As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01727.x

The effect of drought and heat stress on reproductive processes in cereals.

Male reproductive processes in flowering plants take place in the stamen (Esau, 1977). This sporophytic organ system contains diploid cells that undergo meiosis and produce haploid male spores, or microspores. Microspores divide mitotically and differentiate into multicellular male gametophytes, or pollen grains, that contain the sperm cells. Figures 1 and 2 show that the stamen consists of two morphologically distinct partsthe anther and the filament. The filament is a tube of vascular tissue that anchors the stamen to the flower and serves as a conduit for water and nutrients. By contrast, the anther contains the reproductive and nonreproductive tissues that are responsible for producing and releasing pollen grains so that pollination and fertilization processes can occur within the flower. Figure 1 shows that anther development can be divided into two general phases. During phase 1, the morphology of the anther is established, cell and tissue differentiation occur, and microspore mother cells undergo meiosis. At the end of phase 1, the anther contains most of its specialized cells and tissues, and tetrads of microspores are present within the pollen sacs (Figure 1). During phase 2, pollen grains differentiate, the anther enlarges and is pushed upward in the flower by filament extension, and tissue degeneration, dehiscence, and pollen grain release occur (Figure 1). The cellular processes that regulate anther cell differentiation, establish anther tissue patterns, and cause the anther to switch from a histospecification program (phase l) to a cell degeneration and dehiscence program (phase 2) are not known. The developmental events leading to anther formation and pollen release are exquisitely timed and choreographed (Koltunow et al., 1990; Scott et al., 1991). Cell differentiation and dehiscence events occur in a precise chronological order that correlates with floral bud size (Koltunow et al., 1990; Scott et al., 1991). This permits the mechanisms responsible for cell-type differentiation, tissue degeneration, and cellspecific gene activation within the anther to be explored with relative Base. During the past few years, there has been an explosive burst of interest in anther biology, both as a system to dissect plant developmental processes at the molecular and genetic levels (Koltunow et al., 1990; Gasser, 1991; McCormick,

Anther development: basic principles and practical applications.

The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinators driven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selection is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and asking which aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. We reaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruit set is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that do not. Reproductive success is skewed towards few individuals in a population and effective population sizes are often small. Population structure, reproductive success and gene flow among populations suggest that in many situations genetic drift may be as important as selection in fostering genetic and morphological variation in this family. Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversity in this family is largely a consequence of sequential and rapid interplay between drift and natural selection. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 1–54.

Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification

https://www.agroparistech.fr/IMG/pdf/wilcocket_al.pdf

Pollination failure in plants: why it happens and when it matters.

This paper reviews the literature concerning digestion and nutrient content of pollen. Four topics are addressed in detail: 1) The mechanism of pollen digestion by animals; 2) The efficiency of mechanical and digestive removal of pollen content by various animals; 3) Range and taxonomic distribution of pollen nutrients, and 4) Adaptive hypotheses proposed to associate pollen chemistry with pollinator reward. Studies on the mechanism(s) of pollen digestion remain inconclusive, but suggest that differences in digestibility among pollen types may reflect differences in pollen wall porosity, thickness, and composition. Although hummingbirds reportedly digest pollen very poorly, most animals studied, including those that do not regularly consume pollen, can digest 50–100% of ingested grains. Overlooked and recent research of pollen protein content shows that pollen grains may contain over 60% protein, double the amount cited in some studies of pollen-feeding animals. Adaptive hypotheses that associate pollen starch and pollen caloric content with pollinator reward remain unsubstantiated when critically viewed through the lens of phylogeny.

Pollen nutritional content and digestibility for animals

A recent paper by Baker and Baker (1979) on starch reserves in Angiosperm pollen grains only reports finding black and dark blue starch by the IKI test. However other types of insoluble polysaccharides, made not only by amylose and amylopectin, are present: they are composed mainly of amylopectin and dextrin, which stain from dark red to brown with the IKI test. Starch was detected in the shed pollen of 152 plant species and cultivars of economic interest. The cytological detection of starch was effected microscopically either by 1) staining with iodine/potassium iodide (IKI) or 2) observing by polarization microscopy. The occurrence of polysaccharides dispersed in the cytoplasm was detected by PAS preceeded by aldehyde blockade. Plants belong to the following families: Cucurbitaceae, Graminaceae, Labiatae, Leguminosae, Liliaceae, Oleaceae, Rosaceae, Rutaceae, Solanaceae.

Pollen Polysaccharide Reserves in Some Plants of Economic Interest

The PAS reaction was used to stain total insoluble polysaccharides in pollen of two species of Pinus collected in two different years and germinated in vitro at different pH values. PAS-detectable polysaccharides are localized in the intine, cytoplasm and amyloplasts; PAS-undetectable (callose) in the middle of the intine, but only in the distal part of the grain. The total PAS stained polysaccharide content was quantified with a microdensitometer. Values are correlated with germination percentages and pollen tube length. The total polysaccharide content, as well as pollen tube length, varies in each species according to the year, probably due to environmental conditions. Also the various pHs deeply influence pollen germination and tube length, the lowest pH being the most unfavourable condition. At this pH (3.5) the total polysaccharide content is higher than in the control (pH 5.5): this is probably due to a lack of callose synthesis for tube wall or to a depolymerization of this polysaccharide...

Effects of acidity on the insoluble polysaccharide content of germinating pollen of Pinus pinea L. and Pinus pinaster Aiton

Hypothetical phylogenetic trends of the tapetum in sporangia and anthers

Peumus boldus essential oil : new constituents and comparison of oils from leaves of different origin

Sections of Ammi visnaga (L.) Lam. ripe fruits were examined by U.V. microscopy under U.V. 365 nm light and under mixed U.V. 365 nm and visible light. The examination revealed a substance with blue autofluorescence inside the primary rib channels; a blue colour was shown also by the endosperm, whilst vittae were empty. In sections extracted with a selective method for the furanochromones khellin and visnagin, the substance filling primary rib channels was completely dissolved, and the colour of endosperm faded. The extract was analysed by high-performance liquid chromatography: the solubilized materials were composed almost exclusively of khellin and visnagin. These data are in contrast with the previous consensus which considered coumarins and coumarin-like compounds to be located in the vittae of Umbelliferae fruits, and never present in endosperms, whilst primary rib channels were thought empty in ripe fruits.

Ammi Visnaga (L.) Lam.: Occurrence of Khellin and Visnagin in Primary Rib Channels and Endosperm, and Emptiness of Vittae, Revealed by U.V. Microscopy*

Gian Gabriele Franchi

Papers

Pollen Polysaccharide Reserves in Some Plants of Economic Interest

Effects of acidity on the insoluble polysaccharide content of germinating pollen of Pinus pinea L. and Pinus pinaster Aiton

Hypothetical phylogenetic trends of the tapetum in sporangia and anthers

Peumus boldus essential oil : new constituents and comparison of oils from leaves of different origin

Ammi Visnaga (L.) Lam.: Occurrence of Khellin and Visnagin in Primary Rib Channels and Endosperm, and Emptiness of Vittae, Revealed by U.V. Microscopy*