Thermotherapy and Storage Temperature Manipulations Limit the Production of Reactive Oxygen Species in Stored Pedunculate Oak Acorns

Abstract

For many species, seed storage protocols are still being improved to provide viable seeds of the highest quality. Seed storage is extremely problematic for short-lived seeds categorized as recalcitrant, including pedunculate oak (Quercus robur L.), for which the optimal seed storage protocol involves a temperature of –3 °C and 40% acorn moisture content recommendations. The sensitivity of pedunculated oak seeds to temperature manipulations under preparation for long-term storage has been poorly investigated, particularly in terms of the production of reactive oxygen species (ROS), which are assumed to be determinants of seed longevity. Thermotherapy, the pathogen elimination procedure, did not increase the level of three types of ROS: hydrogen peroxide (H2O2), superoxide anion radical and hydroxyl radical (•OH). The temporal heat stress of thermotherapy resulted in slightly reduced levels of H2O2, indicating activation of the antioxidant systems in acorn preparation for storage. The effect of constant storage temperatures (−3, −5, −7 °C) and their combinations (−3 → −5 °C or −3 → −5 →−7 °C) on ROS levels and seed viability was investigated in three provenances. The highest ROS levels were detected in acorns stored at −7 °C, whereas three-step cold acclimation was beneficial for reducing ROS levels. Interestingly, the levels of H2O2 were not affected by temperature in thermotherapized acorns. In contrast, decreasing storage temperature caused a linear increase in •OH levels in all provenances. The effect of heat stress and cold stress on ROS levels in relation to long-term seed storage of pedunculate oak is discussed here in relation to the seed viability evidenced via germination rates, seedling emergence and electrolyte leakage. Thermotherapy and cold acclimation of acorns can improve their viability after storage by decreasing ROS levels.

Effect of Acorn Thermotherapy and Short-Term Storage on Morphological Characteristics of Related Quercus Robur L. Seedlings

“…Other studies on Quercus spp. confirmed the association of the exposure of acorns to thermotherapy with higher acorn viability, germination rates, and the earlier and more uniform germination [29,30,32,36]. The latter was also noticed in this research (personal observation, data not shown).…”

“…Still, thermotherapy is considered to be an efficient method for the control of acorn mycobiota, as the ubiquitous proliferating fungi which can be induced by its application have not yet been proven to reduce the acorn germination after short-term storage [30,33]. Moreover, thermotherapy has been associated with higher acorn viability and germination rates, indicating it does not adversely affect the quality of acorns if combined with the appropriate storage conditions [29,30,33,36]. However, still little is known about the effect of acorn thermotherapy and storage on the characteristics of related seedlings, as there are not many available studies on this topic and most of them are primarily focused on the storage alone [29,[37][38][39][40].…”

Deterioration in the Quality of Recalcitrant Quercus robur Seeds during Six Months of Storage at Subzero Temperatures: Ineffective Activation of Prosurvival Mechanisms and Evidence of Freezing Stress from an Untargeted Metabolomic Study

“…BTO causes strong DNA damage inhibited by antioxidants [67]. Stored acorns display increased ROS levels [21], suggesting that the antioxidant system is less active. Indeed, among the compounds more abundant in seeds stored at −7 • C, the end products of ascorbic acid oxidative metabolism were found (threonic and isothreonic acid) [68].…”

“…Therefore, depending on the provenance, acorns stored at −7 • C for 3 mo exhibited unchanged or decreased germination capacity [21]. Therefore, this subzero storage temperature (−7 • C) appeared promising for more detailed research in relation to the causes of declined germination capacity and deteriorated seedling establishment because the cold acclimation of pedunculate oak seeds can improve their viability after storage by decreasing reactive oxygen species (ROS) levels [21]. Direct seeding is a major practice for oak forest and woodland establishment, but the influence of basic acorn biology during seed storage is still poorly understood for many oak species [22].…”

“…The freezing tolerance of seeds affects their distribution [20]. Therefore, depending on the provenance, acorns stored at −7 • C for 3 mo exhibited unchanged or decreased germination capacity [21]. Therefore, this subzero storage temperature (−7 • C) appeared promising for more detailed research in relation to the causes of declined germination capacity and deteriorated seedling establishment because the cold acclimation of pedunculate oak seeds can improve their viability after storage by decreasing reactive oxygen species (ROS) levels [21].…”