Unfertilized ovary pushes wheat flower open for cross-pollination

Diversity in bread and durum wheat stigma morphology and linkage of increased stigma length to dwarfing gene Rht14

KEY MESSAGE

The dwarfing allele Rht14 of durum wheat associates with greater stigma length, an important trait for hybrid breeding, whilst major dwarfing alleles Rht-B1b and Rht-D1b showed little to no effect. Although much understudied in wheat, the stigma is a crucial component for attaining grain set, the fundamental basis for yield, particularly in hybrid production systems where successful grain set relies on wind-driven pollen dispersal by the male parent and effective pollen capture by the female parent. Females with long stigma that exsert early are thought to be advantageous. Using glasshouse-grown lines, we examined variation in Total Stigma Length (TSL) across diverse panels comprising 27 durum and 116 bread wheat genotypes. Contrasting genotypes were selected for population development and genetic analysis. Quantitative trait loci (QTL) analysis was performed on a durum F2 population and a bread wheat recombinant inbred line (RIL) population. Contrasting with studies of anther length, we found no large effect on TSL of the GA-insensitive semi-dwarfing genes Rht-B1 and Rht-D1 in either durum or bread wheat. However, in durum cultivar Italo, we identified a region on chromosome 6A which is robustly associated with larger TSL and contains the Rht14 allele for reduced plant height, a trait that is favourable for female line development in hybrid systems. This dual effect locus explained 25.2 and 19.2% of TSL phenotypic variation in experiments across two growing seasons, with preliminary results suggesting this locus may increase TSL when transferred to bread wheat. In a bread wheat, RIL population minor QTL on 1A and 2A was indicated, but the strongest association was with Ppd-B1. Methods developed here, and the identification of a TSL-enhancing locus provides advances and further opportunities in the study of wheat stigma.

From the floret to the canopy: High temperature tolerance during flowering

Heat waves induced by climate warming have become common in food-producing regions worldwide, frequently coinciding with high temperature (HT)-sensitive stages of many crops and thus threatening global food security. Understanding the HT sensitivity of reproductive organs is currently of great interest for increasing seed set. The responses of seed set to HT involve multiple processes in both male and female reproductive organs, but we currently lack an integrated and systematic summary of these responses for the world's three leading food crops (rice, wheat, and maize). In the present work, we define the critical high temperature thresholds for seed set in rice (37.2°C ± 0.2°C), wheat (27.3°C ± 0.5°C), and maize (37.9°C ± 0.4°C) during flowering. We assess the HT sensitivity of these three cereals from the microspore stage to the lag period, including effects of HT on flowering dynamics, floret growth and development, pollination, and fertilization. Our review synthesizes existing knowledge about the effects of HT stress on spikelet opening, anther dehiscence, pollen shedding number, pollen viability, pistil and stigma function, pollen germination on the stigma, and pollen tube elongation. HT-induced spikelet closure and arrest of pollen tube elongation have a catastrophic effect on pollination and fertilization in maize. Rice benefits from pollination under HT stress owing to bottom anther dehiscence and cleistogamy. Cleistogamy and secondary spikelet opening increase the probability of pollination success in wheat under HT stress. However, cereal crops themselves also have protective measures under HT stress. Lower canopy/tissue temperatures compared with air temperatures indicate that cereal crops, especially rice, can partly protect themselves from heat damage. In maize, husk leaves reduce inner ear temperature by about 5°C compared with outer ear temperature, thereby protecting the later phases of pollen tube growth and fertilization processes. These findings have important implications for accurate modeling, optimized crop management, and breeding of new varieties to cope with HT stress in the most important staple crops.

Identification of candidate genes responsible for chasmogamy in wheat

BACKGROUND

The flowering biology of wheat plants favours self-pollination which causes obstacles in wheat hybrid breeding. Wheat flowers can be divided into two groups, the first one is characterized by flowering and pollination within closed flowers (cleistogamy), while the second one possesses the ability to open flowers during processes mentioned above (chasmogamy). The swelling of lodicules is involved in the flowering of cereals and among others their morphology, calcium and potassium content differentiate between cleistogamic and non-cleistogamous flowers. A better understanding of the chasmogamy mechanism can lead to the development of tools for selection of plants with the desired outcrossing rate. To learn more, the sequencing of transcriptomes (RNA-Seq) and Representational Difference Analysis products (RDA-Seq) were performed to investigate the global transcriptomes of wheat lodicules in two highly chasmogamous (HCH, Piko and Poezja) and two low chasmogamous (LCH, Euforia and KWS Dacanto) varieties at two developmental stages-pre-flowering and early flowering.

RESULTS

The differentially expressed genes were enriched in five, main pathways: "metabolism", "organismal systems", "genetic information processing", "cellular processes" and "environmental information processing", respectively. Important genes with opposite patterns of regulation between the HCH and LCH lines have been associated with the lodicule development i.e. expression levels of MADS16 and MADS58 genes may be responsible for quantitative differences in chasmogamy level in wheat.

CONCLUSIONS

We conclude that the results provide a new insight into lodicules involvement in the wheat flowering process. This study generated important genomic information to support the exploitation of the chasmogamy in wheat hybrid breeding programs.

New Growth-Related Features of Wheat Grain Pericarp Revealed by Synchrotron-Based X-ray Micro-Tomography and 3D Reconstruction

Wheat (Triticum aestivum L.) is one of the most important crops as it provides 20% of calories and proteins to the human population. To overcome the increasing demand in wheat grain production, there is a need for a higher grain yield, and this can be achieved in particular through an increase in the grain weight. Moreover, grain shape is an important trait regarding the milling performance. Both the final grain weight and shape would benefit from a comprehensive knowledge of the morphological and anatomical determinism of wheat grain growth. Synchrotron-based phase-contrast X-ray microtomography (X-ray µCT) was used to study the 3D anatomy of the growing wheat grain during the first developmental stages. Coupled with 3D reconstruction, this method revealed changes in the grain shape and new cellular features. The study focused on a particular tissue, the pericarp, which has been hypothesized to be involved in the control of grain development. We showed considerable spatio-temporal diversity in cell shape and orientations, and in tissue porosity associated with stomata detection. These results highlight the growth-related features rarely studied in cereal grains, which may contribute significantly to the final grain weight and shape.

Heat stress responses vary during floret development in European spring barley cultivars

The Poaceae, or grasses, include many agriculturally important cereal crops such as rice (Oryza sativa), maize (Zea mays), barley (Hordeum vulgare) and bread wheat (Triticum aestivum). Barley is a widely grown cereal crop used for stock feed, malting and brewing. Abiotic stresses, particularly global warming, are the major causes of crop yield losses by affecting fertility and seed set. However, effects of heat stress on reproductive structures and fertility in barley have not been extensively investigated. In this study we examined three commercial European spring barley varieties under high temperature conditions to investigate the effects on floret development. Using a combination of fertility assays, X-ray micro computed tomography, 3-dimensional modelling, cytology and immunolabelling, we observed that male reproductive organs are severely impacted by increased temperature, while the female reproductive organs are less susceptible. Importantly, the timing of stress relative to reproductive development had a significant impact on fertility in a cultivar-dependent manner, this was most significant at pollen mitosis stage with fertility ranged from 31.6-56.0% depending on cultivar. This work provides insight into how heat stress, when applied during male pollen mother cell meiosis and pollen mitosis, affects barley fertility and seed set, and also describes complementary invasive and non-invasive techniques to investigate floret development. This information will be used to identify and study barley cultivars that are less susceptible to heat stress at specific stages of floral development.

Harnessing intra-varietal variation for agro-morphological and nutritional traits in a popular rice landrace for sustainable food security in tropical islands

Introduction

Rice crop meets the calorie and nutritional requirements of a larger segment of the global population. Here, we report the occurrence of intra-varietal variation in a popular rice landrace C14-8 traditionally grown under the geographical isolation of the Andaman Islands.

Methods

Based on grain husk color, four groups were formed, wherein the extent of intra-varietal variation was studied by employing 22 agro-morphological and biochemical traits.

Results

Among the traits studied, flavonoid and anthocyanin contents and grain yield exhibited a wider spectrum of variability due to more coefficients of variation (>25%). The first five principal components (PCs) of principal components analysis explained a significant proportion of the variation (91%) and the first two PCs explained 63.3% of the total variation, with PC1 and PC2 explaining 35.44 and 27.91%, respectively. A total of 50 highly variable SSR (HvSSR) markers spanning over 12 chromosomes produced 314 alleles, which ranged from 1 to 15 alleles per marker, with an average of 6.28. Of the 314 alleles, 64 alleles were found to be rare among the C14-8 selections. While 62% of HvSSR markers exhibited polymorphism among the C14-8 population, chromosomes 2, 7, 9, and 11 harbored the most polymorphic loci. The group clustering of the selections through HvSSR markers conformed to the grouping based on grain husk coloration.

Discussion

Our studies on the existence and pertinence of intra-varietal variations are expected to be of significance in the realms of evolutionary biology and sustainable food and nutritional security under the changing climate.

Genome-Wide Association Analysis for Hybrid Breeding in Wheat

Disclosure of markers that are significantly associated with plant traits can help develop new varieties with desirable properties. This study determined the genome-wide associations based on DArTseq markers for six agronomic traits assessed in eight environments for wheat. Moreover, the association study for heterosis and analysis of the effects of markers grouped by linkage disequilibrium were performed based on mean values over all experiments. All results were validated using data from post-registration trials. GWAS revealed 1273 single nucleotide polymorphisms with biologically significant effects. Most polymorphisms were predicted to be modifiers of protein translation, with only two having a more pronounced effect. Markers significantly associated with the considered set of features were clustered within chromosomes based on linkage disequilibrium in 327 LD blocks. A GWAS for heterosis revealed 1261 markers with significant effects.

A scalable phenotyping approach for female floral organ development and senescence in the absence of pollination in wheat

In the absence of pollination, female reproductive organs senesce, leading to an irrevocable loss in the reproductive potential of the flower, which directly affects seed set. In self-pollinating crops like wheat (Triticum aestivum), the post-anthesis viability of unpollinated carpels has been overlooked, despite its importance for hybrid seed production systems. To advance our knowledge of carpel development in the absence of pollination, we created a high-throughput phenotyping approach to quantify stigma and ovary morphology. We demonstrate the suitability of the approach, which uses light-microscopy imaging and machine learning, for the analysis of floral organ traits in field-grown plants using fresh and fixed samples. We show that the unpollinated carpel undergoes a well-defined initial growth phase, followed by a peak phase in which stigma area reaches its maximum and the radial expansion of the ovary slows, and a final deterioration phase. These developmental dynamics were consistent across years and could be used to classify male-sterile cultivars. This phenotyping approach provides a new tool for examining carpel development, which we hope will advance research into female fertility of wheat.

Wheat Escapes Low Light Stress by Altering Pollination Types

Although low light stress seriously affects florets fertility and grain number during the reproductive period, crops can be fertilized by heterologous pollen to alleviate the reduction of grain number. However, wheat is strongly autogamous, how to change to outcross after low light remains unclear. To understand the mechanisms of this change process, an approach combined morphological, physiological, and transcriptomic analyses was performed under low light stress imposed at the young microspore stage the booting stage from tetrad to uni-nucleate microspores stage. The results showed that low light stress caused pollen abortion, and the unfertilized ovary is fertilized by heterologous pollen after floret opening. Compared to control, the opening angle of lemma and glume were increased by 11.6-48.6 and 48.4-78.5%, respectively. The outcross of stressed wheat compensated for the 2.1-18.0% of grain number loss. During this process, phytohormones played an important role. Jasmonic acid (JA) and methyl jasmonate (MeJA) levels in spikelets were increased. Meanwhile, lignin and cellulose content decreased, and genes associated with cell wall related GO terms were enriched. Among the differentially expressed genes (DEGs), were identified 88-710 transcription factors genes, of which some homologs in Arabidopsis are proposed to function in lignin and cellulose, influencing the glume and lemma opening. Our finding can provide new insight into a survival mechanism to set seeds through pollination way alteration in the absence of self-fertilization after the stress of adversity.

Mapping open chromatin by ATAC-seq in bread wheat

Gene transcription is largely regulated by cis-regulatory elements. Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) is an emerging technology that can accurately map cis-regulatory elements in animals and plants. However, the presence of cell walls and chloroplasts in plants hinders the extraction of high-quality nuclei, thereby affects the quality of ATAC-seq data. Meanwhile, it is tricky to perform ATAC-seq with different tissue types, especially for those with limited size and amount. Moreover, with rapid growth of ATAC-seq datasets from plants, powerful and easy-to-use data analysis pipelines for ATAC-seq, especially for wheat is lacking. Here, we provided an all-in-one solution for mapping open chromatin in wheat including both experimental and data analysis procedure. We efficiently obtained nuclei with less cell debris from various wheat tissues. High-quality ATAC-seq data from young spike and ovary, which are hard to harvest were generated. We determined that the saturation sequencing depth of wheat ATAC-seq is about 16 Gb. Particularly, we developed a powerful and easy-to-use online pipeline to analyze the wheat ATAC-seq data and this pipeline can be easily extended to other plant species. The method developed here will facilitate plant regulatory genome study not only for wheat but also for other plant species.

Programmed Cell Death in Developing Brachypodium distachyon Grain

The normal developmental sequence in a grass grain entails the death of several maternal and filial tissues in a genetically regulated process termed programmed cell death (PCD). The progression and molecular aspects of PCD in developing grains have been reported for domesticated species such as barley, rice, maize and wheat. Here, we report a detailed investigation of PCD in the developing grain of the wild model species Brachypodium distachyon. We detected PCD in developing Brachypodium grains using molecular and histological approaches. We also identified in Brachypodium the orthologs of protease genes known to contribute to grain PCD and surveyed their expression. We found that, similar to cereals, PCD in the Brachypodium nucellus occurs in a centrifugal pattern following anthesis. However, compared to cereals, the rate of post-mortem clearance in the Brachypodium nucellus is slower. However, compared to wheat and barley, mesocarp PCD in Brachypodium proceeds more rapidly in lateral cells. Remarkably, Brachypodium mesocarp PCD is not coordinated with endosperm development. Phylogenetic analysis suggests that barley and wheat possess more vacuolar processing enzymes that drive nucellar PCD compared to Brachypodium and rice. Our expression analysis highlighted putative grain-specific PCD proteases in Brachypodium. Combined with existing knowledge on grain PCD, our study suggests that the rate of nucellar PCD moderates grain size and that the pattern of mesocarp PCD influences grain shape.

Spikelet movements, anther extrusion and pollen production in wheat cultivars with contrasting tendencies to cleistogamy

BACKGROUND

Cleistogamic flowers are a main barrier in pollen dispersal for cross-pollination necessary in wheat hybrid breeding. The aim of our study was to gain new knowledge on the biology of wheat flowering, in particular on the differences between the cleisto- and chasmogamic forms which has certainly cognitive significance, but it can also be used in practice when seeking a female and male ideotypes for cross hybridization.

RESULTS

We characterized the most significant features defining the flowering specificity in two wheat cultivars with contrasting tendency to cleistogamy: Piko (chasmogamous) and Dacanto (cleistogamous). In the field observations we assessed diurnal pattern of anther extrusion and anther extrusion capacity. For the first time we adapted the time lapse method for measuring kinetics of the spikelet movement and 3-D image correlation technique for the non-invasive measurements of potential deformations of the spikelet lemmas. We found that the two cultivars differ in the potential of pollen dispersion for-cross-pollination and in the spikelet kinetics. We also described some anatomical traits that can have potential functional role in floret opening. None of the cultivars showed any symptoms of lemma surface deformation.

CONCLUSIONS

The cleistogamic and chasmogamic wheat cultivars differ significantly in the potential for pollen dispersion for cross-pollination, which is mainly related to anther extrusion capacity. Although none of these features differentiated the cultivars clearly, we assume, based on spikelet kinetics and the lack of lemmas surface deformation, that the water transport and turgor of cells is essential for the floret opening and anther extrusion in wheat. The search for parental ideotype should be supported by marker assisted selection, e.g. based of polymorphisms in genes related to aquaporin biosynthesis.

Genetic factors associated with favourable pollinator traits in the wheat cultivar Piko

Hybrid breeding in wheat has the potential to boost yields. An efficient hybrid seed production system requires elite pollinators; however, such germplasm is limited among modern cultivars. Piko, a winter wheat (Triticum aestivum L.) cultivar, has been identified as a superior pollinator and has been used in Europe. Piko has favourable pollinator traits for anther extrusion, anther length, pollen mass and hybrid seed set. However, the genetic factors responsible for Piko's favourable traits are largely unknown. Here, we report on the genetic analysis of a Piko-derived F2 mapping population. We confirmed that Piko's Rht-D1a allele for tall stature is associated with large anthers and high anther extrusion. However, Rht-D1 was not found to be associated with anther filament length, confirmed by near isogenic lines. Piko's photoperiod sensitive Ppd-B1b allele shows an association with increased spike length, more spikelets and spike architecture traits, while the insensitive Ppd-B1a allele is linked with high anther extrusion and larger anthers. We identified an anther extrusion quantitative trait locus (QTL) on chromosome 6A that showed significantly biased transmission of the favourable Piko allele amongst F2 progenies. The Piko allele is completely absent in the distal 6AS region and the central 6A region revealed a significantly lower ratio (<8%) of F2 with homozygous Piko alleles. Our study provided further evidence for the effects of Rht-D1 and Ppd-B1 loci on multiple pollinator traits and a novel anther extrusion QTL that exhibits segregation distortion.

Increased expression of the MALE STERILITY1 transcription factor gene results in temperature-sensitive male sterility in barley

Understanding the control of fertility is critical for crop yield and breeding; this is particularly important for hybrid breeding to capitalize upon the resultant hybrid vigour. Different hybrid breeding systems have been adopted; however, these are challenging and crop specific. Mutants with environmentally reversible fertility offer valuable opportunities for hybrid breeding. The barley HvMS1 gene encodes a PHD-finger transcription factor that is expressed in the anther tapetum, which is essential for pollen development and causes complete male sterility when overexpressed in barley. This male sterility is due at least in part to indehiscent anthers resulting from incomplete tapetum degeneration, failure of anther opening, and sticky pollen under normal growth conditions (15 °C). However, dehiscence and fertility are restored when plants are grown at temperatures >20 °C, or when transferred to >20 °C during flowering prior to pollen mitosis I, with transfer at later stages unable to rescue fertility in vivo. As far as we are aware, this is the first report of thermosensitive male sterility in barley. This offers opportunities to understand the impact of temperature on pollen development and potential applications for environmentally switchable hybrid breeding systems; it also provides a 'female' male-sterile breeding tool that does not need emasculation to facilitate backcrossing.

Feeling the heat: developmental and molecular responses of wheat and barley to high ambient temperatures

The increasing demand for global food security in the face of a warming climate is leading researchers to investigate the physiological and molecular responses of cereals to rising ambient temperatures. Wheat and barley are temperate cereals whose yields are adversely affected by high ambient temperatures, with each 1 °C increase above optimum temperatures reducing productivity by 5-6%. Reproductive development is vulnerable to high-temperature stress, which reduces yields by decreasing grain number and/or size and weight. In recent years, analysis of early inflorescence development and genetic pathways that control the vegetative to floral transition have elucidated molecular processes that respond to rising temperatures, including those involved in the vernalization- and photoperiod-dependent control of flowering. In comparison, our understanding of genes that underpin thermal responses during later developmental stages remains poor, thus highlighting a key area for future research. This review outlines the responses of developmental genes to warmer conditions and summarizes our knowledge of the reproductive traits of wheat and barley influenced by high temperatures. We explore ways in which recent advances in wheat and barley research capabilities could help identify genes that underpin responses to rising temperatures, and how improved knowledge of the genetic regulation of reproduction and plant architecture could be used to develop thermally resilient cultivars.

Hybrid breeding in wheat: how shaping floral biology can offer new perspectives

Hybrid breeding in wheat (Triticum aestivum L.) has the potential to deliver major yield increases. This is a requisite to guarantee food security for increasing population demands and to counterbalance the effects of extreme environmental conditions. Successful hybrid breeding in wheat relies on forced outcrossing while preventing self-pollination. To achieve this, research has been directed towards identifying and improving fertility control systems. To maximise cross-pollination and seed set, however, fertility control systems need to be complemented by breeding phenotypically distinct male and female lines. This review summarises existing and novel male sterility systems for wheat hybridisation. We also consider the genetic resources that can be used to alter wheat's floral development and spike morphology, with a focus on the genetic variation already available. Exploiting these resources can lead to enhanced outcrossing, a key requirement in the progress towards hybrid wheat breeding.

CRISPR/Cas9-mediated knockout of Ms1 enables the rapid generation of male-sterile hexaploid wheat lines for use in hybrid seed production

The development and adoption of hybrid seed technology have led to dramatic increases in agricultural productivity. However, it has been a challenge to develop a commercially viable platform for the production of hybrid wheat (Triticum aestivum) seed due to wheat's strong inbreeding habit. Recently, a novel platform for commercial hybrid seed production was described. This hybridization platform utilizes nuclear male sterility to force outcrossing and has been applied to maize and rice. With the recent molecular identification of the wheat male fertility gene Ms1, it is now possible to extend the use of this novel hybridization platform to wheat. In this report, we used the CRISPR/Cas9 system to generate heritable, targeted mutations in Ms1. The introduction of biallelic frameshift mutations into Ms1 resulted in complete male sterility in wheat cultivars Fielder and Gladius, and several of the selected male-sterile lines were potentially non-transgenic. Our study demonstrates the utility of the CRISPR/Cas9 system for the rapid generation of male sterility in commercial wheat cultivars. This represents an important step towards capturing heterosis to improve wheat yields, through the production and use of hybrid seed on an industrial scale.

Effects of Rht-B1 and Ppd-D1 loci on pollinator traits in wheat

Elite wheat pollinators are critical for successful hybrid breeding. We identified Rht-B1 and Ppd-D1 loci affecting multiple pollinator traits and therefore represent major targets for improving hybrid seed production. Hybrid breeding has a great potential to significantly boost wheat yields. Ideal male pollinators would be taller in stature, contain many spikelets well-spaced along the spike and exhibit high extrusion of large anthers. Most importantly, flowering time would match with that of the female parent. Available genetic resources for developing an elite wheat pollinator are limited, and the genetic basis for many of these traits is largely unknown. Here, we report on the genetic analysis of pollinator traits using biparental mapping populations. We identified two anther extrusion QTLs of medium effect, one on chromosome 1BL and the other on 4BS coinciding with the semi-dwarfing Rht-B1 locus. The effect of Rht-B1 alleles on anther extrusion is genotype dependent, while tall plant Rht-B1a allele is consistently associated with large anthers. Multiple QTLs were identified at the Ppd-D1 locus for anther length, spikelet number and spike length, with the photoperiod-sensitive Ppd-D1b allele associated with favourable pollinator traits in the populations studied. We also demonstrated that homeoloci, Rht-D1 and Ppd-B1, influence anther length among other traits. These results suggest that combinations of Rht-B1 and Ppd-D1 alleles control multiple pollinator traits and should be major targets of hybrid wheat breeding programs.

Insights into transcriptional characteristics and homoeolog expression bias of embryo and de-embryonated kernels in developing grain through RNA-Seq and Iso-Seq

Bread wheat (Triticum aestivum L.) is an allohexaploid, and the transcriptional characteristics of the wheat embryo and endosperm during grain development remain unclear. To analyze the transcriptome, we performed isoform sequencing (Iso-Seq) for wheat grain and RNA sequencing (RNA-Seq) for the embryo and de-embryonated kernels. The differential regulation between the embryo and de-embryonated kernels was found to be greater than the difference between the two time points for each tissue. Exactly 2264 and 4790 tissue-specific genes were found at 14 days post-anthesis (DPA), while 5166 and 3784 genes were found at 25 DPA in the embryo and de-embryonated kernels, respectively. Genes expressed in the embryo were more likely to be related to nucleic acid and enzyme regulation. In de-embryonated kernels, genes were rich in substance metabolism and enzyme activity functions. Moreover, 4351, 4641, 4516, and 4453 genes with the A, B, and D homoeoloci were detected for each of the four tissues. Expression characteristics suggested that the D genome may be the largest contributor to the transcriptome in developing grain. Among these, 48, 66, and 38 silenced genes emerged in the A, B, and D genomes, respectively. Gene ontology analysis showed that silenced genes could be inclined to different functions in different genomes. Our study provided specific gene pools of the embryo and de-embryonated kernels and a homoeolog expression bias model on a large scale. This is helpful for providing new insights into the molecular physiology of wheat.

A new opening for wheat seed production

This article comments on: Okada T, Jayasinghe R, Nansamba M, Baes M, Warner P, Kouidri A, Correia D, Nguyen V, Whitford R, Baumann U. 2017. Unfertilized ovary pushes wheat flower open for cross-pollination. Journal of Experimental Botany 69, 399–412.