Abstract: In order to adapt to marine environmental stresses, such as high salinity, high pressure, low temperature and less sunshine, seaweed plants accumulate many kinds of functional active substances in their tissues during growth process. These active substances play important roles in promoting crop growth and improving fruit quality, and enhancing the resistance to environmental stresses. In this paper, the advantages and disadvantages for the methods of physically ultrasonic crushing, chemical acid/alkaline hydrolysis, and complex enzymatic hydrolysis or microbial hydrolysis are discussed in detail according to the extraction technologies of seaweed extracts. It is pointed out that the combination of physical, chemical and biological hydrolysis is the best process for obtaining specific functional substances in seaweed. Meanwhile, the physiological functions of polysaccharides, polyphenols, phytohormones, and amino acid derivatives in seaweed are analyzed so as to explain the internal mechanism of seaweed extracts to promote crop growth. On this basis, the effects of seaweed extracts on soil remediation, root growth, flowering/fruit setting and stress resistance are emphatically discussed. Finally, some problems in the research and application as well as research directions regarding seaweed extracts are put forward.

Key words: dense orchard, canopy structure., thinning and reshaping, leaf quality, apple