Seed production,dispersal and germination in Cryptostegia grandiflora and Ziziphus mauritiana,two invasive shrubs in tropical woodlands of northern Australia

Abstract Cryptostegia grandiflora and Ziziphus mauritiana are exotic shrubs that are invading tropical woodlands of northern Australia. Although they are widespread, they currently occupy only small proportions of their respective potential ranges. Large C. grandiflora can produce more than 8000 wind-dispersed seeds in a single reproductive episode and can set seed at least twice per year. More than 90% of seeds will germinate within 10 days of moisture becoming available. Few, if any, seeds survive for more than 12 months in the soil. Large Z. mauritiana can produce more than 5000 fruits per year. The fruit is a drupe that contains a single seed (sometimes two) enclosed in a woody endocarp. Less than 10% of fresh seeds will germinate without removal of the endocarp. Removal of the endocarp increased germination to 56%. Less than 10% of seeds of Z. mauritiana remain viable after burial in the soil for 12 months. Seeds of Z. mauritiana are dispersed by several mammalian vectors, including wallabies and feral pigs, but domestic cattle are probably the major means by which large numbers of seeds reach new sites. Propagules pass intact through the digestive tract of cattle, feral pigs and wallabies and contain viable seeds that germinate more readily than seed in fresh intact fruits. Controlling movement of cattle that have had recent access to mature fruit of Z. mauritiana could significantly reduce the likelihood of new infestations. For both species, management that reduces seed production will be important for containing spread.     

Regional and Landscape-scale Patterns of Shrub Invasion in Tropical Savannas

The shrubby vine Cryptostegia grandiflora and the shrub Ziziphus mauritiana were both introduced to northern Australia over 100 years ago and have become invasive in savanna woodland environments. Data from a land resource survey were used to examine regional- and landscape-scale distribution patterns of these species in the Dalrymple Shire, an area of over 6 1/2 million hectares in northeast Queensland. Each species was present at 10% of the 2362 sites examined and most frequent and abundant close to Charters Towers, the major settlement of the regions. C. grandiflora was recorded at 50 % of sites within 20 km of the town and in 14 out of 21 of the region's major sub-catchments. Z. mauritiana was recorded at 32 % of sites within 20km of Charters Towers, but in only three sub-catchments. Little of the variation in frequency and abundance of C. grandiflora and Z. mauritiana was accounted for by landscape factors, including geology, soils, or vegetation. While survey results do not absolutely distinguish between history, habitat and disturbance in explaining the weed's current distributions within the region, a strong influence of historical factors is suggested. Both exotic species were much less abundant than Carissa spp., a native taxon that has purportedly increased in the region in recent decades. In spite of their current prominence as weeds, there is potential for further increase by both C. grandiflora and Z. mauritiana. This increase could include expansion from the zone of high abundance and proliferation within that zone. While the results of such surveys must be interpreted with caution, they can yield useful information about regional patterns of plant invasion.     

Biological control of Ziziphus mauritiana (Rhamnaceae): feasibility,prospective agents and research gaps

The tropical fruit tree, Ziziphus mauritiana (Rhamnaceae), a native of the Indian subcontinent, is a pasture and environmental weed in northern Australia and Fiji. In their native range, Ziziphus spp., including commercially cultivated Z. mauritiana and Z. jujuba, are subjected to a wide range of pests and diseases. The feasibility of classical biological control of this weed has not been explored to date. Effective biological control could reduce plant vigour and seed output, thereby limiting the spread of Z. mauritiana in Australia. Two Ziziphus species are native to Australia, hence, any prospective biological control agent should be specific to Z. mauritiana. Opportunistic field surveys and literature searches identified 133 species of phytophagous insects, 9 species of phytophagous mites and 12 plant pathogens on Ziziphus spp. Host records suggest the following are possibly specific to Z. mauritiana and hence are prospective biological control agents in Australia: the seed‐feeding weevil Aubeus himalayanus; the leaf‐feeding gracillariid moth Phyllonorycter iochrysis; the leaf‐mining chrysomelid beetle Platypria erinaceus; the leaf‐folding crambid moth Synclera univocalis; the leaf‐galling midge Phyllodiplosis jujubae; and the gall‐mites Aceria cernuus and Larvacarus transitans. Host range of the rust Phakopsora zizyphi‐vulgaris includes many Ziziphus species, including the native Z. oenoplia and hence would not be a suitable biological control agent in Australia. The powdery mildew Pseudoidium ziziphi, with a host range restricted to Ziziphus species, has not been reported on Z. oenoplia. All available information on the pests and diseases of Z. mauritiana are from cultivated varieties. Hence, future surveys should focus on wild Z. mauritiana in the Indian subcontinent in areas that are climatically similar to the regions of northern Australia, where it is currently most abundant.     

Impacts of changed fire regimes on tropical riparian vegetation invaded by an exotic vine

Abstract Riparian habitats are highly important ecosystems for tropical biodiversity, and highly threatened ecosystems through changing disturbance regimes and weed invasion. An experimental study was conducted to assess the ecosystem impacts of fire regimes introduced for the removal of the exotic woody vine, Cryptostegia grandiflora, in tropical north‐eastern Australian woodlands. Experimental sites in subcatchments of the Burdekin River, northern Queensland, Australia, were subjected to combinations of early wet‐season and dry‐season fires, and single and repeated fires, with an unburnt control. Woody vegetation was sampled using permanent quadrats to record and monitor plants species, number and size‐class. Sampling was conducted pre‐fire in 1999 and post‐fire in 2002. All fire regimes were effective in reducing the number and biomass of C. grandiflora shrubs and vines. Few woodland or riparian species were found to be fire‐sensitive and community composition did not change markedly under any fire regime. The more intense dry‐season fires impacted the structure of non‐target vegetation, with large reductions in the number of sapling trees (<5 cm d.b.h.) and reductions in the largest tree size‐class and total tree basal area. Unexpectedly, medium‐sized canopy trees (10–30 cm d.b.h.) appear to have been significantly benefited by fires, with decreases in number of trees of this size‐class in the absence of fire. Although the presence of C. grandiflora as a vine in riparian forest canopies changed the nature and intensity of crown combustion patterns, this did not lead to the initiation of a self‐perpetuating weed–fire cycle, as invaders were unable to take advantage of gaps caused by fire. Low intensity, early wet‐season burning, or early dry‐season burning, is recommended for control of C. grandiflora in order to minimize the fire intensity and risk of the loss of large habitat trees in riparian habitats.     

Biology and host plant range of the soft scale,Steatococcus new species [Hem.: Margarodidae] for the biological control of the weedCryptostegia grandiflora [Asclepiadaceae]

The soft scale,Steatococcus new species, occurs onCryptostegia grandiflora R. Br. in Madagascar. Studies on its biology and host plant range showed colonies could survive for up to 6 months on some genera in theApocynaceae and indefinitely on many genera in the familyAsclepiadaceae. The species was therefore rejected as a biological control agent forC. grandiflora, a serious weed in northern Queensland, Australia.      

Geographic variation in morphological and reproductive characters of coastal and tableland populations ofBlandfordia grandiflora (Liliaceae)

For tenBlandfordia grandiflora populations spanning about 90% of the species' range, univariate and multivariate analyses on 14 vegetative and reproductive characters separated plants into distinct coastal and tableland groups. Distinguishing characters were number of flowers and leaves, leaf length and width, and inflorescence stalk height and diameter; coastal plants were larger than tableland plants. In a transplant experiment, coastal and tableland plants retained their phenotypic distinctness, indicating that vegetative morphology was genetically determined. Coastal plants exhibited clinal variation with latitude. Compared to tableland plants, coastal plants had higher pollen: ovule ratios, and produced fewer but heavier seeds per flower. Tableland and coastal plants are phenotypically distinct, indicating that separate subspecific status is warranted.     

The beneficial role of potassium in Cd-induced stress alleviation and growth improvement in Gladiolus grandiflora L.

Heavy metal contaminated agricultural soils are one of the most important constraints for successful cultivation of crops. The current research was conducted to evaluate the role of potassium (K) on plant growth and amelioration of cadmium (Cd) stress in Gladiolus grandiflora under greenhouse conditions. G. grandiflora corms were sown in media contaminated with 0 (C), 50 (Cd50) and 100 (Cd100) mg Cd kg^?1 soil. The plants growing in Cd-contaminated media exhibited reduced gas exchange attributes, chlorophyll (Chl) contents, vegetative and reproductive growth as compared to control. The plants raised in Cd contaminated media showed reduced nutrition yet higher Cd contents. However, supplementation of 60 mg Kg^?1 K in treated plants (C+K, Cd50+K and Cd100+K) improved quantity of total soluble protein and proline (Pro) along with activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) under Cd stress. Similarly, K supplementation reduced the level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) in treated plants. Potassium supplemented plants exhibited better vegetative and reproductive growth. The improved stress tolerance in K supplemented plants was attributed to the reduced quantity of MDA and H₂O₂, enhanced synthesis of protein, proline, phenols, flavonides and improved activity of antioxidant enzymes. The present research supports the application of K for alleviation of Cd stress in G. grandiflora.     

Effect of tree leaves with or without urea as a feed supplement on nutrient digestion and nitrogen balance in sheep

Two nitrogen balance experiments in a 4 × 4 Latin square design were conducted to compare leaves of Grewia oppositifolia and Ziziphus mauritiana containing condensed tannins (CT) 0.08 g and 34.7 g per kg DM, respectively as supplement on feed intake, nutrients digestibility and nitrogen (N) retention in adult wethers and to examine the benefits of adding urea to Z. mauritiana leaves containing higher level of CT on N utilization in the animals. In experiment 1, the basal diet of oat hay was fed to sheep either un-supplemented or supplemented with 320 g dried leaves of Z. mauritiana or 320 g dried leaves of G. oppositifolia or 160 g dried leaves each of Z. mauritiana and G. oppositifolia. In vivo dry matter (DM) digestibility was not different among the four diets while N digestibility remained significantly lower (P<0.05) in Z. mauritiana supplemented diets. Daily intake of oat hay reduced (P<0.05) and that of total diet DM increased (P<0.001) with feeding of the leave supplements. Mean N retention on control diet was 4.39 g/d and increased to 7.51 g/d with inclusion of G. oppositifolia leaves but did not change with the other two supplements. In experiment 2, a basal diet of sorghum hay was fed ad libitum without any supplement (control diet) and the other three diets were supplemented with 320 g dried leaves of Z. mauritiana with no urea, 5 g urea/d or 10 g urea/d. Daily intake (g/d) of sorghum hay or total DM intake did not respond to inclusion of urea in the diets. Supplementation of Z. mauritiana with or without urea did not affect digestibility of DM, organic matter and acid detergent fiber. Nitrogen retention increased (P<0.05) from 0.57 g/d on control diet to 3.72 g/d with supplementation of Z. mauritiana leaves. Addition of urea 5 g/d did not further increase the N retention (4.78 g/d) but was significantly increased to 7.16 g/d in response to 10 g urea/d in the diet. It was concluded that response to urea feeding in the presence of tannin rich Z. mauritiana leaves was dose-dependant and that feeding 10 g urea/d increased the capacity of sheep to consume more feed and retain more N in the body.     

栽培与野生化血丹不同部位中两种化学成分含量分析

为探讨并分析栽培与野生化血丹植株中不同部位中两种化学成分的含量差异,该研究采用超声法提取、高效液相色谱法(HPLC)测定栽培与野生化血丹根、茎、叶、花、混合样等部位中桃叶珊瑚苷和梓醇的含量,并进行比较。结果表明:(1)桃叶珊瑚苷在栽培与野生化血丹植株内均有分布,含量均以根中最高,其在栽培与野生化血丹植株内的含量表现分别为根叶混合样茎花、根混合样茎花叶,栽培化血丹不同部位中桃叶珊瑚苷的含量均高于野生化血丹。(2)梓醇在栽培化血丹的茎中未检出,在栽培与野生化血丹其他部位均有分布,含量均以叶中最高,其在栽培与野生化血丹植株内的含量分别表现为叶花混合样根、叶混合样茎花根,野生化血丹不同部位中梓醇的含量均高于栽培化血丹。(3)桃叶珊瑚苷和梓醇在栽培和野生化血丹植株不同部位中的含量均存在显著差异(P0.05),栽培与野生同一部位间总体上无显著差异,为该濒危药用植物资源药用部位选择和合理开发利用提供实验参考。     

Lack of fire and insect herbivory constrain recruitment in a rare legume from Western Australia

We investigated factors influencing recruitment in the rare endemic shrub Acacia insolita subsp. recurva (Fabaceae) from the heavily cleared south‐west of Western Australia. We examined annual seed production over 4 years, including the impact of herbivory on reproductive output. Conversion of bud to fruit was low (overall 0.02%). The lack of significant difference in canopy dimensions between caged and uncaged plants suggested that vertebrate herbivore grazing was negligible, but invertebrate predation had a negative impact on seed production (loss of >80% of fruiting potential). Soil cores determined the presence of soil‐stored seeds and an experimental burn confirmed that plants are fire‐killed and can regenerate from the seed bank. This seed reserve was found to contain <5 seeds m^?2, and both freshly collected seeds and seeds retrieved from the soil had high viability (99% vs. 91%) when subjected to a germination test. Seedling recruitment 29 months post‐fire resulted in a ratio of three seedlings for every adult killed by fire. We also compared reproductive success in this rare Acacia with its common conspecific and although the rare species produced more flowers, the success of flowering did not translate into better fruit set. We conclude that insect damage to reproductive branchlets and lack of appropriate disturbance are major factors constraining recruitment. Active site management may be required for the continued persistence of this fire‐dependent legume.     

<Emphasis Type="Italic">Codium fragile</Emphasis>: rhizomatous growth in the<Emphasis Type="Italic"> Zostera</Emphasis> thief of eastern Canada

A rhizomatous growth form of Codium fragile is described for the first time. Plants were collected in the Gulf of St. Lawrence in estuaries dominated by Zostera marina. Rhizomatous plants developed from propagules of whole plants that settled horizontally. Horizontal axes of C. fragile were up to 1 m long in plants collected in situ. Plants developed several to dozens of erect axes at right angle to the base. Horizontal growth of up to 0.2 m was found in field experiments where fragments were tied to plastic mesh and left in situ for 4 months. The unconsolidated filaments at the base of C. fragile often wrapped around the rhizomes of Z. marina and up to five separate attachment sites to eelgrass were found in single plants of C. fragile. In four estuaries, 57–100% of Codium plants with identifiable substratum were attached to shoots and rhizomes of Z. marina. The rhizomatous growth form was found in plants identified as C. fragile ssp. tomentosoides (Nova Scotia and Prince Edward Island) and C. fragile ssp. atlanticum (Prince Edward Island), suggesting that this is a phenotypic response to growth in soft bottom environments.Communicated by K Lüning     

Microsite requirements,population structure and growth of the cushion plant Azorella compacta in the tropical Chilean Andes

Abstract In the altiplano of northern Chile, we investigated the microsite habitats, adult population structure and growth rates over 14 months for the giant cushion plant Azorella compacta. The majority of plants (52.0%) were found growing downslope of large (>50 cm diameter) boulders and on north‐facing rocky slopes with a slope angle greater than 10° inclination (44.4%). Size was used as a surrogate for age. The youngest size class was most common; remaining size classes were roughly equal in frequency; but very large cushions were scarce. Growth rates were expressed as height, diameter and perimeter over a 14‐month period. Height increased an average of 1.40 cm, perimeter increased an average of 10.28 cm and diameters actually decreased 2.30 cm, although this was not a significant difference. Using perimeter averages based on mean plant size, we estimated a radial growth rate of 1.70 cm per 14 months. Trenches 4 cm wide were cut over the length of five plants, and these trenches were nearly repaired after 1 year, indicating that growth is overall and not just on the perimeter of the plant. These findings indicate that although Azorella compacta populations may be recovering from past exploitation, future monitoring is still necessary to ensure viable populations.     

Plant architecture,growth and biomass allocation effects of the invasive pathogen myrtle rust (Austropuccinia psidii) on Australian Myrtaceae species after fire

In 2010, the parasitic fungus Austropuccinia psidii (myrtle rust) was detected in Australia. Austropuccinia psidii infects immature growth of myrtaceous species. Many of Australia’s myrtaceous species occur within fire‐prone vegetation communities and have the capacity to resprout after fire. Therefore, it is likely that new post‐fire growth may be vulnerable to A. psidii infection, causing subsequent flow‐on effects to species’ persistence and community dynamics. The aim of this study was to test the impacts of A. psidii on native Australian Myrtaceae species after fire. We grew eight native susceptible species in a glasshouse experiment before burning them and inoculating the resprouting new growth of half the plants with A. psidii. We assessed the effect of A. psidii on the architecture, growth and biomass allocation of our study species. Although general patterns were observed across species, results were found to be species‐specific. Austropuccinia psidii significantly reduced the height of two of the eight species (Callistemon citrinus and Eucalyptus moluccana), but none of the species had increased branching. As expected, specific leaf area was lower (9%) in inoculated plants – although only significant for C. citrinus and E. dalrympleana – and leaf biomass was greater (15%), but significant for Angophora costata only. Finally, biomass allocation did not significantly differ between infection treatments. We can conclude that the effect of A. psidii infection on fire‐damaged plants has significant impacts on plants at the species level, which may have flow‐on effects at the community level, especially after repeated infections. Furthermore, these impacts may be exacerbated in the future under climate change, as the predicted increase in frequency and intensity of fires across Australia will result in more frequent new growth availability, providing more opportunities for A. psidii infection.     

Vegetative and seedling regeneration after fire in planted Sardinian pinewood compared with that in other areas of Mediterranean-type climate

Aim Fire is a key disturbance in Mediterranean‐type climates. It has effects on plant community structure and composition and on the evolution of different groups of the flora. This study aimed to quantify changes in demography and vegetative regeneration caused by fire in key species in Mediterranean vegetation and to examine the hypothesis of convergence in characteristics of species in relation to fire by comparing the occurrence of bioecological groups of plants in the Mediterranean basin with groups of plants in other areas of Mediterranean‐type climate. Methods Changes in the demography and species composition after fire were studied in a plant community developed from a pinewood (Pinus halapensis Miller plantation) in north‐western Sardinia. The demography of the three main species dominating the community after fire, Cistus monspeliensis L., Pistacia lentiscus L. and Chamaerops humilis L., was quantified by quadrat measurements in areas of the plantation which had never been burnt and in different parts 1 and 8 years after fire. Vegetative regeneration of resprouting species was also quantified. From the results of the survey and examination of published material, comparison was made between bioecological groups of species in different regions with Mediterranean‐type climate to assess the similarities and differences in the biogeographic distribution of what can be broadly considered fire‐related functional groups of plants. Results Seeder and sprouter species showed contrasting population responses to fire in an area planted to pines over 50 years ago but subsequently in part subjected to fire. Mean species richness at a 25 m² scale decreased by c. 28% over 8 years after fire. C. monspeliensis in the unburnt pinewood was sparse and sporadic but recruitment after fire was prolific and produced plant densities similar to those of similar seeder species in other Mediterranean Basin areas and in Californian chaparral. However density was very variable between burnt areas of different ages. In particular, seedling recruitment was much lower under the canopies of P. lentiscus than away from the canopy. Sprouter species (P. lentiscus and C. humilis) had recruited into the pinewood over time since planting of the pines but did not recruit after fire. Above‐ground parts of plants were killed by fire and resprout stem density per plant increased initially but later decreased slightly due to self thinning of stems on each plant. Main conclusions Plant demographic and floristic changes after fire in the Sardinian pinewood were similar to those reported for other Mediterranean plant communities. Resprouting species only recovered by vegetative means and, in contrast to seeder species, underwent thinning of stems within, rather than between, plants, so plant density did not change after fire in resprouting species. The Tertiary relic, strongly resprouting, animal‐dispersed group of species in the Mediterranean Basin has a homologous group in the North American mediterraneoid flora but not in the Southern Hemisphere. In contrast, the general group of resprouting species which recruit after and/or between fires in the Southern Hemisphere mediterraneoid areas corresponds to only a small group of taxa in the Mediterranean basin. The bioecological group of seeder species, which are killed by and establish prolifically after fire, are represented in both Northern and Southern Hemisphere Mediterranean‐type heathlands but are much more species rich in the south.     

Introduction of asymbiotically propagated seedlings of <Emphasis Type="Italic">Cephalanthera falcata</Emphasis> (Orchidaceae) into natural habitat and investigation of colonized mycorrhizal fungi

Asymbiotic seedling propagation and introduction of seedlings into a natural habitat were achieved for Cephalanthera falcata. For immature seeds collected 65 days after pollination, high germination rate (av. 50%) was achieved on Hyponex agar medium plates. Root development occurred in about 10% of the protocorms 5 months after seed sowing. Rooted protocorms were transferred to a culture bottle containing 100 ml of the Hyponex agar medium and incubated continually. In about 30% of the transferred individuals, shoot height reached 1.5–2 cm 8 months after the transfer. After acclimatization in wet vermiculite at 4°C for 6 months, 135 individuals were planted in a natural stand of C. falcata in mid February 2001. Shoot appearance rate was 44.4% at the first year and flowering was noted in some plants. At the fifth year, shoots with an average height of 21.6 cm still appeared in four plants, and flowering was noted in three of them. Colonization of mycorrhizal fungi was examined in two of them as well as one wild plant, in which the mycorrhizal fungi were identified to be in Thelephoraceae or Russulaceae. These fungi are known to form ectomycorrhiza with trees, and thus a tripartnership symbiosis consisting of C. falcata, mycorrhizal fungi and trees was suggested. The involvement of ectomycorrhizal fungi might be the reason for the low survival rate in the field experiment, because the distribution of ectomycorrhizal fungi relevant to this orchid is assumed to be uneven. The possibility of introducing artificially propagated orchids into natural habitats was discussed.     

A new subspecies ofArenaria grandiflora (Caryophyllaceae) from the Rif mountains (Northern Morocco)

A new subspecies,A. grandiflora subsp.gomarica (Caryophyllaceae), is described from the mountains of northern Morocco. The new taxon is tetraploid and has been confused in the past either with the Balearic endemicA. grandiflora subsp.glabrescens or with the widespreadA. grandiflora subsp.grandiflora, which is also present in the African continent. Detailed morphological analysis showed that plants from the Rif mountains are closely related toA. grandiflora subsp.glabrescens, but differ from it in having a longer calyx, smaller seed size, and a glandular indument on pedicels, leaves and calyx.Arenaria grandiflora subsp.gomarica can be distinguished from all infraspecific taxa of theA. grandiflora complex by the combined presence of a heterotrichous (simple and pluricellular eglandular hairs, glandular hairs) indument on the inflorescences and glandular hairs on the calyx.     

Ant foraging on extrafloral nectaries of Qualea grandiflora (Vochysiaceae) in cerrado vegetation: ants as potential antiherbivore agents

Summary Qualea grandiflora is a typical tree of Brazilian cerrados (savanna-like vegetation) that bears paired extrafloral nectaries (EFNs) along its stems. Results show that possession of EFNs increases ant density on Q. grandiflora shrubs over that of neighbouring non-nectariferous plants. Frequency of ant occupancy and mean number of ants per plant were much higher on Qualea than on plants lacking EFNs. These differences resulted in many more live termitebaits being attacked by foraging ants on Qualea than on neighbours without EFNs. Termites were attacked in equal numbers and with equal speeds on different-aged leaves of Qualea. The greatest potential for herbivore deterrence was presented by Camponotus ants (C. crassus, C. rufipes and C. aff. blandus), which together attacked significantly more termites than nine other ant species grouped. EFNs are regarded as important promoters of ant activity on cerado plants.     

Chilling stress and oxygen metabolizing enzymes in Zea mays and Zea diploperennis*

Abstract. The activities of five active-oxygen scavenging enzymes were compared for cold-lability and three were compared for chilling induction in two Zea genotypes of contrasting susceptibility to photoinhibition during chilling. Activities of superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GTR, EC 1.6.4.2) in leaf extracts from plants grown without chilling stress were assayed at 19°C and 5°C. Enzymes from the chilling-susceptible Z. Mays cv. LG11 had lower specific activities at 5°C than did enzymes from the chilling-tolerant Z. diploperennis, except for MDHAR where no significant differences were observed. The activities of SOD and APX from Z. diploperennis were double those of Z. mays at both assay temperatures. Monodehydroa-scrobate reductase and glutathione reductase activities in both species were reduced by 63–78% at a 5°C assay temperature. The dehydroascorbate reductase (DHAR) showed the greatest low-temperature lability losing 96% (Z. diploperennis) and 100% (Z. mays) of its activity at 5°C. To examine possible chilling-induced changes in levels of enzyme activity, plants of both Zea genotypes were transferred to growth chambers at 10°C at moderate light intensities. Glutathione reductase activity was found to increase within 24h in Z. diploperennis, but it decreased slightly in Z. mays. MDHAR activity decreased by 50% in Z. diploperennis but showed only a transient increase in activity in Z. mays.     

Reproductive ecology and the persistence of an endangered plant

Amsinckia grandiflora (large-flowered fiddleneck)is an extremely rare California annual wildflower, known only from threepopulations. We conducted field and greenhouse experiments to compare the rareheterostyle with a cryptic self-incompatibility system (A.grandiflora) to a common, self-compatible, homostylous, sympatriccongener (A. tessellata). Inter-species comparisons ofadult plants suggested that in the greenhouse, A.grandiflora balances low floral seed set (seeds per flower) withincreased floral output (flowers/plant) and a greater number of flowers perinflorescence. Seed set from active self-, intra- and inter-morph pollinationswas high in A. grandiflora, indicating that the crypticself-incompatibility system does not prevent seed set in the species. In thefield, A. grandiflora floral output was only slightlygreater than for A. tessellata, and did not fully balance lowerfloral seed set. Amsinckia tessellata average seed weightwas lower than that of A. grandiflora, which, along with the lowernumber of flowers produced, indicated lower maternal investment per nutlet thanfor A. grandiflora. Under conditions of unlimited resources, itappears that A. grandiflora fitness is not intrinsicallylimited when compared to its weedy relative A. tessellata.The differences in nutlet output between A. grandiflora andA. tessellata under field conditions are more likely due todifferential responses to extrinsic factors such as competition and pollinatoravailability.     

Severed stems of Amaranthus palmeri are capable of regrowth and seed production in Gossypium hirsutum

Field studies were conducted to evaluate the capacity of Amaranthus palmeri to grow and reproduce following incomplete physical control in Gossypium hirsutum fields. A. palmeri plants that emerged simultaneously with a G. hirsutum crop were selected for use. Treatments included severing the main stem of flowering plants at heights of 0, 3 and 15 cm above the soil level. A non‐cut/intact control, in which the apical meristem was not removed, was also included. Six weeks after treatment, intact A. palmeri plants had grown to a mean height of 210 cm (SE = 38) and produced 477 408 (SE = 81 250) seeds per plant. Thirty‐five percent of the A. palmeri plants cut back to a height of 15 cm above the soil level did not recover from the treatment; survivors regrew to a mean height of 102 cm (51% reduction, compared to intact plants) and produced 116 000 seeds per plant (73% reduction). A. palmeri plants cut to 3 cm above the soil level had a mortality rate of 64%, an 82% reduction in final plant height, and produced 28 000 seeds per plant. When stems were severed at the soil surface, plant mortality was 95%; final plant height and seed production of survivors were reduced by 95 and 99%, respectively, relative to the control. G. hirsutum seeded yields exceeded 3 t ha^?1 when A. palmeri plants were cut back to at least 15 cm, whereas yield was reduced 50% in the control treatment, where A. palmeri growth was not interrupted by cutting. In conclusion, while there is immediate benefit of removing A. palmeri plants in terms of G. hirsutum yield, incomplete stem removal can have multi‐season implications. Results demonstrate that severely pruned A. palmeri plants can resume growth, reach reproductive maturity and produce viable seed, which have the potential to repopulate soil seedbanks.