Tissue composition and arrangement in sugar beet genotypes of different tissue strength with regard to damage and pathogen infestation

Abstract

Drought stress affects yield formation and quality of sugar beet. The aim of this study was to identify the growing period, in which drought stress has the greatest impact on growth, and furthermore, to analyze the response of different sugar beet genotypes. Causes for a different response should be identified. In pot experiments in the greenhouse, drought stress was simulated by reducing irrigation to 60% of the water holding capacity (WHC) for four weeks at various growth stages followed by re-watering. Growth reduction was greatest when drought stress occurred early in the season: the content of the quality-determining non-sugars was highest, sugar yield and beet diameter were lowest. Responses of the genotypes in sugar yield, but primarily in the accumulation of osmotically active substances differed. Despite re-watering after drought stress the restrictions could not be compensated during growth. The transpiration coefficient of the drought-stressed treatments was only slightly different to the control, because water consumption in the control did not either increase at average air temperatures beyond 23 °C. The strong effect of early drought stress could be attributed to the high growth rates, so that a limited water supply affected yield formation more than at later growth stages. The storage losses of sugar beet genotypes are closely related to damage during harvest and subsequent infestation with mould and rots. Genetic variation for storability seems to be primarily linked to textural properties of the roots such as the resistance against mechanical damage. However, no information is available about the tissue strength, tissue composition and structural organization leading to an enhanced resistance against damage and pathogen attack. Therefore, the aims of the study were the identification of genotypic differences concerning tissue strength of the beet, the relation to damage and pathogen infestation and the underlying physiological basis of tissue strength. Field trials were carried out with 6 genotypes at 2 locations in 2018. The roots were harvested in August and November. After harvest in November, a storage trial was carried out. The root strength increased from August to November. Beets with a high puncture resistance of the periderm also had a firm inner tissue. Genotypic differences in puncture resistance were not affected by the harvest time, indicating that this trait is stable throughout the growing period. A higher puncture resistance of the beet was related to a lower mould growth during storage. Genotypes with varying tissue strength also differed in fiber content (AIR), but the composition of AIR was stable over genotypes. The number of cambium rings seems not to essentially influence the tissue strength of the beet. In the further course of the project, microscopic analyzes will clarify, whether genotypic differences in tissue strength can be attributed to cell size or cell wall thickness.

Tissue strength of sugar beet root genotypic variation and environmental impact

“…They described a relation between the puncture resistance and the tissue firmness, both measured with a puncture probe, which was confirmed by Nause et al. (2020). As these studies were conducted at only two sites, the relationship between the parameters has to be tested across a wider set of environments.…”

“…The puncture test measured the maximum resistance of the root periderm as puncture resistance and the strength of the underlying tissue as tissue firmness. They described a relation between the puncture resistance and the tissue firmness, both measured with a puncture probe, which was confirmed by Nause et al (2020). As these studies were conducted at only two sites, the relationship between the parameters has to be tested across a wider set of environments.…”

“…Thus, there is no information on possible interactions between different measuring methods and/or measuring positions and environments. Kleuker and Hoffmann (2019) and Nause et al (2020) described a close correlation between puncture resistance and tissue firmness in sugar beet roots and suggested to reduce the measurements to one parameter. Puncture resistance was regarded as the most meaningful parameter as it describes the rupture of the periderm which offers an entry for pathogens during storage.…”

“…Furthermore, it has to be investigated if a higher cell wall content is also accompanied by a different cell wall composition. Recently, Nause et al (2020) analyzed the tissue composition of sugar beet genotypes. They came to the conclusion that the total amount of cell wall compounds is more important for the tissue strength than the composition.…”

“…Tissue strength is the force required to destroy the sugar beet root and may thus represent its response to external impacts. In long‐term storage trials, the tissue strength of the root turned out to be a possible indicator for the susceptibility of sugar beet genotypes to damage during harvest and for their storability (Kleuker & Hoffmann, 2020; Nause et al., 2020). Harvest, especially lifting and cleaning of the root, is mechanical intensive and leads to injuries which may act as entry points for pathogens during storage.…”