Michael G. Mullins

Dormancy in somatic embryos and seeds ofVitis: changes in endogenous abscisic acid during embryogeny and germination

Abscisic acid (ABA) in extracts of somatic embryos and seeds of Gloryvine (Vitis vinifera L.xV. rupestris Scheele) was measured by gas chromatography-mass spectrometry-selected ion monitoring using deuterated ABA, (±)-[C-3Me-2H3]ABA, ([2H3]ABA) as internal standard. The ABA content increased rapidly during embryogeny (0.035 ng/embryo at the globular stage to 0.22 ng/embryo at the mature stage). The level of ABA in the tissues of somatic embryos, expressed in ng/mg dry weight, decreased from the globular stage (0.76 ng/mg) to the mature stage (0.25 ng/mg). Chilling (4° C) induced normal germination of seeds and mature somatic embryos and precocious germination of globular, heart-shaped and torpedoshaped somatic embryos. In all cases chilling led to a marked reduction in endogenous ABA. Exogenous (±)-ABA inhibited the germination of chilled somatic embryos.

Effects of serial subculture in vitro on the endogenous levels of indole-3-acetic acid and abscisic acid and rootability in microcuttings of ‘Jonathan’ apple

Proliferating axillary shoots of the difficult-to-root apple cultivar ‘Jonathan’ acquired an enhanced ability to form adventitious roots with increasing number of subcultures in vitro. The transition between the difficult-to-root and the easy-to-root condition occurred at the fourth subculture.Endogenous levels of free IAA and ABA in shoot tissues were analysed by gas chromatography/mass spectrometry/single ion monitoring (GC/MS/SIM) using negative ion chemical ionisation. Tissues from the mother plants grown in the glasshouse contained more IAA and ABA than those from tissue-culture material. After establishment in vitro there was no variation in the IAA content throughout the subcultures but a decrease in ABA content was observed after the fourth transfer. The IAA/ABA ratio increased from 0.2 in difficult-to-root shoots from the initial culture up to 0.7 in easy-to-root shoots from the long-term subculture.

Seasonal changes in endogenous ABA and IAA and the influence of applied ABA and auxin in relation to shoot growth and abscission in Valencia Orange (Citrus sinensis (L.) Osbeck)

Seasonal changes in endogenous IAA and ABA were measured by gas chromatography/mass-spectrometry. Highest concentrations of ABA occurred in leaves. There was a major ABA peak in early spring (up to 1360 ng g−1 dw). Levels were low in summer (90 ng g−1 dw). There was a minor ABA peak in autumn. Endogenous IAA in leaves was highest in winter/spring (up to 76 ng g−1 dw). Applied ABA promoted abscission of leaves and shoots while applied NAA delayed abscission. The main peak in leaf-ABA content was followed by extensive shoot abscission. The involvement of ABA and IAA in regulation of flush growth was not clear.

Endogenous indole-3-acetic acid and abscisic acid in apple microcuttings in relation to adventitious root formation

The effects of applying indole-3-butyric acid (IBA) for periods up to 48 h were examined in difficult-to-root microcuttings (from newly-established cultures) and in easy-to-root microcuttings (from long-term subcultures) of ‘Jonathan’ apple (Malus X domestica Borkh). In easy-to-root material, 20% of the microcuttings produced roots in the absence of IBA, while 6 h exposure to 10 μM IBA gave 100% rooting of microcuttings. In contrast, root formation in difficult-to-root material was IBA-dependent. Maximum rooting of these microcuttings (50%) required 24 h exposure to 10 μM IBA.Variation in the endogenous levels of free indole-3-acetic acid (IAA) during the course of root induction was similar in microcuttings of both types but there were marked differences in endogenous abscisic acid (ABA) levels. In easy-to-root microcuttings ABA remained at a constant low level, but in difficult-to-root material ABA exhibited marked fluctuations and was present at higher concentrations than in easy-to-root microcuttings.

PHASE CHANGE IN CITRUS : THE EFFECTS OF MAIN STEM NODE NUMBER, BRANCH HABIT AND PACLOBUTRAZOL APPLICATION ON FLOWERING IN CITRUS SEEDLINGS

SummaryTwo studies were carried out to identify the effects of several factors including plant size (defined as the number of main stem nodes), growth habit and paclobutrazol application on the length of the juvenile period of citrus seedlings. In the first group of experiments, seedlings were grown from seed of ‘West Indian’ lime (Citrus aurantifolia (Christm.) Swingle), ‘Marsh’ grapefruit (Citrus paradisi Macf.) and ‘Meiwa’ kumquat (Fortunella × crassifolia Swingle (pro sp.)). Seedlings were initially grown under rapid-growth controlled environment conditions of 30/25°C day/night temperatures, 16 h days of 650 µmol s-1 m-2 and high relative humidity for 5–8 months and maintained as single stemmed plants until the main stems had more than 100 nodes. They were then treated with one of four rates of paclobutrazol as a root-drench and transferred to mild environmental conditions in a greenhouse (25°C maximum/15°C minimum) to assess subsequent branch development and flowering response. ‘West Indian’ lime and...

Endogenous gibberellin- and cytokinin-like substances in cultured shoot tissues of apple, Malus pumila cv. Jonathan, in relation to adventitious root formation

The frequencies of adventitious root formation in vitro of isolated shoots from bud cultures of apple (Malus pumila cv. Jonathan) after 1, 7 and 31 subcultures (weeks 5, 29 and 109 after the initial culture) were 5, 78 and 95% respectively. Endogenous gibberellin-like substances (GA) were extracted, chromatographed on SiO2 partition columns, and assayed on dwarf rice (Oryza sativa cv. Tan-ginbozu). The levels of GA in shoots from the 1st, 7th and 31st subcultures were 40, 19 and 14 ng GA3 eq./g dry weight of tissue, respectively, a trend which suggests an inverse relationship between endogenous GA level and rooting ability. This is consistent with the fact that applied GA3 inhibits rooting in apple and many other species. The major peak of GA activity eluted coincidentally with GA1/GA3/GA19. Endogenous cytokinin-like substances (CK) were chromatographed on paper and assayed with soybean hypocotyl sections. In contrast to the decrease in GA activity, CK activity increased 1.5–2.7 fold in the later subcultures (cytokinin activity per shoot, however, declined).

Micropropagation of Telopea speciosissima R. Br. (Proteaceae). 1: Explant establishment and proliferation

Axillary bud explants of 11 selected mature waratah clones were established in vitro on a modified Murashige & Skoog medium. Adequate proliferation of axillary shoots was achieved by optimisation of the growth regulator status of the culture medium. For the majority of clones, a three to six times rate of proliferation was achieved with 1.25 μM BA and 1.0 μM GA3 without the occurrence of abnormalities. The white flowering clone did not respond favourably to the addition of GA3 to the medium.

Phase change in citrus: Growth and flowering of citrus seedlings from thirteen genetically diverse seedling families

SummarySeedlings of 13 families belonging to the citrus group were grown under greenhouse and natural conditions at Camden, Australia and at Palmerston North, New Zealand. Lateral branches were removed for 24 months so that the seedlings had a single main stem. After 24 months, the plants pregrown under greenhouse conditions were transferred to natural outside conditions. Seedlings were subsequently allowed to form lateral branches from axillary buds, some of which eventually bore flowers. Floral branches were found predominantly in the apical region of seedlings, indicating a relationship between large plant size and the end of the juvenile phase. Only populations that had previously been grown in the greenhouse environments flowered. Within 30 months of germination some seedlings of some families formed flowers (e.g. ‘West Indian’ lime, ‘Eureka’ lemon, ‘Rangpur lime’, ‘Palestine’ citron and ‘Cleopatra’ mandarin). Within 48 months, some seedlings of all families had formed flowers, but the proportion of ...