High-temperature tolerance of Citrus maxima ‘Mansailong’ seeds at different developmental stages

Abstract

The seeds are invariably exposed to high-temperature conditions since global warming frequently causes abnormally high temperature. In order to investigate the physiological basis of high-temperature tolerance in seeds, we used intermediate Citrus maxima ‘Mansailong’ seeds as the research material, heated the seeds at different developmental stages, and simultaneously detected changes in seed morphology, contents of soluble protein and heat-stable protein, and cellular ultrastructure. The results were as follows: (1) The seed moisture content dramatically dropped between 23 and 49 weeks after flowering (WAF), whereas the percentage of DW/FW and fresh weight increased significantly. All of these indicators began to change quickly from 31 WAF and stabilized around 41 WAF. (2) At 29 WAF, the seeds acquired full seedling formation ability and a preliminary tolerance to high temperature; from there, the high-temperature tolerance gradually grew and improved quickly between 37 and 49 WAF. The improvement in high-temperature tolerance was accompanied by a steady increase in contents of soluble protein and heat-stable protein of seeds, from 23 to 49 WAF. The results of the correlation analysis showed a substantial positive correlation between the accumulation of heat-stable protein and soluble protein and the ability of seeds to tolerate high temperature. (3) Ultrastructural observation showed that the number of mitochondria progressively dropped as the seed developed, the volume of the embryonic axis cells gradually decreased, and the number of lipid bodies in the cells gradually rose and their arrangement became more and more regular. In addition, the vacuoles enlarge at the same time, and at a later stage, they were packed with black floccules. In conclusion, the C. maxima ‘Mansailong’ seeds reach physiological maturity at 41 WAF without any noticeable maturation drying; the ability to tolerate high temperature is acquired during seed development and is further enhanced until a later stage; changes in cellural ultrastructure and an increase in contents of soluble protein and heat-stable protein of seeds are essential in helping the seeds develop their high-temperature tolerance.

Keywords

• high-temperature stress
• intermediate seeds
• cellular ultrastructure
• soluble protein
• heat-stable protein
• seed development