Polyamines (PAs) are low molecular weight aliphatic nitrogenous bases containing two or more amino groups. They are produced by organisms during metabolism and are present in almost all cells. Because they play important roles in diverse plant growth and developmental processes and in environmental stress responses, they are considered as a new kind of plant biostimulant. With the development of molecular biotechnology techniques, there is increasing evidence that PAs, whether applied exogenously or produced endogenously via genetic engineering, can positively affect plant growth, productivity, and stress tolerance. However, it is still not fully understood how PAs regulate plant growth and stress responses. In this review, we attempt to cover these information gaps and provide a comprehensive and critical assessment of the published literature on the relationships between PAs and plant flowering, embryo development, senescence, and responses to several (mainly abiotic) stresses. The aim of this review is to summarize how PAs improve plants' productivity, and to provide a basis for future research on the mechanism of action of PAs in plant growth and development. Future perspectives for PA research are also suggested.

Anoectochilus roxburghii was grown under different shade treatments–50%, 30%, 20%, and 5% of natural irradiance–to evaluate its photosynthetic characteristics, chloroplast ultrastructure, and physiology. The highest net photosynthetic rates and stomatal conductance were observed under 30% irradiance, followed in descending order by 20%, 5%, and 50% treatments. As irradiance decreased from 50% to 30%, electron transport rate and photochemical quenching increased, while non-photochemical quenching indexes declined. Reductions in irradiance significantly increased Chl a and Chl b contents and decreased Chl a/b ratios. Chloroplast ultrastructure generally displayed the best development in leaves subjected to 30% irradiance. Under 50% irradiance, leaf protein content remained relatively stable during the first 20 days of treatment, and then increased rapidly. The highest peroxidase and superoxide dismutase levels, and the lowest catalase activities, were observed in plants subjected to the 50% irradiance treatment. Soluble sugar and malondialdehyde contents were positively correlated with irradiance levels. Modulation of chloroplast development, accomplished by increasing the number of thylakoids and grana containing photosynthetic pigments, is an important shade tolerance mechanism in A. roxburghii.

The aim of this study was to investigate the effects of light quality on the morphological traits, leaf anatomical characteristics, antioxidant enzyme (superoxide dismutase, catalase, and peroxidase) activities, photosynthetic pigments content, and bioactive compounds (phenols, flavonoids, and polysaccharides) content in Anoectochilus roxburghii. Plants of A. roxburghii were grown under light filtered through four differently colored films for 8 months. The four treatments were red film (RF), blue film (BF), yellow film (YF), and colorless plastic film (CLF; control, CK). Compared with the A. roxburghii plants in CK, those in the BF treatment showed significantly greater stem diameter, fresh weight, leaf area, stomatal frequency, chlorophyll content (Chl a, Chl b, Chl a+b), antioxidant enzyme activities, and active compound (polysaccharides, flavones) content. The plants in the RF treatment showed the greatest plant height and phenolics contents. These results show that growing A. roxburghii plants under blue film is a useful technique to improve quality. This technique is conducive to achieving large-scale sustainable production of high-quality plant materials.