Optimum harvest maturity for guayule seed

Guayule (Parthenium argentatum Gray) is a rubber-producing shrub native to the semi-arid region of north central Mexico and southwestern Texas. Timely harvest is critical to achieve maximum seed viability, vigour, and yield. The objective of this study was to investigate possible indicators of optimum seed maturity in guayule. The optimum harvest maturity time for guayule was studied by comparing quality parameters at different times after flowering. Heat units expressed as growing degree-days after flowering were calculated and related to seed development stages and quality. Seed quality at different stages of development was assessed by germination, capitulum dry mass, 1000 seed mass, and percentage of filled seeds. The maximum seed quality was recorded at 329 growing degree-days (GDD). This was 28 days from time of flowering. At this date, the moisture content of the capitulum was 48% on a wet basis and the colour was comparable to cinnamon (Code 165C) on the Royal Horticultural Society (R.H.S.) standard colour chart. Of all the parameters GDD, 1000 seed mass, and percentage of filled seeds provided a more rapid and reliable measure of optimum seed maturity. Colour identification can be used as an additional indicator. (C) 2005 Elsevier B.V. All rights reserved.

Transpiration efficiency in a segregating population of sunflower: Inheritance, correlation with other traits and association with hybrid grain yield

Plants incorporate isotopes of carbon into their tissue at different rates because of discrimination against 13C relative to 12C during photosynthesis. This difference in discrimination has been negatively correlated with transpiration efficiency (TE) in many C3 species and so, carbon isotope discrimination (Δ) of leaf tissues has been proposed as a potential tool for selecting genotypes with improved performance under water limited conditions. The relationship between Δ and TE in sunflower has been described previously using diverse genotypes, but this relationship has not been investigated with material selected from a segregating population. In this study, the TE of twenty recombinant inbred lines from a population (HAR4 x SA52) segregating for Δ was evaluated in a rainout shelter experiment. A strong negative genetic correlation between TE and Δ was observed (rg = -0.58), confirming previous studies of sunflower with unrelated lines. In addition, TE was strongly correlated to plant height at the final harvest (rg = 0.64) and TDW (rg = 0.58), and moderately correlated to SLW (rg = 0.46) and SPAD (rg = 0.21) but not leaf number (rg = 0.02). Estimates of narrow sense heritability of TE and Δ were very high (0.82 and 0.77, respectively) suggesting that selection for these traits could occur in early generations of segregating populations. Grain yield evaluations under field conditions of hybrids contrasting for Δ showed that low Δ (high TE) hybrids had a yield advantage between 22-35% in dry environments where the yield was less than 2t/ha. While this level of yield advantage may not be realized in commercial breeding programs, computer simulations suggest that 10-15% yield improvements may be possible. Low Δ material selected from the population HAR4 x SA52 has been distributed to private seed companies for further evaluation.

Gut content analysis to distinguish between seed feeding and mycophagy of a biphyllid beetle larva found on Acacia melanoxylon

In a search for potential biocontrol agents for Acacia melanoxylon R. Br. (Mimosaceae), larvae of the beetle Diplocoelus dilataticollis Lea (Coleoptera; Biphyllidae) were found within damaged seeds of A. melanoxylon. The gut contents of larvae and adults were examined to determine whether their diet included seeds, in apparent contradiction to the known mycophagous diet of members of this family of beetles. Calcofluor M2R White, a plant cell-wall staining optical brightener was used to differentiate between plant cell fragments and fungal tissue in the gut content smears. Gut contents of adults of a known seed predator of A. melanoxylon, a weevil of the genus Melanterius, were examined in the same way to provide a benchmark. The gut contents of D. dilataticollis differed from those of Melanterius sp. Fungal structures and microbes were found in the gut of D. dilataticollis, in contrast to plant cell fragments found in the gut of the weevil and from scrapes made directly from seeds. We conclude that larvae of D. dilataticollis feed primarily on fungi associated with damaged seed and therefore may not be the proximate cause of seed damage.

Effects of boron foliar applications on vegetative and reproductive growth of sunflower

Foliar application may be used to supply boron (B) to a crop when B demands are higher than can be supplied via the soil. While B foliar sprays have been used to correct B deficiency in sunflower (Helianthus annuus L.) in the field, no studies have determined the amount of B taken up by sunflower plant parts via foliar application. A study was conducted in which sunflower plants were grown at constant B concentration in nutrient solution with adequate B (46 mum) or with limited B supply (0.24, 0.40 and 1.72 mum) using Amberlite IRA-743 resin to control B supply. At the late vegetative stage of growth (25 and 35 d after transplanting), two foliar sprays were applied of soluble sodium tetraborate (20.8 % B) each at 0, 28, 65, 120 and 1200 mm (each spray equivalent to 0, 0.03, 0.07, 0.13 and 1.3 kg B ha(-1) in 100 L water ha(-1)). The highest rate of B foliar fertilization resulted in leaf burn but had no other evident detrimental effect on plant growth. Under B-deficient conditions, B foliar application increased the vegetative and reproductive dry mass of plants. Foliar application of 28-1200 mm B increased the total dry mass of the most B-deficient plants by more than three-fold and that of plants grown initially with 1.72 mum B in solution by 37-49 %. In this latter treatment, the dry mass of the capitulum was similar to that achieved under control conditions, but in no instance was total plant dry mass similar to that of the control. All B foliar spray rates increased the B concentration in various parts of the plant tops, including those that developed after the sprays were applied, but the B concentration in the roots was not increased by B foliar application. The B concentration in the capitulum of the plants sprayed at the highest rate was between 37 and 93 % of that in the control plants. This study showed that B foliar application was of benefit to B-deficient sunflower plants, increasing the B status of plant tops, including that of the capitulum which developed after the B sprays were applied. (C) 2003 Annals of Botany Company.

Storage protein mobilization and thiol protease up-regulation by solid matrix priming in loblolly pine (Pinus taeda) seed embryos

Experiments were conducted to investigate physiological mechanisms of solid matrix priming (SMP) on germination enhancement of loblolly pine (Pinus taeda) seeds. During SMP, osmotic potential in the embryo decreased by 0.65 MPa, concentration of crystalloid proteins decreased to 62% and concentrations of buffer soluble proteins and free amino acids increased by 22% and by 166%, respectively. Observations under an electron microscope demonstrated protein bodies in the embryo were mobilized. Inhibitor analysis indicated thiol protease was the dominant enzyme among endopiptidases to degrade the reserved proteins. A fragment of thiol protease was cloned from the primed seed embryos and it has high identities to those thiol proteases responsive to water stress. RNA get blot analysis showed a 1.5 kb thiol protease gene was up-regulated by SMP. Treatment with E64, a thiol protease inhibitor, negated SMP effects on germination performance, water potentials and protein profiles. Based on the experimental results, reserve protein mobilization induced by SMP in the embryo before radicle emergence might be one of the mechanisms to enhance germination in loblolly pine seeds.

Enzymatic cyclization of a potent Bowman-Birk protease inhibitor, sunflower trypsin inhibitor-1, and solution structure of an acyclic precursor peptide

The most potent known naturally occurring Bowman-Birk inhibitor, sunflower trypsin inhibitor-1 (SFTI-1), is a bicyclic 14-amino acid peptide from sunflower seeds comprising one disulfide bond and a cyclic backbone. At present, little is known about the cyclization mechanism of SFTI-1. We show here that an acyclic permutant of SFTI-1 open at its scissile bond, SFTI-1[ 6,5], also functions as an inhibitor of trypsin and that it can be enzymatically backbone-cyclized by incubation with bovine beta-trypsin. The resulting ratio of cyclic SFTI-1 to SFTI1[6,5] is similar to9:1 regardless of whether trypsin is incubated with SFTI-1[ 6,5] or SFTI-1. Enzymatic resynthesis of the scissile bond to form cyclic SFTI-1 is a novel mechanism of cyclization of SFTI-1[ 6,5]. Such a reaction could potentially occur on a trypsin affinity column as used in the original isolation procedure of SFTI-1. We therefore extracted SFTI-1 from sunflower seeds without a trypsin purification step and confirmed that the backbone of SFTI-1 is indeed naturally cyclic. Structural studies on SFTI-1[ 6,5] revealed high heterogeneity, and multiple species of SFTI-1[ 6,5] were identified. The main species closely resembles the structure of cyclic SFTI-1 with the broken binding loop able to rotate between a cis/trans geometry of the I7-P8 bond with the cis conformer being similar to the canonical binding loop conformation. The non-reactive loop adopts a beta-hairpin structure as in cyclic wild-type SFTI-1. Another species exhibits an isoaspartate residue at position 14 and provides implications for possible in vivo cyclization mechanisms.

Seed viability and dormancy mechanisms of Leucopogon melaleucoides Cunn. ex DC. (Epacridaceae)

Leucopogon melaleucoides, a flowering shrub, is desired by floricultural markets but is difficult to propagate. Seed viability was tested and dormancy mechanisms were studied to develop a commercial propagation system. Although around 56% of seed were viable, germination was completely inhibited unless the endocarp was removed. After-ripened seed (8 months after collection) germinated faster than fresh seed (2 days after collection), but germination occurred over a prolonged period (155 days). Germination of after-ripened seed was promoted with GA(3) or a commercial smoke product containing unknown plant growth regulators. All viable seed treated with GA(3) at 1000 mg L-1 had germinated after 24 days. The results suggest that both a physical and physiological dormancy mechanism occur for this species, with removal of the endocarp and pretreatment with 1000 mg L-1 GA(3) promoting complete germination of viable seed.

An investigation into effects of long-distance seed dispersal on organelle population genetic structure and colonization rate: a model analysis

A simulation-based modelling approach is used to examine the effects of stratified seed dispersal (representing the distribution of the majority of dispersal around the maternal parent and also rare long-distance dispersal) on the genetic structure of maternally inherited genomes and the colonization rate of expanding plant populations. The model is parameterized to approximate postglacial oak colonization in the UK, but is relevant to plant populations that exhibit stratified seed dispersal. The modelling approach considers the colonization of individual plants over a large area (three 500 km x 10 km rolled transects are used to approximate a 500 km x 300 km area). Our approach shows how the interaction of plant population dynamics with stratified dispersal can result in a spatially patchy haplotype structure. We show that while both colonization speeds and the resulting genetic structure are influenced by the characteristics of the dispersal kernel, they are robust to changes in the periodicity of long-distance events, provided the average number of long-distance dispersal events remains constant. We also consider the effects of additional physical and environmental mechanisms on plant colonization. Results show significant changes in genetic structure when the initial colonization of different haplotypes is staggered over time and when a barrier to colonization is introduced. Environmental influences on survivorship and fecundity affect both the genetic structure and the speed of colonization. The importance of these mechanisms in relation to the postglacial spread and genetic structure of oak in the UK is discussed.

Fruit processing, seed viability and dormancy mechanisms of Persoonia sericea A. Cunn. ex R. Br. and P-virgata R. Br. (Proteaceae)

The morphology of the fruit and difficulties with fruit processing impose major limitations to germination of Persoonia sericea and P. virgata. The mesocarp must be removed without harming the embryo. Fermentation of fruit or manual removal of the mesocarp was effective but digestion in 32% hydrochloric acid (HCl) completely inhibited germination. The endocarp is extremely hard and therefore very difficult and time consuming to remove without damaging the seeds. The most efficient method was cracking the endocarp with pliers, followed by manual removal of seeds. Germination was completely inhibited unless at least half of the endocarp was removed. Microbial contamination of the fruit and seeds was controlled by disinfestation and germination of the seed under aseptic conditions. The results suggest that dormancy in these species is primarily due to physical restriction of the embryo by the hard endocarp.

Segregation distortion for seed testa color in mungbean (Vigna radiata L. Wilcek)

Genetic segregation experiments with plant species are commonly used for understanding the inheritance of traits. A basic assumption in these experiments is that each gamete developed from megasporogenesis has an equal chance of fusing with a gamete developed from microsporogenesis, and every zygote formed has an equal chance of survival. If gametic and/or zygotic selection occurs whereby certain gametes or zygotic combinations have a reduced chance of survival, progeny distributions are skewed and are said to exhibit segregation distortion. In this study, inheritance data are presented for the trait seed testa color segregating in large populations (more than 200 individuals) derived from closely related mungbean (Vigna radiata L. Wilcek) taxa. Segregation ratios suggested complex inheritance, including dominant and recessive epistasis. However, this genetic model was rejected in favor of a single-gene model based on evidence of segregation distortion provided by molecular marker data. The segregation distortion occurred after each generation of self-pollination from F-1 thru F-7 resulting in F-7 phenotypic frequencies of 151:56 instead of the expected 103.5:103.5. This study highlights the value of molecular markers for understanding the inheritance of a simply inherited trait influenced by segregation distortion.

Germination response of Phalaris paradoxa L. seed to different light qualities

The influence of different light regimes on the germination of Australian and English populations of Phalaris paradoxa L. (awned canary-grass) seed was investigated to determine the impact of changing tillage practices on weed infestation. Seeds of all biotypes were highly viable, but differed in levels of innate dormancy (26-99%). In one experiment seed from a single Australian biotype, either enclosed in the spikelet glumes or having the spikelet glumes removed, were exposed to nine light treatments. Germination was stimulated by red and white light, but was inhibited by far-red light. Time to 50%, germination was less for seed enclosed in the spikelet glumes than for naked caryopses, although the final percentage of seed germinating when still enclosed in the spikelet glumes was significantly lower than for naked caryopses. In another experiment, six Australian and English biotypes with varying dormancy characteristics were exposed to eight light treatments. Red light did not stimulate germination in the deeply dormant biotype, however stimulated all other biotypes. Germination in darkness was below 20% in all biotypes except for one where germination was 51%. To overcome dormancy seeds were imbibed and placed in darkness at 16degreesC for either 7 or 14 days prior to exposure to red or white light for a single 15-min period. Dormancy in all biotypes was overcome indicating that a period of burial may decrease the dormancy level and increase seed sensitivity to light. This increased light sensitivity suggests that exposure to light during tillage may stimulate germination in P. paradoxa seed.

Sunflower trypsin inhibitor-1

SFTI-1 is a bicyclic 14 amino acid peptide that was originally isolated from the seeds of the sunflower Helianthus annuus. It is a potent inhibitor of trypsin, with a sub-nanomolar K, value and is homologous to the active site region of the well-known family of serine protease inhibitors known as the Bowman-Birk trypsin inhibitors. It has a cyclic backbone that is cross-braced by a single disulfide bridge and a network of hydrogen bonds that result in a well-defined structure. SFTI-1 is amenable to chemical synthesis, allowing for the creation of synthetic variants. Alterations to the structure such as linearising the backbone or removing the disulfide bridge do not reduce the potency of SFTI-1 significantly, and minimising the peptide to as few as nine residues results in only a small decrease in reactivity. The creation of linear variants of SFTI-1 also provides a tool for investigating putative linear precursor peptides. The mechanism of biosynthesis of SFTI-1 is not yet known but it seems likely that it is a gene-coded product that has arisen from a precursor protein that may be evolutionarily related to classic Bowman-Birk inhibitors.

Genetic variation for carbon isotope discrimination in sunflower: Association with transpiration efficiency and evidence for cytoplasmic inheritance

Plants accumulate isotopes of carbon at different rates because of discrimination against C-13 relative to C-12. In plants that fix carbon by the C-3 pathway, the amount of discrimination correlates negatively with transpiration efficiency (TE) where TE is the amount of dry matter accumulated per unit water transpired. Therefore, carbon isotope discrimination (Delta) has become a useful tool for selecting genotypes with improved TE and performance in dry environments. Surveys of 161 sunflower (Helianthus spp.) genotypes of diverse origin revealed a large and unprecedented range of genetic variation for Delta (19.5-23.8parts per thousand). A strong negative genetic correlation (r(g)) between TE and Delta (r(g) = -0.87, P < 0.001) was observed in glasshouse studies. Gas exchange measurements of field grown plants indicated that Delta was strongly correlated with stomatal conductance to water vapor (g), (r(g) 0.64, P < 0.01), and the ratio of net assimilation rate (A) to g, (r(g) = 0.86, P < 0.001), an instantaneous measure of TE. Genotype CMSHA89MAX1 had the lowest TE (and highest Delta) of all genotypes tested in these studies and low yields in hybrid combination. Backcrossing studies showed that the TE of this genotype was due to an adverse effect of the MAX1 cytoplasm, which was inherited from the diploid perennial H. maximiliani Schrader. Overall, these studies suggested that there is an excellent opportunity for breeders to develop sunflower germplasm with improved TE. This can be achieved, in part, by avoiding cytoplasms such as the MAX1 cytoplasm.

Discovery, structural determination, and putative processing of the precursor protein that produces the cyclic trypsin inhibitor sunflower trypsin inhibitor 1

Backbone-cyclized proteins are becoming increasingly well known, although the mechanism by which they are processed from linear precursors is poorly understood. In this report the sequence and structure of the linear precursor of a cyclic trypsin inhibitor, sunflower trypsin inhibitor 1 (SFTI-1) from sunflower seeds, is described. The structure indicates that the major elements of the reactive site loop of SFTI-1 are present before processing. This may have importance for a protease-mediated cyclizing reaction as the rigidity of SFTI-1 may drive the equilibrium of the reaction catalyzed by proteolytic enzymes toward the formation of a peptide bond rather than the normal cleavage reaction. The occurrence of residues in the SFTI-1 precursor susceptible to cleavage by asparaginyl proteases strengthens theories that involve this enzyme in the processing of SFTI-1 and further implicates it in the processing of another family of plant cyclic proteins, the cyclotides. The precursor reported here also indicates that despite strong active site sequence homology, SFTI-1 has no other similarities with the Bowman-Birk trypsin inhibitors, presenting interesting evolutionary questions.