No abstract

Sumimary. The transport of indole-3-acetic acid-1-14C (IAA) through 4 mm segments of etiolated Avena coleoptiles was studied as a function of time by applying IAA in apical agar blocks and measuring the basal IAA export rate at 5-minuite intervals. The transport velocity found in this way is at least 15 mm/hour at 260. Following a 30-minute equilibration period, the export rate is nearly constant for at least 50 minutes at physiological donor concentrations. Exposure to about 3 X 105 ergs/cm2 blue light for 15 minutes leads to a transient reduction in the export. The export rate reaches a minimum about 25 mintutes after the onset of illumination, then rises to reach a maximum by 35 minutes, and subsequently declines again. The result is a net export depression during the first 80 minutes, amounting to some 12 to 17 %. Its timing closely matches the timing of the light growth response elicited by the same light dosage.At higher IAA concentrations (0.5 and 1.8 mg/l), the export rate reaches a peak about 60 minutes after the initial application of auxin, and thereafter declines rapidly. Light increases this decline in export rate, without causing peaks and troughs, and even at 0.25 mg/l IAA the transient changes in export appear to be superimposed on a gradtual decline in export rate after illumination. Blue light is effective in all these phenomena; the red far-red system appears to exert no effect. The results are discussed in relation to the mechanism of action of light both in the light growth response and in phototropism.WVhen the stubapical part of the etiolated oat coleoptile is exposed to a tunilateral pulse of strong blue light, its response includes a transient depression in growth rate as well as eventual cturvature toward the light souirce. The light-growth response (LGR) does not need to occur for the plant to cturve (4), and it is well established that phototropic curvatuire involves lateral movement of auxin (3,16) rather than the changes in tissue sensitivity, atuxin produiction, or auxin movement that might be expected to produce a growth response. Thus there is little basis for an assumption that the LGR produices phototropism.OIn the other hanid, the close similarity between the LGR and phototropism in terms of dose-response relations (51) stuggests that both responses might spring from the same basic mechanism. If

No abstract

(4,10). Kinetic analyses have likewise pointed to the possibility that adaptation of the growth rate involves relatively large changes in metabolic pool levels (3, 7). Thus it seems that a study of constitutional changes in response to light may yXield insights into the still elusive earlx steps in the photores-ponses of this fungus. Toward that end, the present report descrilbes light-induced changes in the accumulation of crv-stalloids in the sporangiophore.The occurrence of octahedral cry-stalloids in the sl)orangiophores of mucoraceous fungi has been kinowni since 1872 (12,18). The crystalloids of P. blakeslceanus have been found by electron microscopic examination to consist of approximately isodiamiietric subunits in a cubic space lattice with spacings in the vicinity of 120A (18, 23). In the same studies, crystalloids were found in s,porangiophores that were as small as 1 % of the terminal size, but not in vegetative hyphae or storage vesicles.

Developmental distinctions between giant and dwarf fruiting bodies of Phycomyces blakesleeanus (Burgeff) were studied by means of size measurements and growth analyses. Histograms of fruiting body lengths showed a bimodal distribution, with peaks around 0.3 millimeter (dwarfs) and 30 millimeters (giants). Individual cultures contain both giants and dwarfs. Differences between giants and dwarfs appear in the first phase of development; the apex of the giant is tapered, whereas the dwarf apex is dome-shaped. Probable cytological distinctions at this stage are cited in discussion. The dwarfs terminate enlargement upon expansion of the sporangium, thus lacking the subsequent phase of rapid elongation (stage IV) that contributes 90% of the length in the case of giants. It was concluded that P. blakesleeanus maintains two developmental patterns for asexual fruiting, with dwarfs and giants differing radically in growth regulation.The giant asexual fruiting bodies of Phycomyces blakesleeanus have long been studied in the search for basic mechanisms of growth regulation (2). Experiments in this laboratory have led to the impression that P. blakesleeanus can produce two types of asexual fruiting bodies, one of which has previously been ignored in the literature. The second type may conveniently be termed the "dwarf" fruiting structure owing to its small stature compared to the better known "giant" type. Our interest has been drawn by the possibility of elucidating developmental mechanisms by comparing the physiology of the dwarf and giant fruiting pathways. A precedent for this anticipation may be found in the aquatic fungus Blastocladiella emersonii, where the ability to form two types of vegetative spore has been given intensive metabolic study (5). Relevant work on the giant and dwarf fruiting systems in P. blakesleeanus is in progress in this laboratory.The first question must be whether the giant and dwarf fruiting bodies of P. blakesleeanus are truly distinct in development, or whether they are merely quantitative variants arising from a single pathway. At least two related species are known to have binary fruiting capabilities: Choanephora cucurbitarum produces both conidia and sporangia (1); and Thamnidium elegans produces sporangia as well as sporangioles (10). But the situation in P. blakesleeanus is less clear. The giant and dwarf fruiting bodies in this species are similar in general morphology. Further, the giant fruiting system can be made to produce dwarf-sized variants by germinating large