The only ultraviolet rays that affect plant growth are UVB and UVA, which are an important part of life on earth. And UVC is filtered out by the ozone layer and never really enters our plants naturally outdoors. 

Faster photosynthesis 

Ultraviolet light helps speed up the process of photosynthesis and promotes plant growth. Exposure to UV-A light increases photosynthesis by 12%. UV-A light promotes increased leaf size, dry weight and growth potential. 

Increase resin production 

When intense UV wavelengths hit a plant, the plant produces a substance called resin in order to protect itself from more UV rays. Resin is a protective substance that prevents plants from dehydration or infection, and also adds protection from pests or fungi. These resins give plants their vibrant colors, rich taste and characteristic smell. 

speed up seed germination 

When grown indoors, UV light can speed up the germination process of the seeds. When transplanting seedlings to a more intense light source, the UV light boosts the plants' growth power, making them better prepared for high-intensity light. Exposing the plant's seeds to UV light shortens germination time and speeds up plant growth. 

But overexposure to UV rays can also wreak havoc on plants. Plant leaves and buds lose their taste and smell when exposed to too much light. Plant surface cells can become damaged and discoloured, leading to stunted growth and poor yields.