horticulturae
horticulturae
Article
Standardization of an Effective Scarification and Germination Protocol for Strawberry Seeds That Is Useful for Gamic Propagation
Abstract: In strawberry breeding programs, seed dormancy adversely affects germination, resulting in delayed seedling emergence and low germination rates. This study investigated the best solution to enhance strawberry seed germination both in terms of efficiency and timing by evaluating the effect of three key factors: genotype, pre-sowing treatment, and germination medium. Chemical scarification treatment with the sulfuric acid of seeds from three different genotypes was optimized; treated seeds were placed to germinate on three germination media (Murashige and Skoog medium, peat, and filter paper) in a growth chamber. Seedlings obtained were acclimatized for evaluating post-acclimatization survival rate and possible phenotypic differences regarding seedling development. Chemical scarification treatment produced the best results, with germination rates of around one hundred percent and the highest speed of germination compared to the not-treated controls. Indeed, more than ninety percent of the seeds germinated fourteen days after sowing, regardless of the genotype or germination medium tested. Seedlings germinated on filter paper gave the poorest performances in terms of post-acclimatization survival rate and showed lower average plant height. In conclusion, it was demonstrated that excellent germination rates can be achieved through proper seed scarification, which is not genotype dependent; furthermore, when this method is combined with the correct germination medium, excellent seedling quality can be achieved.
One. Introduction
One. Introduction
Strawberry fruits are currently consumed all over the world due to their highly appreciated taste and aromatic profiles; moreover, they belong to the broader category of small fruits that are recognized by the scientific community as an important source of vitamins, minerals, and antioxidants which have beneficial effects on consumers' health. Strawberry consumption has increased significantly over the last decade together with consumer demand for a qualitatively superior product, both from an organoleptic and nutraceutical point of view. In this evolving context, farmers rely on scientific research to provide them with technical and scientific means to cope with the current socio-economic and climate change impacts on the entire sector. The answer to these challenges can be achieved through the development of new, low-impact cultivation techniques that give better results in terms of quality and yield of production, adapted to the different cultivation systems, and through the development of new strawberry cultivars that are genetically advanced, resilient, and able to adapt to the environmental and cultural changes in the new millennium by using traditional and new breeding techniques. Genetic studies on seedling populations are now essential for identifying genes that control traits that are important for strawberry breeding but also open the possibility of obtaining F one hybrid lines that can be propagated by seeds.
In recent years, in addition to public strawberry breeding programs affiliated with research institutions and or universities, a diversification of breeding lines on this species has been observed due to the implementation of several private breeding programs carried out by companies of reference for the strawberry nursery and or production sector. This change has increased the number of cultivars licensed annually and has expanded the varietal offer, differentiating varieties by market sectors and increasingly distinguishing them through registered trademarks. Thus, it is necessary to improve all the critical steps in a breeding program to make the whole process efficient, from the production of new genotypes, to their evaluation, selection and their long-term conservation for future progress in breeding programs. One of the crucial initial steps for strawberry breeding is the process of controlled hybridization. Problems in this phase often arise from the interaction of several factors, including the viability of the pollen to be used as a male parent, gametic incompatibility between the pollen and the gynoecium of a different genotype or a different genus (interspecific hybridization), as well as the growing environment of the parent plants and their agronomic quality.
The second criticality is encountered in the next step, namely the germination of seeds obtained from controlled crossbreeding; strawberry achenes indeed present an accentuated tegumentary dormancy that greatly affects the germination phase. The germination rate is usually around thirty percent, with high-scalar seedlings sprouting. At present, all stakeholders, especially private institutions, are struggling to reach a high percentage of seed germination and, in order to obtain the requested number of seedlings for a representative breeding population, are obliged to pollinate a large number of flowers in order to obtain a large number of seeds to be used in the in vivo germination standard procedure. Therefore, the low germination rate contributes to increasing costs for the germination program and also to restricting the characterization to only the low-rate germinated seedlings. This leads to a loss of genetic variability because each seedling presents a different, potentially superior, genotype. Scalar seedling emergence, on the other hand, results in the evaluation of seedlings with different vegetative development.
Several research groups have already evaluated and validated different methods, which are more or less efficient, to overcome seed dormancy that were mainly based on either chemical or mechanical scarification processes. This method consists of removing or weakening the outer tegument of seeds, allowing them to be imbibed with water and promoting germination.
Concerning physical or mechanical methods of scarification, seeds have been subjected to cold treatments, exposure to light, and cutting of achenes, while the best results obtained through chemical scarification were produced by the use of sodium hypochlorite, hydrochloric acid, and sulphuric acid.
Tegument dormancy affects the germination of strawberry seeds due to the outer hard shelter, which can be removed by several techniques to facilitate the germination. El Hamdouni and collaborators used chemical treatment in the achenes of two commercial cultivars which were subjected to different treatments, such as scarification with sulphuric acid, hydrogen peroxide for different timings (from five to sixty minutes), tegument cutting, and light scarification with sulphuric acid. Finally, all treated embryos were isolated and placed to germinate on a Murashige and Skoog medium free of hormones. The highest germination rate was obtained by scarifying seeds with sulphuric acid, demonstrating, along with the treatments involving lesioning or complete removal of the outer integument, the effect of the scarification protocols on strawberry achene dormancy. From these treatments, in addition to maximizing the germination rate, germination time was significantly reduced by up to fifty percent.
Another important aspect to be considered is the growing medium to be used in germination trials; there are many different substrates that are useful for germination and they are referred to under the generic term of "Growing media". A distinction is made between paper, sand, and other materials, such as organic mixtures of peat, sand, perlite, etc.
It is important to use the culture medium that best reflects the parameters of water retention, pH, and conductivity specific to the species, and it must also meet the prerequisites of cleanliness and harmlessness.
The use of in vitro growing media for strawberry seed germination has also been evaluated by some researchers as a valuable option for more standard substrates. Miller and collaborators increased the germination rate up to one hundred percent and decreased the germination delay by longitudinal cutting of achenes on Murashige and Skoog medium free of plant growth regulators. This technique was particularly efficient from the point of view of germination rate but much too laborious for the cutting of the achenes.
Hongxiang and collaborators adopted the same technique of removing the outer integument of the achenes by cutting and germinating them in vitro or on moist filter paper. Again, the best results were obtained through in vitro germination on MS media. In this study, different concentrations of PGRs were used and an almost one hundred percent germination rate was obtained. The different germination rates observed were imputed to be genotype dependent.
The current private and small breeding programs for the genus Fragaria require standardized and effective seed scarification and germination protocols that enable a high germination percentage to be achieved with short germination times without the need for particularly expensive facilities and equipment. This study investigated the best solution, also on a technical-practical level, to achieve these results using repeatable methodologies and readily available materials. In particular, we investigated the efficiency and practicality of using some of the commonly used culture media for strawberry seed germination, comparing them with an in vitro substrate. The combination of chemical scarification combined with in vitro germination seems to positively influence the results in terms of germination rate and speed of germination on the strawberries genotypes tested.