No abstract

Low concentrations of auxin (e.g. 10(-10)m) do not promote the growth of intact seedling roots of maize (Zea mays L. Bear Hybrid WF 9 x 38). Higher concentrations are inhibitory. When the roots are pretreated with the ethylene biosynthesis inhibitors, cobalt and aminoethoxyvinylglycine, auxin (10(-10) to 10(-8)m) strongly promotes their growth. The promotion of growth by auxin in pretreated roots is preceded by enhanced hydrogen ion secretion from the roots. The data indicate that hormone-enhanced hydrogen ion secretion may play a role in the rapid promotion of root growth by auxin. The ability of auxin to promote the growth of intact roots is discussed in relation to the Cholodny/Went hypothesis of hormonal control of root geotropism.

Using an auxanometer and time-lapse cinematography we have studied the timing of abscisic acid (ABA) effects on elongation, gravitropic curvature, and hydrogen-ion efflux in several cultivars of maize (Zea mays L.). The effect of high concentrations (e.g. 0.1 mM) of ABA on root elongation is triphasic, including 1) a period of promotion lasting approximately 12 h, 2) a subsequent period of increasing inhibition lasting approximately 12h, and 3) gradual recovery to a rate within approximately 80% of the control rate. With lower concentrations of ABA (e.g. 0.1 μM) only the transient promotive phase is seen. Abscisic acid enhances ethylene biosynthesis in roots of maize but suppression of ethylene biosynthesis does not prevent the long-term inhibitory action of ABA on growth. Application of ABA (0.1 mM) to the upper surface of horizontally placed roots accelerates positive gravitropism. Application of ABA to the lower surface retards gravitropism and in some cases causes the roots to curve upward against the direction of gravity. These observations are consistent with our finding that the initial effect of ABA on root elongation is stimulatory. Since root gravitropism is rapid enough to be completed within the stimulatory phase of ABA action, the data argue against hypotheses of gravitropism based upon accumulation of ABA to inhibitory levels on the lower side of a hirizontal root.

Recent evidence indicates a role for calcium and calmodulin in the gravitropic response of primary roots of maize (Zea mays, L.). We examined this possibility by testing the relationship between calmodulin activity and gravitropic sensitivity in roots of the maize cultivars Merit and B73 x Missouri 17. Roots of the Merit cultivar require light to be gravitropically competent. The gravitropic response of the Missouri cultivar is independent of light. The occurrence of calmodulin in primary roots of these maize cultivars was tested by affinity gel chromatography followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with bovine brain calmodulin as standard. The distribution of calmodulin activity was measured using both the phosphodiesterase and NAD kinase assays for calmodulin. These assays were performed on whole tissue segments, crude extracts, and purified extracts. In light-grown seedlings of the Merit cultivar or in either dark-or light-grown seedlings of the Missouri cultivar, calmodulin activity per millimeter of root tissue was about 4-fold higher in the apical millimeter than in the subtending 3 millimeters. Calmodulin activity was very low in the apical millimeter of roots of dark-rown (gravitropically nonresponsive) seedlings of the Merit cultivar. Upon illumination, the calmodulin activity in the apical millimeter increased to a level comparable to that of light-grown seedlings and the roots became gravitropically competent. The time course of the development of gravitropic sensitivity following illumination paralleled the time course of the increase in calmodulin activity in the apical millimeter of the root. The results are consistent with the suggestion that calmodulin plays an important role in the gravitropic response of roots.

The spectral sensitivity and the irradiance dependence of anthocyanin synthesis, a "high irradiance response," in cabbage (Brassica oleracea, cv. Red Acre) and tomato (Lycopersicon esculentum, cv. Beefsteak) seedlings exposed to continuous irradiation depend upon the length of the exposure. In cabbage, blue and red are more effective than far red when the irradiations are shorter than 12 hours and less effective than far red when the irradiations are longer than 12 hours. The irradiance dependence is negigble under red and becomes evident under blue and far red red only for exposures longer than 12 hours. Anthocyanin synthesis under intermittent Ught treatments, of effidency comparable to that of continuous treatments, obeys the Bunsen-Roscoe reciprocity law and is a function of the dose (irradiance x time), rather than of the irradiance alone. The validity of the reciprocity relationships suggests that only one photoreceptor is responsible for the photocontrol of the response in the blue, red, and far red spectral regions. The characteristics of the response suggest that the photoreceptor is phytochrome, at least in cabbage.The nature of the pigments active in the photocontrol of the HIR2 responses of plant photomorphogenesis (1, 13, 16) is still a topic for debate. Phytochrome is active in HIR processes, but it is not definitively known if it is the only photoreceptor involved. Several factors are responsible for the difficulties encountered in identifying the HIR photoreceptors(s): (a) the variability of the spectral sensitivity of the HIR: some responses show maximum action in the FR region (1,2,7,8), some others in the BL (2,3,5,6), and some in the R (2); (b) the irradiance dependence of the HIR: it can be theoretically explained on the basis of phytochrome (1,7,14,17), but the assumptions made are not completely consistent with the variability of the spectral sensitivity of the HIR in different response-systemn combinations; (c) the reciprocity failure of the HIR under continuous irradiation: equal TRDs, obtained by reciprocal changes of irradiance and time (I x t = nI x tin), do not produce the same extent of the response (3, 13). The reciprocity failure suggests the possibility of a cooperation between different photoreceptors and creates difficulties in the interpretation of action spectroscopy studies (15).In a previous note (10), we suggested that the reciprocity failure of the HIR could be explained in terms of phytochrome, and that the irradiance dependence perhaps was no more than (8,10), the fact that phytochrome was involved in the photocontrol of anthocyanin production under inductive conditions (8, 9), and some ancillary observations (9, 11, 12) strongly suggested that the photoreceptor was phytochrome.Considerable amounts of anthocyanin are formed in cabbage seedlings exposed to continuous BL and R radiation (8,11,12). The objectives of the present study were to determine if reciprocity would be retained under intermittent BL and R irradiations, and if a comparative study of the actio...