SHORT VEGETATIVE PHASE Research Articles

Molecular cloning of SVP: a negative regulator of the floral transition in Arabidopsis.

Flowering time mutants represent genetic functions in control of the floral transition, an important developmental phase switch in the life cycle of higher plants. Many such mutants have been identified and characterized, particular in Arabidopsis. Here we describe the identification and initial characterization of a new early flowering mutant of Arabidopsis. The corresponding gene, SVP (SHORT VEGETATIVE PHASE), was cloned through transposon tagging and represents a new member of the MADS-box gene family of transcription factors. Analysis of its transcriptional activity revealed the presence of differently sized transcripts that were confined to vegetative tissues and floral primordia and absent from developed flowers and siliques. The function of SVP as a repressor of the floral transition is discussed.

Seed production in cluster clover (Trifolium glomeratum L.). 2 Effect of sowing time and sowing rate on flowering time, abortion, seed size, and hardseededness

An ecotype of cluster clover (Trifolium glomeratum L.) common to the wheatbelt of Western Australia was sown in pure undefoliated swards at rates in the range 2-256 kg/ha, on 2 dates to give growing seasons 13 and 18 weeks in length. Irrigation was phased out at the time of flowering of late-sown plots. The treatments generated a scale of seed production in cluster clover from 148 kg/ha (43 000 seeds/m2) to 1640 kg/ha (389 000 seeds/m2). Delayed sowing was more important in reducing seed production than was increasing sowing rate. By comparison with the early-sown plots, the late-sown plots had less above-ground biomass at flowering (2053 v. 2728 kg/ha); 48-66% fewer inflorescences per m2; a shorter vegetative phase (9 days); fewer seeds per inflorescence (26 v. 31); and lesser seed mass (337 µg v. 436 µg). There was little effect of the treatments on hardseededness except that hardseededness after 1 season in the field was higher in seeds from late-sown swards at high sowing rate (58%) than from late-sown swards at low sowing rate or early-sown swards (40%). Neither sowing rate nor sowing time influenced the within season pattern of hardseed breakdown. The results indicate that cluster clover is capable of massive seed production, and that even under highly stressed conditions seed production is maximised by a high reproductive allocation and small seed size. Despite conditions which reduced seed production by up to 90%, cluster clover is able to maintain its seed quality. Relatively few inviable seeds are produced and hardseededness is either unaffected or enhanced.

Developmental Implications of Photoperiod Sensitivity in Soybean (Glycine max [L.] Merr.)

Although the effects of photoperiod and temperature on flowering time are well established, their underlying physiology and implications for other aspects of growth and development require further investigation. The pattern of development in soybean (Glycine max L. Merr.) cultivars Biloxi (photoperiod sensitive) and Fiskeby V (photoperiod insensitive) are compared in an environment where photoperiod sensitivity was not expressed (11-h photoperiod) and in an environment where photoperiod sensitivity was maximally expressed (15.5-h photoperiod). A floral staging scale was developed to study the pattern of floral initiation and to compare rates of floral development. There were inherent differences between cultivars in earliness of flowering, vegetative development, and, therefore, in yield potential. When photoperiod sensitivity was not expressed, cv Biloxi had a short vegetative phase and floral primordia were apparent 14 d after sowing. Expression of photoperiod sensitivity, beginning 8-9 d after sowing, resulted in a much extended vegetative phase with increased rates of leaf initiation. Cultivar Fiskeby V had a minimal vegetative phase; floral primordia were apparent 9 d after sowing and initiation of primary nodes was complete 3 d later. There were inherent differences between cultivars in the duration of the transitional period between the initiation of the first lateral floral primordium and the initiation of the terminal inflorescence, a period that is important in determining yield potential. The pattern of floral initiation observed was primarily acropetal and reflected the competence of meristems to respond to the floral promoter. Meristems developing in the prophyll axes had consistently lower rates of leaf initiation and remained vegetative longer than other lateral meristems, indicating that there are subtle differences in the distribution of floral signals or that these meristems have different threshold requirements.

Influence of pH on Sporulation, Spore Germination and Germling Survival in Blue‐green Algae

AbstractAn investigation is made on the influence of the pH‐value in the range of pH = 4.5 to 11.0 on sporulation, germination and germling development of Anabaena fertilissima and Anabaenopsis arnoldii, isolated from the Sambhar Salt Lake, Rajasthan. Maximum rates of germination were obtained with Anabaena at pH = 7.0… 10.5, with Anabaenopsis at pH = 7.0 to 8.5. With both species, sporulation occurs in the same pH‐ranges, Anabaena dying without sporulation in the range of pH = 5.5… 6.5 and above pH = 10.5 and Anabaenopsis surviving without sporulation above pH = 9.5. With Anabaena the germling growth steadily increases from pH = 7 to 10.5, whereas Anabaenopsis shows a maximum at pH = 7 and at any other pH‐value growth is considerably lower. Moreover, spore germination is bound to an exchange of the medium. In agreement with these experimental findings concerning the cultures, from July to February Anabaenopsis occurs in the Sambhar Lake as a vegetative form with a partial mass development, and from March to June it survives as a spore. Anabaena, however, has only a short vegetative phase in June, and it survives the remaining part of the year as a spore. Thus, the fast germination and sporulation in the range of higher alkalinity is the ecological strategy for Anabaena to resist the competition offered by Anabaenopsis.

Néoformation de Bourgeons Végétatifs et Inflorescentiels à Partir de Disques Foliaires du Crepis capillaris L. Wallr. cultivés in vitro

Summary Foliar discs excised from plants of Crepis capillaris are remarkably able to regenerate buds when cultured in vitro on Murashige and Skoog's medium added with indole acetic acid (2 mg/l) and kinetin (1 or 5 mg/l). Dedifferentiation begins by the reactivation of the parenchyma located near the veins, at the vicinity of the excised margins of the explants. It leads to the establishment of a callus on which primary meristematic cells organise, from place to place, numerous shoot meristems. The vegetative or reproductive nature of the meristems depends upon the season of the excision. Foliar discs born of vegetative plants give rise only to vegetative buds. On the same medium, during summer, discs belonging to flowering plants put in place reproductive buds, directly or after a short vegetative phase. In both cases, the dedifferentiation is caracterised by a great nuclear stability and by the preservation of the diploid state pre-existent in the mother-plant. The retention of an induced floral condition in the foliar discs after their excision is also discussed.

Genetic control of the pre-anthesis development of spring rape (Brassica napus L.). II.* Identification of individual genes controlling developmental pattern

Controlled environment studies of variation in the duration of the vegetative phase of spring rape were conducted with the objective of detecting major genes determining vernalization response. These studies involved analyses of variation within early generation segregating populations and between inbred-backcross lines. Distributions of unvernalized F2 plants from crosses between Target, Bronowski, and Isuzu were clearly bimodal when grown under continuous light at 25°C. Duplicate recessive genes were found to determine the vernalization requirement of Bronowski. In Isuzu, however, the vernalization requirement was determined by two independent genes, one having a markedly greater effect than the other on vernalization response. The genes in Bronowski occurred at different loci from those in Isuzu, and Target, which had no vernalization requirement, possessed the dominant alleles of all four genes. Plants having a significantly shorter vegetative phase than Target were also detected in F2 and backcross populations of crosses between Target and the other two cultivars. Analyses of the distribution of inbred-backcross lines showed that this character was determined by genes from Isuzu and Bronowski which were not expressed when associated with the parental genotype. ______________________ *Part I, Aust. J. Agric. Res., 30: 251 (1979).

Variation in budding and flowering-time of commercial cultivars of Antirrhinum majus

Seven F1 hybrids of Antirrhinum majus were grown in a range of environments and measurements were made of the time from sowing to budding and of the time from budding to flowering. The time to budding of all cultivars varied with environment, but was always longer for late-flowering types. The time from budding to flowering changed with environment in the early-flowering cultivars. This period was shorter and apparently independent of environment in the late-flowering types. The possibility of selecting types which combine the short vegetative phase of the early-flowering cultivars with the lower time for inflorescence development of the late ones is discussed.