Light Pollution's Impact: Plants' Growth And Health

Light pollution is a growing problem, with around a quarter of the planet currently affected and this figure increasing by 6% every year. It is caused by outdoor lighting, such as streetlamps, and can have a detrimental impact on plants and their pollinators.

Plants rely on light as a source of energy and information, and their growth and development are determined by the quality, intensity and duration of light. Light pollution can disrupt this, with artificial light at night altering the natural photoperiod and affecting plants' seasonal rhythms and their relationship with pollinators.

For example, in 2014, a group of scientists in the UK observed that 70% of moths were drawn towards streetlights and away from flowering plants, resulting in reduced plant pollination. Another study found that nocturnal plant-pollinator interactions were 62% lower in brightly lit areas.

Light pollution can also impact the food chain, with one-third of all human food requiring a pollinator. It can also affect the health of humans, with disruption to the circadian rhythm linked to an increased risk of sleep disorders, diabetes, depression, heart conditions and breast cancer.

Light pollution disrupts the natural sleep-wake cycle of plants

Light pollution has a significant impact on plants, affecting their growth, reproduction, and overall health. One of the primary ways it does this is by disrupting their natural sleep-wake cycle, known as the circadian rhythm or photoperiod.

The circadian rhythm is a 24-hour cycle that governs the daily activities of most living things, including plants. It is regulated by the presence or absence of light and helps plants carry out essential processes, such as dormancy, shoot growth, flowering, and leaf shedding.

Artificial light at night, or light pollution, interferes with this natural cycle, causing what is known as "photoperiodism." Photoperiodism can influence a range of plant processes, including:

• Pigment formation: The presence of artificial light can alter the production of pigments in plants, such as chlorophyll, which is essential for photosynthesis.
• Leaf shedding: Artificial lighting can cause deciduous trees to retain their leaves longer than normal, making them more susceptible to damage from winter storms and increasing the risk of branch breakage.
• Bud dormancy: Artificial light can disrupt the timing of bud dormancy and growth, affecting the overall health and development of plants.
• Flowering and reproduction: Many plant species rely on specific day lengths as a cue to flower and reproduce. Artificial light can alter these cues, disrupting their natural flowering patterns and reducing reproduction.
• Pollination: Light pollution can also impact plant-pollinator relationships. Nocturnal pollinators like moths and bees may be attracted to artificial lights, drawing them away from flowering plants and reducing pollination success. This, in turn, can affect the amount of pollen available for diurnal pollinators, further disrupting plant reproduction.

The disruption of the natural sleep-wake cycle in plants can have far-reaching consequences, not only for the plants themselves but also for the entire ecosystem that depends on them. It can impact food sources for other organisms, including humans, and disrupt the delicate balance of ecological communities. Therefore, understanding and mitigating the effects of light pollution on plants is crucial for maintaining the health and sustainability of our natural world.

It can prevent flowering and pollination in certain plants

Light pollution can prevent flowering and pollination in certain plants. Many plant species, such as night-blooming cacti, bloom only at night and depend on nocturnal pollinators for pollination. Increasing lighting conditions can prohibit these plants from ever flowering and reproducing.

The whiteness and aroma of certain plants' flowers help nocturnal pollinators find them in the night. For example, Selenicereus grandiflorus (Queen of the Night) is pollinated by moths and bats and is only open for about two hours each night. Increasing lighting conditions around these plants may prohibit them from ever flowering and reproducing.

Light pollution can also affect the pollinators themselves. Moths, for example, are drawn to artificial light and away from flowering plants, resulting in a reduction in plant pollination and the number of pollen types transported. This can have wide-reaching consequences, as one-third of all human food requires a pollinator.

Artificial light can also affect the timing of flowering. In some plants, flowering is induced by long nights. A flash of artificial light can interrupt this process, preventing flowering.

It can reduce native plant growth, reproduction and fruit production

Light pollution can reduce native plant growth, reproduction and fruit production in several ways.

Firstly, artificial light can distract, repel or kill pollinators, reducing flower visits and the amount of pollen transported. This can have a ripple effect on the wider ecosystem. For example, in 2014, scientists in the UK observed that over 70% of moths were drawn towards streetlights and away from flowering plants, resulting in a reduction of plant pollination and the number of pollen types transported. This also impacted bees and other diurnal pollinators, as they had less pollen to fertilise plants during the daytime.

Secondly, artificial light can restrict the movement of seed-dispersing animals across the landscape. For example, artificial lighting along roads or paths can fragment surrounding habitats, isolating populations on either side.

Thirdly, light pollution can reduce nutrient cycling by soil-digging nocturnal mammals, such as bandicoots, bettongs and bilbies. It can also reduce the activity of invertebrates that break down dead organic material, such as beetles, marine amphipods and saltmarsh crabs.

In addition, light pollution can change food webs by causing prey to gather around light sources, reducing their dark refuges and making them easier targets for predators. It can also enable daytime predators to hunt at night.

Finally, for plant-eaters, light pollution can reduce food availability by changing the timing of plant reproduction, growth, flowering and fruiting.

It affects the timing of plant reproduction, growth, flowering and fruiting

Light pollution has a significant impact on the timing of plant reproduction, growth, flowering, and fruiting. It disrupts the natural sleep-wake cycle, known as the circadian rhythm, which governs the 24-hour sleep-wake cycle in plants and other living organisms.

Plants rely on light as a source of energy and information. They have evolved under a stable regime of 24-hour cycles of light and darkness, with seasonal variations in day length outside the tropics. The rapid spread of artificial lighting has disrupted these natural light cycles, affecting the timing of plant reproduction and growth.

For example, cacti species that bloom only at night depend on nocturnal pollinators for reproduction. Increased lighting conditions can prohibit them from flowering and reproducing. Similarly, plants that are strongly scented and have white flowers are typically pollinated at night. Altering the lighting conditions can disrupt their reproduction and, consequently, our food sources.

Artificial light can also affect the timing of plant growth. Night lighting, particularly from sources emitting red to infrared light, extends the day length and can change flowering patterns. It can promote continued growth beyond the safe period before winter, leaving trees vulnerable to frost damage and branch breakage from snow and ice buildup.

Furthermore, artificial light can impact the timing of flowering and fruiting. For instance, a study in the UK found that branches of large-leaved lime trees near LED lighting experienced budburst up to two weeks earlier than those in darker areas. In another example, Metrosideros excelsa trees in New Zealand were observed to flower more profusely next to streetlights than between them.

Light pollution also affects the timing of plant reproduction by disrupting plant-animal interactions. Nocturnal pollinators like moths and bees are drawn towards artificial lights, reducing their pollination activities. This, in turn, affects the stimulation of plants to produce pollen, impacting diurnal pollinators like bees.

In summary, light pollution has far-reaching consequences for plants, affecting the timing of their reproduction, growth, flowering, and fruiting. These disruptions to natural cycles can have cascading effects on ecosystems and our food supply.

It can alter the photoperiods of plants

Plants have the ability to sense the amount of light and duration of this light, using a light-sensitive receptor known as phytochrome. This allows them to respond appropriately by flowering at suitable times of the year.

Photoperiodism can be defined as the specific day length required by plants to induce or prevent flowering. The best photoperiod for plants varies between different species. A critical photoperiod, in simple words, is the minimum or maximum amount of daylight that is required for flowering.

In the case of long-day plants, the day length must exceed the critical photoperiod while for short-day plants, the day length needs to be lesser than the critical photoperiod.

Plants can be classified into three groups based on their photoperiods:

• Short-Day Plants: These plants flower or bloom when they experience more than 12 hours of uninterrupted darkness and less than 12 hours of light.
• Long-Day Plants: These plants flower when exposed to more than 12 hours of light and less than 12 hours of dark.
• Day-Neutral Plants: Day-neutral plants are not affected by the amount of daylight they are exposed to. Such plants initiate flowering after reaching a certain point such as their vegetative growth.

Photoperiod or the day length plays a significant role in plant growth. It can have desirable effects on some plants and undesirable effects on others depending on how long they are exposed to periods of light and dark.

There may be some plants that bloom under multiple different photoperiods but bloom much faster under one specific photoperiod.

Understanding the effects of light on plant growth can help growers to maintain the right environment for their plants.

• Results In Developmental Responses: Photoperiod for plants can be beneficial in preventing some plants from flowering or blooming. They can be kept in the vegetative stage of growth and this leads to a high and stable yield of plants.
• Affects The Seasonal Flowering Of Plants: Photoperiodism is the reason why plants flower and experience fruiting in certain seasons. The day length changes all year round and affects the productivity of plants.
• Higher Yield By Keeping Plants In Vegetative Stage Of Growth: By understanding the photoperiod of a particular plant, growers can prevent undesirable responses such as flowering, seed germination, leaf and stem growth, or bulb initiation. Instead, they can have a higher yield by keeping their plants in specific stages of growth.

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