Crop plant domestication has targeted a variety of traits, including synchronous development of ovules and stamens to maximize fertilization and seed production. Wheat plant bears autogamous, self-pollinating flower which is very attractive for guaranteeing yield but at the same time frustrating for the researchers and breeders trying to develop hybrids through cross-pollination. A new discovery, characterizing the developmental physiology of wheat florets opening after a few days post-anthesis (second opening), provides an additional opportunity for pollination, facilitates out-crossing and provides a method to further understand the regulation of wheat flower architecture and development. (Okada T, 2017)
MECHANISM OF FLOWER OPENING:
First flower opening:
Generally, at anthesis in wheat, floret first opening driven by lodicule swelling, anther dehiscence and filament elongation occur simultaneously, that also, only for less than 30 minutes and then closes ensuring self-fertilization (Heslop-Harrison Y, 1996). This flower opening is caused due to increased turgidity of lodicules which pushes the anthers outward. Since this extrusion occurs post-anthesis and flower opens only for short duration, there is very less chance (<1%) to produce F1 hybrid (Adelaide, 2018). More the duration of flower opening, more will be the probability for out-crossing.
Second flower opening:
The second opening of flower occurs due to the enlargement of unfertilized ovary. The fertilized ovaries increase in size vertically with the degradation of pericarp whereas the unfertilized ovaries swell up radially along with the pericarp cells remaining intact and enlarging but show only a slight increase in vertical direction. This enlargement exceeds the breadth of lemma and forces lemma and palea apart resulting the opening of flower for the second time (Nansamba, 2017). This second opening can last for several days providing better opportunities for cross-pollination.
To address the question, when and how does the ovary detect the absence of pollination and begin to enlarge, different researches were held and it was found that the width and depth of male-sterile ovaries were already significantly larger than male-fertile ovaries at 3 DF (days after fertilization). This indicates that the enlargement of ovary initiates at 3 DF in male-sterile lines.
Now, there is a dilemma, how the ovary could remain unfertilized till the second opening of flower?
Wheat flower is autogamous and self-pollinating and hence, the pollination is completed prior to extrusion of anthers i.e. first opening. Therefore, there is insignificant probability of flower being out-crossed.
There are two popular ways to prevent self-fertilization in wheat (Mette MF, 2015):
- First is, chemical application to the female receptor line or emasculation. This is expensive, can give variable results and requires precision spraying in favorable weather conditions to avoid triggering sterility in the male donor lines, which are grown in close proximity (Whitford R, 2013).
- The second method involves the use of male-sterile lines that do not produce pollen. It is favorable because it needs no chemical manipulation, however, the use of male-sterile lines only is not sufficient as they must also open to allow cross-pollination by neighboring male pollen donor line. The male-sterile lines have prolonged second opening facilitated by the swollen ovary (Okada T, 2017).
The row of male donor line and female receptor line is arranged in a close proximity with male pollen donor line taller than the female receptive line which extrudes anther prior to dehiscence to release pollen. The pollen gets dispersed over male-sterile florets. The male-sterile florets open due to radial swelling of unfertilized ovary so that it can receive the pollen and increase the chance of fertilization and survival. In this whole process, breeder should keep in mind that the female receptive line is isolated from the contact of all other undesired pollen. For this, the spikes should be covered with glycine bag immediately after emasculation. Isolation can also be done as spatial isolation (isolation by distance) or temporal isolation (isolation by time i.e. by altering the sowing time or varietal time etc.)
You may wonder, why not the laboratory-work then??
Well, we are all acquainted with the fact that, first opening of flower lasts only for few hours and capturing the pollen at right stage is a lottery. Thus, collecting pollen from the field and fertilizing with ovule at correct stage with success is impracticable. Instead, using lines with an open receptive female flower would make it possible to pollinate by wind, dramatically increasing the chance for successful fertilization.
Fertilization is crucial in plant’s life cycle mostly for evolution and to continue its race. When fertilization fails, plant tends to increase the probability of new fertilization by altering their behavioral pattern like flowering pattern and so on. One of the noble examples is second opening of flower in wheat. This research characterizes the cross-talk between fertilization, ovule development and physiology of flowers in self-pollinating cereal crops. Moreover, it reveals the alteration in function of different floral parts such as the nutrients accumulated in pericarp for seed development is redirected for a different function, specifically the opening of the flower. The outcomes and findings of this research can also be applied to different plant species which would further aid in sustainable grain production.
Adelaide, U. o. (2018, February 15). Open flower for Wheat Seed Production. Research Stories.
Heslop-Harrison Y, H.-H. J. (1996). Lodicule function and ﬁlament extension in the grasses: potassium ion movement and tissue specialization. Annals of Botany 77, 573-582.
Mette MF, G. M. (2015). Hybrid breeding in wheat. In: Ogihara Y, Takumi S, Handa H, . Advances in wheat genetics: from genome to field.eds.Tokyo:Springer.
Nansamba, M. &. (2017). Unfertilized ovary pushes wheat flower open for cross-pollination. Journal of Experimental Botany.
Okada T, J. R. (2017). Unfertilized ovary pushes wheat flower open for cross-pollination. Journal of Experimental Botany68, 395–408.
Whitford R, F. D. (2013). Hybrid breeding in wheat: technologies to improve hybrid wheat seed production. Whitford R ,Fleury D,Reif JC,Garcia M,Okada T,Korzun V,Langridge P. 201Journal of Experimental Botany64,, 5411–5428.