Costas A. Thanos

CISTACEAE: A PLANT FAMILY WITH HARD SEEDS

ABSTRACT An examination of 203 seed lots from 42 Old World species (or 54 taxa), with representatives from all five genera (Cistus, Fumana, Halimium, Heliarthemum, and Tuberaria) and an additional seed lot of the New World Lechea maritima shows that the phenomenon of haidseededness is a prominent characteristic of the entire Cistaceae family. Promotion of seed germination in the laboratory can be obtained by mechanical scarification and thermal pretreatment, while light seems to be totally ineffective. A seed ecology syndrome for Cistaceae is postulated, featuring small seed size, hardcoatedness (primary dormancy), short-distance dispersal, long-term persistence in soil seed banks, fire- or heat- induced seed “softening”, an opportunistic strategy of germination (germination at a wide range of temperature and light conditions), and a slow germination rate of “softened” seeds. This syndrome is suggested to endow Cistaceae, a principally Mediterranean family, with a considerable ecological advantage in the ...

Fire-Followers in Chaparral: Nitrogenous Compounds Trigger Seed Germination

1 Application of nitrates (optimal concentration c. 10mM) promoted seed germination in the fire annuals Emmenanthe penduliflora and Phacelia grandiflora and to a lesser extent in the fire-adapted shrub Salvia mellifera. Ammonium ions, although virtually inactive in E. penduliflora, were as effective as nitrate in both P. grandiflora and S. mellifera. 2 Diurnal illuminations and a chilling pretreatment, though of little effect by themselves, significantly enhanced the nitrate-mediated promotion of germination in the two fire annuals. 3 Nitrate was the principal factor inducing germination in E. penduliflora (nitrate or ammonium in P. grandiflora) whereas in S. mellifera light was the principal agent. It was also shown in E. penduliflora that nitrates are not required during chilling, i.e. seed sensitisation by chilling and nitrate-mediated induction of germination are entirely different mechanisms. 4 In the three species tested, the effect on seed germination caused by nitrogenous substances was nearly identical to that produced by an extract of charred wood, although chemical analysis of the extract showed that the combined concentrations of nitrates, ammonium ions and free amino acids could not account for the promotive action of charate. However, the nitrate and ammonium concentrations required to induce germination are very close to the increased values encountered after a fire in the otherwise nitrogen-poor chaparral soil. Therefore, in addition to the possible effect of charred wood, the postfire germination flush observed in chaparral may be induced by the increased levels of available nitrogen as well.

Early post‐fire regeneration of a Pinus halepensis forest on Mount Párnis, Greece

. The post-fire regeneration of a 45-yr-old Pinus halepensis (Aleppo pine) forest, burned in July 1989, has been studied on Mount Parnis, Attiki, Greece. Four experimental plots at various slopes and exposures were established at altitudes of 400 - 450 m, and monitored for 3 yr at 3-month intervals. Early regeneration took place abundantly, through both resprouting and seed germination of mostly hard-seeded herbs and shrubs; the floristic richness was high with 80 taxa. Pine seedling emergence took place during the winter of the first post-fire year. The mean pine seedling density by the end of the recruitment period (March 1990) was 5–6 seedlings/m2. This density decreased slightly during late spring and considerably during summer. During the second post-fire year only a relatively slight decline was observed; thereafter the density was stabilized to 1 - 2 seedlings/m2. Mortality follows a negative exponential curve that levels off at ca. 20 %. Height distributions throughout the three post-fire years were all positively skewed as a result of the presence of few very tall saplings. A considerable fraction (20 %) of very short (5–15 cm) saplings were still alive 39 months after the fire; these may constitute the sapling bank. Based on the analysis of height distribution curves, it is concluded that the taller seedlings survived significantly better than the shorter ones.

Seed stratification and germination strategy in the Mediterranean pines Pinus brutia and P. halepensis

The ecophysiology of germination in Pinus brutia and P. halepensis was studied in seeds collected from different areas of Greece. In regard to the temperature range of germination, both P. halepensis and the southern provenance (Lasithi, Crete) of the East Mediterranean pine, P. brutia , follow a typical Mediterranean pattern. In the latter species, dramatic differences in the degree of dormancy were noted among the three provenances investigated; in all seed lots however, 20°C was clearly the optimal temperature for germination. Stratification resulted in a considerable promotion of P. brutia seed germination. Nevertheless, the inductive effect of stratification was shown to differ among the three provenances used, escalating from a simple increase of germination rate (in the southern seed lot from Lasithi, Crete) through a broadening of the temperature range of germination (in the intermediate lot from Thasos Island) to, finally, a dramatic release from a particularly deep dormancy (in the northern lot from Soufli). These deeply dormant seeds of the latter provenance displayed an absolute stratification requirement; prolonged illumination or seed coat scarification could not substitute for the promotive effect of prechilling. A considerable interaction between far-red light and stratification was revealed in the dormant seeds of P. brutia (Soufli provenance); far-red pulses during stratification could either cancel or diminish the germination promotion induced by low temperatures. The differences observed in the germination behaviour among the various P. brutia provenances may be attributed to a variable ecophysiological strategy in regard to the temporal pattern of seedling emergence and establishment. According to the variants of this strategy, seed germination is timed to occur during either spring (in regions with relatively cold and moist climates), or autumn and early winter (in southern, mild and dry areas) or both (in intermediate conditions).

Ecophysiology of germination in the aromatic plants thyme, savory and oregano (Labiatae)

Ecophysiological aspects of seed germination were investigated in the widely distributed Mediterranean-endemic, aromatic plants thyme (Coridothymus capitatus), savory (Satureja thymbra) and oregano {Origanum vulgare subsp. hirtum). Thyme seed germination is light indifferent, whereas oregano seeds have an absolute light requirement; their germination can be promoted even by green safelight or far-red light. In savory, a portion of the seeds germinates in the dark and germination can be either increased or decreased by the appropriate illumination. All three species show more germination at a relatively low temperature range, a Mediterranean characteristic, with an optimum around 15-20°C. The rate of germination is considerably higher in the tiny seeds of thyme and oregano than in the larger seeds of savory. In the latter species, germination is also dependent upon the age of the seeds; old seeds germinate to a higher percentage than fresh ones, as already observed by Theophrastus, possibly as a result of the volatilization of the essential oils present on the nutlet coat. Seeds of thyme, savory and oregano are dispersed within the persistent fruiting calyces and the seeds eventually germinate within it. Essential oils in the calyx strongly inhibit germination of the enclosed seeds; germination is much suppressed in thyme and to a lesser extent in savory and oregano. This diaspore dormancy caused by essential oils apparently is overcome under natural conditions by leaching of the inhibitors with rainwater. It is suggested that this dormancy operates as an adaptation strategy that delays germination by acting as a rain gauge. In this way, seed germination and subsequent seedling

Variable desiccation tolerance in Acer pseudoplatanus seeds in relation to developmental conditions: a case of phenotypic recalcitrance?

Nine seedlots of the widely planted southern and central European native tree species Acer pseudoplatanus L. were collected along a north-south gradient spanning 21° of latitude in Europe. We investigated how the heat sum during seed development influences seed maturity as assessed by physical, physiological and biochemical traits. Using principal component analysis we found predictable and consistent patterns in all traits, which correlated with heat sum. For example, compared with fruits from their native range (Italy and France, heat sum >3000°C d), fruits from the coldest location (Scotland; heat sum of 1873°C d) were shorter (c. 30 v. 42 mm), germinated over a narrower temperature range (5-20 v. 5-35°C) and had smaller embryos (28 v. > 70 mg) with a higher water content (c. 63 v. 48%), less negative solute potentials (c. -2.4 v. -4.1 MPa) and were more desiccation sensitive (critical water potential of -20.2 v. -55.4 to -60.7 MPa). The observed level of desiccation-tolerance for the French and Italian seedlots is more consistent with the intermediate category than the previous classification of A. pseudoplatanus as recalcitrant. Our results demonstrate that a lower heat sum causes fruits from northern Europe to be dispersed before maximum potential seed quality is achieved.

Ecophysiology of seed germination in Mediterranean geophytes. 1. Muscari spp

Ecophysiological aspects of seed germination were investigated in four Mediterranean geophytes of the genus Muscari (Liliaceae): M. comosum (tassel hyacinth), M. neglectum (common grape hyacinth), M. commutatum and M. weissii. Experiments were performed at constant temperatures in the dark and under temperature and light conditions simulating those prevailing in nature during November‐January, i.e. well into the rainy season of the Mediterranean climate. In all species, no primary dormancy was revealed, and germination occurred in a rather narrow range of cool temperatures (optimum at 10 or 15°C) and at a remarkably slow rate; both germination characteristics seem to be associated with autumn/winter seed germination and seedling establishment. Such a postulated strategy is ecologically advantageous within an unpredictable rainfall regime, known to prevail during the start of the rainy period of the Mediterranean climate. This strategy may also explain the spread of germination of M. comosum seeds over two consecutive years, observed by Theophrastus. Far-red light, simulating light conditions under a dense canopy, resulted in only a slight delay of germination compared to dark controls. Diurnal white light, qualitatively simulating natural daylight, caused a significant decrease of the germination rate in all four species studied. Moreover, white light was found to suppress considerably final seed germination (photoinhibition) in M. weissii and M. neglectum; in the latter species, prolonged imbibition under white light also led to the induction of secondary dormancy.

ECOPHYSIOLOGY OF SEED GERMINATION IN ENDEMIC LABIATES OF CRETE

ABSTRACT Ecophysiological aspects of seed germination were investigated in four aromatic labiate plants of Crete Origanum dictamnus (dittany), Sideritis syriaca L. ssp. syriaca (Cretan mountain tea), Salvia pomifera L. ssp. pomifera (gall-bearing sage), and Salvia fruticosa (three-lobed sage). Experiments were performed both at constant temperatures and darkness as well as under temperature and light conditions simulating those prevailing in nature during the main germination periods (i.e., start and middle of the rainy season, November and February-March, respectively). In three out of the four species, no particular dormancy was revealed and germination occurred rather promptly, although in a rather narrow range of cool temperatures and at a relatively slow rate; both characteristics determine and/or support an early, autumn seed germination and seedling establishment. In the fourth plant, Sideritis syriaca, germination was manifested at relatively warm temperatures and at a considerably faster rate, in...

Seed bank composition and above-ground vegetation in response to grazing in sub-Mediterranean oak forests (NW Greece)

We investigate the persistent soil seed bank composition and its relation to the above-ground flora of grazed and non-grazed sub-Mediterranean deciduous oak forests of NW Greece. Twenty-eight taxa were recorded in the soil seed bank and 83 taxa (70 taxa in plots of seed bank sampling) in the above-ground vegetation. The dominant tree species and many woodland species found in the above-ground vegetation were absent from the soil seed bank. Similarity between the soil seed bank and the above-ground vegetation decreased with grazing, and grazing led to a decrease of species richness in above-ground vegetation and soil seed bank. Beta diversity of vegetation among grazed and among non-grazed plots did not differ, but was significantly higher between grazed and non-grazed areas. Beta diversity of the soil seed bank declined with grazing. When applying classification tree and logistic regression analyses, non-grazed forest sites are clearly differentiated by the presence of Phillyrea latifolia, Euphorbia amygdaloides and Brachypodium sylvaticum. PCA ordination of above-ground species composition reflected a gradient from sites grazed by ruminants to non-grazed sites, but no clear structure was detected in the seed bank.

Campanulaceae: a family with small seeds that require light for germination

BACKGROUND AND AIMS The Campanulaceae is a large cosmopolitan family, but is understudied in terms of germination, and seed biology in general. Small seed mass (usually in the range 10-200 µg) is a noteworthy trait of the family, and having small seeds is commonly associated with a light requirement. Thus, the purpose of this study was to investigate the effect of light on germination in 131 taxa of the Campanulaceae family, from all five continents of its distribution. METHODS For all taxa, seed germination was tested in light (8 or 12 h photoperiod) and continuous darkness under constant and alternating temperatures. For four taxa, the effect of light on germination was examined over a wide range of temperatures on a thermogradient plate, and the possible substitution of the light requirement by gibberellic acid and nitrate was examined in ten taxa. KEY RESULTS For all 131 taxa, seed germination was higher in light than in darkness for every temperature tested. Across species, the light requirement decreased significantly with increasing seed mass. For larger seeded species, germination in the dark reached higher levels under alternating than under constant temperatures. Gibberellic acid promoted germination in darkness whereas nitrates partially substituted for a light requirement only in species showing some dark germination. CONCLUSIONS A light requirement for germination, observed in virtually all taxa examined, constitutes a collective characteristic of the family. It is postulated that smaller seeded taxa might germinate only on the soil surface or at shallow depths, while larger seeded species might additionally germinate when buried in the soil if cued to do so by fluctuating temperatures.