Effect of Dark Pretreatment on the Kinetics of Response of Barley Pulvini to Gravistimulation and Hormones

Abstract

Starch in pulvinus amyloplasts of barley (Hordeum vulgare cv Larker) disappears when 45-day-old, light-grown plants are given 5 days of continuous darkness. The effect of this loss on the pulvinus graviresponse was evaluated by following changes in the kinetics of response during the 5-day dark period. Over 5 days of dark pretreatment, the lag to initial graviresponse and the subsequent half-time to maximum steady state bending rate increased significantly while the maximum bending rate did not change. The change in response to applied indoleacetic acid (100 micromolar) plus gibberellic acid (10 micromolar) without gravistimulation, under identical dark pretreatments, was used as a model system for the response component of gravitropism. Dark pretreatment did not change the lag to initial response following hormone application to vertical pulvini, but both the maximum bending rate and the half-time to the maximum rate were significantly reduced. Also, after dark pretreatment, significant bending responses following hormone application were observed in vertical segments with or without added sucrose, while gravistimulation produced a response only if segments were given sucrose. These results indicate that starch-filled amyloplasts are required for the graviresponse of barley pulvini and suggest that they function in the stimulus perception and signal transduction components of gravitropism.

Hormonal and Gravitropic Specificity in the Regulation of Growth and Cell Wall Synthesis in Pulvini and Internodes from Shoots of Avena sativa L. (Oat)

“…They also found that about 32% of the pulvinar cell wall material was cellulose, a value quite similar to that reported here (about 35%) derived using an alternative extraction method and measuring incorporation of label rather than absolute amount. In the present work, enhanced cell wall synthesis and elongation of the lower half of gravistimulated pulvini were both initiated between 3 and 6 h. This is significantly later than the 0.5 to 1 h time of initiation of bending found with 80-mm segments used by others (Kaufman and Dayanandan, 1985;Brock and Kaufman, 1988b). Unfortunately, accurate measurement of the initiation of bending was not possible with the relatively short 10-mm segments used here; therefore, the results are not strictly comparable.…”

“…Other workers (Kaufman and Dayanandan, 1985;Brock and Kaufman, 1988b), using 80-mm Avena segments to study the response of the pulvinus to gravity, have reported the initiation of bending by 0.5 to 1 h. Although preliminary experiments also showed significant gravistimulated bending of the 10-mm segments used here, this response was difficult to measure accurately, especially at early times, because the segments were so short. Therefore, elongation of the lower half of gravistimulated pulvini was chosen as an alternative method to determine the kinetics of the pulvinar growth response.…”

“…Of course, the difference in time of initiation could be due at least in part to the greater amount of tissue present in the 80-mm segments. Note that the lag time is certainly influenced by the developmental stage of the plant (Brock and Kaufman, 1988b). In addition, no conclusions should be drawn from these data about an absolute requirement for enhanced cell wall synthesis in the pulvinar graviresponse.…”

“…This agronomically important bending response is initiated within 0.5 to 1 h following appropriate gravistimulus of the pulvinus (Kauf- man and Dayanandan, 1985;Kaufman, 1988b, 1988~). Strong evidence exists that the initial graviperception occurs through the movement of starch statoliths (amyloplasts) located in the statenchyma cells found near each vascular bundle in the pulvinus Song et al, 1988).…”

“…Stem segments were ordinarily prepared from shoots that contained the internode immediately below the peduncular node (p-1 internode) with a length between approximately 10 and 40 mm, which ensured maximum growth response to GA, (Adams et al, 1973), as well as adequate gravitropic response. This developmental staging was a compromise between maximum internodal elongation potential (Adams et al, 1973), high pulvinar graviresponsiveness (Brock and Kaufman, 1988b), and reduction in segment variability.…”