Sucrose Accumulation in Sugar Beet Under Drought Stress

Drought stress may affect sucrose accumulation of sugar beet by restricting leaf development and storage root growth. The objective of this study was to identify changes occurring in the storage root of Beta beets in growth characteristics and ions and compatible solutes accumulation under drought with regard to sucrose accumulation. Two pot experiments were conducted: (1) sugar beet well supplied with water (100 % water capacity), under continuous moderate (50 %) and severe drought stress (30 %), (2) sugar beet and fodder beet well supplied with water (100 %) and under continuous severe drought stress (30 %). Under drought stress, the ratio of storage root to leaf dry matter of sugar beet decreased indicating a different partitioning of the assimilates. The sucrose concentration of the storage root was reduced. In the root, the number of cambium rings was only slightly affected, although drought stress was implemented already 6 weeks after sowing. In contrast, the distance between adjacent rings and the cell size was considerably restricted, which points to a reduced expansion of existing sink tissues. The daily rate of sucrose accumulation in the root showed a maximum between 16 and 20 weeks after sowing in well-watered plants, but it was considerably reduced under drought stress. The concentration of compatible solutes (K, Na, amino acids, glycine betaine, glucose and fructose) decreased during growth, while it was enhanced because of drought. However, when sucrose concentration was added, a constant sum of all examined solutes was found throughout the vegetation period. It was similar in sugar beet and in fodder beet despite different concentrations of single solutes, and the total sum was not affected by water supply. A close negative relationship between the concentration of compatible solutes and sucrose occurred. It is therefore concluded that the accumulation of compatible solutes in the storage root of Beta beets under drought might be a physiological constraint limiting sucrose accumulation.