plant were inserted through a hole in the lid of a foil-wrapped 30-ml vial filled with the buffered sucrose solution. The sleeve and plant were firmly held by the lid, with about 0.8 cm of the coleoptile protruding above the lid. A small translucent conical cap was placed on the tip of the coleoptile, and the transducer arm rested on this cap. For application of test solutions to the plants, we used a slightly larger cap, containing a wad of cotton soaked in the solution.Experiments with Peas. Seedlings (Pisum sativum L. cv. Alaska) were grown as d...

Rapid Suppression of Extension Growth in Dark-Grown Wheat Seedlings by Red Light

“…With etiolated seedlings, the most rapid phytochrome-mediated response yet recorded is that of Vanderhoef et al (13) who demonstrated a latent period of 20 min for the inhibition of maize mesocotyl extension. Pike et al (12), using isolated tip segments of oat coleoptiles, found that R promoted extension after a latent period of 46 experiments with etiolated seedlings, green light was used as a 'safelight'. It is known, however, that seedlings grown in the absence of light are exceptionally sensitive to irradiation (1,8) and that exposure even to brief periods ofgreen safelight is often sufficient to induce a physiological response.…”