Growth Cycles in Lemna gibba Cultures and Their Effects on Growth Rate and Ultrastructure

A long-established axenic culture of Lemna gibba G3 was maintained in exponential growth phase under controlled conditions. Weekly analyses for 2 years showed that the individual plants of the Lemna gibba clone fluctuated between two forms. One extreme consisted of plants light in weight, small in size, and with a high relative growth rate (RGR), the other of heavy, large, and more slowly growing plants. At intervals plants having intermediate characteristics dominated in the stock culture. Indication of an annual growth-cycle was also found. The magnitude of growth response (weight, RGR, area, and dry matter content) after treatment with abscisic acid (ABA), 6-benzylaminopurine (BAP), and a combination of the two was different for low-weight and heavy plants. The heavy plants were more sensitive to ABA and BAP treatment than the low-weight ones. The accumulation of starch was least in small untreated plants and greatest in ABA treated plants. Large electron transparent globules were found in the chloroplasts of the ABA treated plants and in heavy plants regardless of how they had been treated. The different physiological and ultrastructural characteristics of the two forms of Lemna plants probably reflect an ageing-rejuvenation cycle. Emphasis is placed on the importance of this cycle when Lemna is used as a model plant in physiological experiments.     

Role of Ethylene in Abscisic Acid-induced Callus Formation in Citrus Bud Cultures

The induction of callus formation in cultured buds of Shamouti orange (Citrus sinensis [L.] Osbeck) by abscisic acid (ABA) is a multiphasic process. (Altman, and Goren 1974 Physiol Plant 32: 55.) A study of the mediation by ethylene on this effect of ABA was undertaken. It was found that: (a) ethylene and (2-chloroethyl) phosphonic acid, as well as ABA, induced callus formation; (b) callus induction is best attained when explants are exposed to ethylene during the 1st day after excision; and (c) ABA-induced callus formation is inhibited by rhizobitoxine analog, an inhibitor of ethylene biosynthesis. It is concluded that the effect of ABA on callus formation is mediated via ethylene.     

Hormonal Regulation of Dormancy in Developing Sorghum Seeds

The role of abscisic acid (ABA) and gibberellic acid (GA) in determining the dormancy level of developing sorghum (Sorghum bicolor [L.] Moench.) seeds from varieties presenting contrasting preharvest sprouting behavior (Redland B2, susceptible; IS 9530, resistant) was investigated. Panicles from both varieties were sprayed soon after pollination with fluridone or paclobutrazol to inhibit ABA and GA synthesis, respectively. Fluridone application to the panicles increased germinability of Redland B2 immature caryopses, whereas early treatment with paclobutrazol completely inhibited germination of this variety during most of the developmental period. Incubating caryopses in the presence of 100 [mu]M GA4+7 overcame the inhibitory effect of paclobutrazol, but also stimulated germination of seeds from other treatments. IS 9530 caryopses presented germination indices close to zero until physiological maturity (44 d after pollination) in control and paclobutrazol-treated particles. However, fluridone-treated caryopses were released from dormancy earlier than control and paclobutrazol-treated caryopses. Incubation in the presence of GA4+7 stimulated germination of caryopses from all treatments. Our results support the proposition that a low dormancy level (which is related to a high preharvest sprouting susceptibility) is determined not only by a low embryonic sensitivity to ABA, but also by a high GA content or sensitivity.     

Hormonal Regulation of Cormel Dormancy in Gladiolus grandiflorus

The germination of dormant gladiolus corrnels is promoted bycold storage and by treatment with 6-benzyladenine (BA). Itis inhibited by abscisic acid (ABA) and by gibberellin A₃. Ethrelpromotes the germination of dormant cormels but inhibits thegermination of non-dormant ones. ABA was identified by ORD,GLC, TLC, and wheat coleoptile bioassay as the major endogenousgrowth inhibitor controlling cormel germination. The ABA levelin dormant cormels (summer crop stored at 25 °C) is fiveto 10 times that in non-dormant ones (winter crop, or cold-storedsummer crop). The ABA content of the deeply dormant varietyTexas is higher than that of the less-dormant variety Friendship.     

Effect of Purine Analogs and Their Interactions with Benzyladenine on Bud Burst and Shoot Growth in Apple Bud Explants

The purine analogs 8-azaadenine, 8-azaguanine, 4-aminopyrazolo [3,4-d] pyrimidine, 2-fluoroadenine and 6-methylpurine inhibited bud burst in non-dormant vegetative axillary bud explants obtained from apple (Malus domestica Borkh.) seedlings. The reversal of inhibition was attempted by incorporating increasing concentrations of benzyladenine (BA) in the culture medium. The inhibition caused by 8-azaadenine and 8-azaguanine was reduced especially when BA was added to the media at equimolar ratios along with the inhibitors. The inhibition due to 4-aminopyrazolo [3,4-d] pyridine was reduced only with equimolar BA, BA was ineffective or only minimally effective in reducing inhibition due to 2-fluoroadenine and 6-methypurine. Short pretreatment with 8-azaadenine and 8-azaguinine failed to cause any inhibition when explants were subsequently transferred to BA -containing medium. The effectiveness of 2-fluoroadenine and 6-methylpurine was maintained or only slightly reduced, indicating their long-term inhibitory action.     

Interrelationship of Abscisic Acid and Gibberellic Acid in the Promotion of Callus Formation in the Abscission Zone of Citrus Bud Cultures

The mechanism of ABA-induced callus formation was studied in sterile bud cultures of Citrus [Citrus sinensis (L.) Osbeck] on defined media. ABA was found to promote callus formation in the abscission zone between the petiole and the branch while inhibiting bud growth. The promoting effect of ABA was dependent on the physiological state of the shoot from which buds were excised, and on the size of the explant. Callus formation was highest in autumn and summer (i.e. younger) buds, and lowest in older buds excised from previous summer flush. GA was only slightly active in promoting callus formation when applied separately, but showed a highly synergistic effect when applied with ABA: maximal callus formation was attained at a combination of 10^?5M ABA and 10^?6 MGA in the medium. Subcultures of ABA-induced callus revealed that ABA inhibited the growth of isolated subcultured callus, while IAA and kinetin, and especially GA, promoted its rapid proliferation. A general decrease in protein synthesis was found in the abscission zone during the first 5 days of induction, while total protein content changed only slightly. The results suggest that ABA-induced callus formation in Citrus bud explants is a multiphasic phenomenon involving, at least, two stages: (1) activation of certain cells in the abscission zone by ABA, resulting in the formation of callus layers, and (2) subsequent proliferation of the callus tissue, which is dependent on the hormonal balance in the explant. This growth-promoting effect of ABA seems to be a general phenomenon in explants exposing a cut-surface.     

Sexual and Hormonal Cycles in Geriatric Gorilla gorilla gorilla

As our closest living relatives, great apes likely experience behavioral and physiological patterns associated with reproductive aging and menopause that are similar to human patterns. We present results from a nationwide zoo-based study on behavioral and hormonal changes in female western gorillas. We evaluated progestogen concentrations via daily fecal sampling in 30 gorillas, 22 of which were geriatric (≥30). We collected concurrent behavioral data 1–3 times weekly on 16 of the females. While control females cycled regularly, ca. 23% of geriatric females are acyclic (menopausal), and another 32% show variable hormonal patterns suggesting perimenopause. Patterns included increased variability in cycle length and peak progestogen values, and frequent insufficient increases in progestogen levels during the luteal phase. Acyclic females have significantly lower overall progestogen concentrations than the self cycling females, though differences are not significant when cycle phase is incorporated. We detected behavioral estrus in 9 of 10 cycling females for which data were available. In all but 1 case, proceptive behavior occurred during the follicular phase, preceding ovulation on average by 6.6 d. Females spent more time in proximity to the silverback male while in behavioral estrus than during other periods. To date, maximum longevity in captive female gorillas is 52 yr, with poor reproductive prognosis beginning from the age of 37. We demonstrate that both perimenopause and menopause characterize aged female gorillas, which may experience a postreproductive lifespan of >25% of their lives. Continued study of aging apes is warranted, and apes may serve as models for age-related reproductive changes in humans.     

Bud Dormancy in the Kiwi Fruit, Actinidia chinensis Planch

A study of lateral bud dormancy in Actinidia chinensis has shownthat true dormancy can be induced, especially in short daysat warm and constant temperatures This dormancy can be brokenquantitatively by chilling but temperatures as high as 10 °Care effective The dormancy appears to be due to an inhibitor(possibly ABA), apparently stored in the special bud cover aspecial structure in Kiwi fruit which may represent fused stipulesRemoval of the cover also admits oxygen and light, both of whichhave promoting effects on bud break Application of ABA enhancesdormancy (as do crude extracts tentatively identified as ABA)while GA₃ application enhances dormancy before chilling andpromotes bud break only after chilling Actinidia chinensis, Kiwi fruit, dormancy, abscissic acid, gibberellic acid, chilling     

Lateral Bud Dormancy in the Blackcurrant Ribes nigrum (L.)

Previous investigations have shown that the inhibition of lateralbuds in blackcurrants soon after their formation is due to apicaldominance and this is followed by leaf inhibition. The presentstudies have revealed that bud scales are the main cause ofinhibition even before leaf fall. The inhibitor level assayedby the wheat coleoptile test shows large seasonal changes; itis maximal in early autumn and minimal in spring. Inhibitor levels are reduced by low-temperature treatment inparallel with the promotion of dormancy breaking; temperatureswell below freezing, applied for very short times, almost certainlyact through the killing of bud scales in the same way as doesrapid high-temperature treatment. Other treatments causing budbreak in the presence of high inhibitor levels are: continuousillumination, ethylene vapour, and application of gibberellicacid. It is suggested that bud inhibition in the blackcurrant is dueto the formation of an inhibitor produced by the leaves andaccumulated in the bud scales. The inhibitor (probably abscisicacid) may then interact with internal gibberellin levels, summerand winter dormancy differing only quantitatively.     

Growth Substances and Dormancy in Ceratophyllum demersum

Ceratophyllum demersum L. occurs in winter in the dormant form, in summer in the vegetative form. Factors that affect growth and dormancy in Ceratophyllum were studied. After several weeks of severe winter conditions the plants changed from dormant to quiescent state. Under natural conditions Ceratophyllum plants remain quiescent for several months, due to unfavourable growth conditions. Experimentally the dormant could also be broken by high and low temperature treatments (shocks), and most effectively by addition of GA, An attempt to induce dormancy in full grown plants by the addition of ABA under extreme summer or winter conditions proved unsuccessful. The IAA and ABA contents in the plants were measured during the year. In winter the concentration of ABA was high and that of IAA low, whereas in summer the IAA concentration increased and that of ABA was variable. IAA only slightly antagonized the inhibition of growth by ABA. Both the growth regulators were readily taken up from the culture medium, as was confirmed by a study with the radioactive labelled compounds. The uptake rate of IAA was significantly higher than that of ABA. being 762 μg and 3.26, μg per plant in 24 h, respectively. GA, was found to have a strong antagonistic effect on the ABA induced growth inhibition. The total GA activity in dormant and quiescent plants was similar, in full grown plants it was much lower. In the dormant state a large part of GA was in a bound form, whereas during quiescence relatively more GA occurred in a free state in the plants.     

The Dormancy of Buds of Citrus sinensis (L.) Osbeck Inserted into Rootstock Stems: Factors Intrinsic to the Inserted Bud

Buds of sweet orange, harvested from shoots of different timeof flushing and from different positions along the shoot, wereused to examine whether lack of burst of inserted buds was acharacteristic of the bud. Bursting of inserted buds was significantlyslower in buds taken from (a) older branches (b) shoots producedunder winter conditions, and (c) basal rather than apical budson the same shoot. The slowness to burst when transferred matched a tendency todormancy in buds on shoot segments grown in vitro, suggestingthat the variation in budburst was intrinsic to the bud. Budburstwas correlated with the extent of secondary bud development;the majority of buds from apical regions of the shoot had developeda secondary bud by the time of implantation, but basal budshad not. Adequate vascular connections with the host tissueswere found in both burst and unburst buds. Citrus sinensis (L.) Osbeck, sweet orange, buds, endodormancy, budding