Plants flowering is an important part of their life cycle. When plants flower, it leads to the production of seeds and fruits which are essential for reproduction and dispersal. There are several factors that can influence flowering in plants.
Light
Light is one of the most important factors that induces flowering in plants. Plants need to receive signals from light that it is the appropriate time of year to flower. These light signals are sensed by photoreceptors in plants such as phytochromes and cryptochromes. The duration of light exposure, intensity of light, and wavelengths of light sensed can all influence flowering.
Many plants flower in spring or summer when days start to become longer. Long days stimulate flowering in what are called long day plants. Other plants are short day plants that flower when days become shorter in fall. The exact duration of light that induces flowering differs between plant species. Besides daylength, higher light intensity or quantities of certain wavelengths of light can promote flowering.
Some common garden and crop plants that are long day plants include potatoes, oats, lettuce, spinach, and beets. Short day plants include crops like rice, soybeans, chrysanthemums, poinsettias, and Christmas cactus.
How light induces flowering
Light signals are perceived in leaves, which then send hormones like florigen to meristems where flowers develop. In long day plants, a pigment called phytochrome absorbs red wavelengths of light. This converts phytochrome to an active form that induces the expression of genes involved in flowering. Long days keep high enough levels of active phytochrome to promote flowering. In short day plants, phytochrome also plays a role. Short days lead to more inactive phytochrome which allows flowering in these species.
Besides phytochrome, the pigment cryptochrome also absorbs blue light wavelengths that induce flowering. Cryptochromes stimulate flowering by regulating circadian clocks and the expression of flowering genes. The interplay of phytochromes, cryptochromes, and the pigment chlorophyll all allow plants to gauge seasons based on light cues.
Temperature
Ambient air temperatures also have a strong effect on flowering, especially in biennial and perennial plants. Many plants need to experience cooler temperatures for a certain period in order to flower. This cold treatment mimics winter conditions and readies the plant to flower when warmer weather returns.
The duration of required cold temperatures varies. For example, biennials like carrots and beets need around 2 weeks below 50°F (10°C) to flower the following season. Apples, pecans, and lilacs need much longer cold periods around 800-1000 hours under 45°F (7°C) to flower properly.
Cooler root zone temperatures rather than just air temperatures can also stimulate flowering. This allows container plants to experience cooler suboptimal root temperatures while shoots remain warmer.
After cold treatment, warmer temperatures need to return for floral induction to occur. Warmer temperatures initiate new growth and flowering. Each plant species has optimal temperature ranges to flower after cold exposure based on its native climate.
How temperature triggers flowering
Cool temperatures increase plant hormone levels like gibberellic acid that induce flowering. Extended cold also leads to changes in gene expression and processes like vernalization that make plants competent to flower. Once plants have fulfilled their needed chill hours and warm up occurs, flowering pathways are activated.
If plants do not receive enough chilling, flowering can be delayed or not occur at all. Insufficient chilling is a problem for growers in warmer winter climates. Providing optimal cold conditions, even artificially, is key to allowing flowering in many perennial crops.
Photoperiod
Photoperiod refers to the relative lengths of light and dark periods that plants experience. As mentioned earlier, photoperiod is a critical signal for flowering in many plants. Short day plants flower when night length exceeds a critical duration. Long day plants flower when dark periods are shorter than a critical length.
Being sensitive to photoperiod allows plants to coordinate flowering with favorable seasonal conditions. Shortening days indicate winter is coming, while lengthening days signify spring and summer. Responding to photoperiod prevents plants from flowering during stressful times of year or when pollinators are not available.
Some common short day plants that flower in late summer or fall as days shorten include poinsettias, chrysanthemums, and Christmas cactus. Long day plants like irises, spinach, and tomatoes flower in spring or early summer as days lengthen.
How photoperiod regulates flowering
Photoperiod is perceived in leaves by photoreceptors like phytochrome and cryptochrome. Under inductive short or long days, these photoreceptors trigger the expression of flowering genes. This leads to the production of florigen hormone that moves through the plant’s phloem to meristems and initiates flower development.
There are also day-neutral plants whose flowering is not controlled by photoperiod. Examples include cucumbers, roses, and tomatoes. Day-neutral plants may flower continuously as long as temperatures are suitable.
Vernalization
As mentioned, many plants need to go through a period of cold exposure before they can flower. This cold period is referred to as vernalization. It serves as a signal that winter has passed and the return of spring is an appropriate time to flower.
Vernalization is most critical in biennials. Biennials grow vegetatively in the first year, then flower, set seed and die in the second year after experiencing winter cold. Common biennial crops include carrots, beets, celery, onions and cabbage.
In perennials, vernalization encourages flowering after plants become established. Fruit trees, strawberries, and clovers are examples of perennials that benefit from vernalization to flower yearly.
The process of vernalization
During vernalization, gradual cold exposure causes changes in gene expression that make plants competent to flower. Important vernalization genes control processes like chromatin structure, circadian rhythms, and hormone pathways. Prolonged cold alters these processes so that plants transition from vegetative growth to reproductive growth when warm conditions return.
If plants do not receive enough cold to satisfy vernalization requirements, flowering can be delayed or may not occur. Growers sometimes provide artificial vernalization by exposing plants to cold temperatures in refrigeration units to ensure flowering happens on schedule.
Gibberellins
Gibberellins are a class of plant hormones that regulate various aspects of growth and development. They play an important role in stimulating flowering.
Bioactive gibberellins increase during vernalization. Exposure to cold induces the expression of gibberellin biosynthesis genes. The accumulation of gibberellins during and after cold then promotes the transition to flowering.
Applied gibberellins can also substitute for cold exposure and induce flowering. They are sometimes sprayed on trees to induce early flowering or on bolting crops to synchronize flowering. However, too much gibberellin can be detrimental and cause excessive stem elongation.
Gibberellin interactions
Gibberellins interact with other hormones like abscisic acid and ethylene to control flowering. They regulate genes involved in the flowering pathway to stimulate inflorescence and flower development. Gibberellins also induce flowering signals like LEAFY.
Gibberellin biosynthesis inhibitors or compounds that inactivate gibberellins are used to prevent flowering. These are applied to keep ornamental plants vegetative or delay flowering until the desired time.
Stress
Environmental stresses can influence the flowering response in plants. Stress conditions that can impact flowering include drought, heat, flooding, nutrient deficiency, and salinity.
Moderate stress often promotes flowering as plants try to complete their life cycle under suboptimal conditions. However, severe stress usually suppresses flowering because resources are focused on survival instead of reproduction.
Drought stress tends to hasten flowering, especially in annual plants. Water deficit signals conditions are deteriorating and induces earlier flowering. Flooding stress also shortens vegetative growth and promotes flowering.
Heat stress encourages flowering in some plants, but becomes detrimental past optimal temperatures. Excessively high temperatures during flower bud development can prevent flowering from occurring.
Stress effects on hormones
Stresses alter plant hormone levels like abscisic acid, ethylene, and salicylic acid. These changes in hormone concentrations and signaling pathways regulate stress-induced flowering responses.
Stress also impacts gibberellin and jasmonic acid levels. Jasmonic acid typically delays flowering but can stimulate it under stress. Stress-induced reactive oxygen species that form in plants also play signaling roles to modify flowering.
Carbohydrates
The balance between carbon and nitrogen levels influence flowering, especially in biennials and monocarpic perennials. These plants need abundant carbon resources stored up to fuel the demanding process of flowering.
Many plants that flower once and die like agaves and bananas form a vegetative rosette first. The large leaves photosynthesize and build up carbohydrate reserves. Once carbon resources reach a sufficient level, the plants begin mobilizing resources and flowering.
Defoliation, shading, or other stress during vegetative growth that limits carbon assimilation delays flowering. Having adequate carbohydrates provides plants the energy to undergo the metabolic transition into flowering.
How carbohydrates promote flowering
High carbon levels increase cytokinins that stimulate flowering. Cytokinins regulate gene expression changes involved in the floral transition. Trehalose-6-phosphate, a precursor of the sugar trehalose, also signals carbon status and carbohydrate accumulation to influence flowering.
Sucrose transporter proteins help modulate flowering in response to carbohydrate conditions. These transporters provide sucrose to meristems to fuel growth. Their activity is upregulated when carbon reserves increase to support flowering.
Flowering genes and proteins
The major external signals that induce flowering all exert their effects by influencing flowering genes and proteins. Key genes and gene products that control the floral transition and flower formation include:
- CONSTANS (CO) – A transcription factor induced by long days that activates flowering genes
- FLOWERING LOCUS T (FT) – Encodes florigen hormone that initiates flowering
- LEAFY (LFY) – Transcription factor that regulates flower organ development
- SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) – Integrates signals with CONSTANS to promote flowering
- AGAMOUS (AG) – Controls floral organ identity and flower formation
These and many other flowering genes form complex genetic networks. They regulate each other through pathways like vernalization and photoperiod perception to control the transition from vegetative to reproductive development based on environmental cues.
Flowering integrators
Certain flowering genes act as integrators that bring together multiple flowering signals. For example, SOC1 integrates inputs from photoperiod, vernalization, hormones, and age-related signals to regulate CO and LFY activity. AGAMOUS-LIKE 24 (AGL24) is another integrator that responds to endogenous and environmental stimuli.
These integrators analyze different inputs and convey information to floral pathway genes. They serve as hubs to coordinate the plant’s overall flowering response based on a variety of factors rather than a single stimulus.
Age
The age of a plant can influence its flowering response. Some plants have a juvenility phase where they are unable to flower until reaching a certain developmental stage or age.
Woody perennials like fruit trees commonly have a juvenile non-flowering period. Seeds need to germinate and seedlings establish before gaining the capacity to flower. This prevents small seedlings from flowering prematurely before they are mature enough.
Biennials also have a juvenile vegetative phase their first year before transitioning to flowering in their second year. Factors like vernalization help biennials determine when their juvenile phase has ended so flowering can commence.
Regulation of juvenility
Juvenility prevents flowering until plants become reproductively competent and are a suitable size. This period is regulated by transcription factors, epigenetic changes, and microRNAs that repress flowering genes.
For example, the gene EARLY FLOWERING 6 (ELF6) is silenced by polycomb group proteins during juvenility. As plants age and overcome juvenility, ELF6 becomes activated to induce flowering. Other genes control juvenility in response to factors like nutrients, hormones, and sugars.
Nutrients
The availability of macronutrients like nitrogen, phosphorus, and potassium impacts flowering. Deficiencies in these nutrients often promote flowering, while an overabundance suppresses flowering.
Nitrogen has a particularly strong influence on flowering. High nitrogen fertilization prolongs vegetative growth and delays flowering, especially in biennials and perennials. Lower nitrogen levels help shift plants from the vegetative to reproductive mode.
Phosphorus deficiency hastens flowering and fruit set. Potassium deficiency also shortens plant lifespan and accelerates flowering. Proper nutrition early on followed by controlled nutrient stress can be used to regulate flowering.
Nutrient signaling
Nutrients modulate hormone levels like cytokinins and gibberellins to control flowering. Nitrogen regulates gene expression of CO, FT and LFY. MicroRNAs also respond to nitrogen status and target flowering time regulators.
Sugar signaling related to photosynthesis and carbon metabolism interacts with nitrogen status. Nitrate transporters and assimilation enzymes help transmit nutritional signals to influence flowering pathways.
Damaging treatments
Pruning, girdling, grafting and other damaging treatments sometimes promote flowering. These shocks stress plants and divert resources toward reproduction.
Pruning removes apical shoots and manipulates hormone balances in ways that can stimulate flowering. For example, pruning encourages flower bud formation in apple trees. Girdling involves removing a ring of bark to stress plants.
Grafting appropriate flowering rootstocks onto plants can induce earlier flowering. Root pruning also causes stress that shortens vegetative growth and hastens flowering. Such damaging treatments alter source-sink relationships and hormone signaling to accelerate flowering.
Response to damage
Damaging practices like pruning trigger changes in plant hormones. Removing apical buds disrupts auxin transport and cytokinin production from roots to promote flowering. Jasmonic acid levels also increase in response to these damages.
The stress of girdling, grafting and root pruning modifies gene expression of flowering regulators like FT to induce flowering. Damaging treatments must be applied properly to achieve desired flowering effects.
Chemical applications
Applying certain chemical compounds can influence flowering, especially in ornamental plants. These chemicals supplement or substitute for environmental cues that induce flowering.
For example, spraying gibberellic acid can promote flowering and is used commercially to synchronize flowering in pineapples. Applications of nitrate salts substitute for vernalization and lead to earlier flowering.
Hydrogen peroxide applications speed up flowering by imposing controlled oxidative stress. Chlormequat chloride and paclobutrazol inhibit gibberellin synthesis to suppress flowering when desired.
Chemical flowering effects
Applied chemicals alter hormone levels or signaling pathways to modify gene expression that controls flowering. Gibberellins and their inhibitors have especially pronounced flowering effects by regulating genes like LFY.
Chemicals also exert anti-juvenility effects in some plants, overcoming genetic repression of flowering genes. Proper concentrations and application timing are needed for chemical treatments to have desired flowering impacts.
Conclusion
Flowering is a complex process involving numerous external signals and internal regulators. Factors like light, temperature, photoperiod, hormones, nutrients and age all converge through flowering control genes to initiate the reproductive phase.
Understanding the stimuli that promote flowering allows the timing of flowering in crops and ornamentals to be optimized. While flowering is strongly environmentally controlled, growers can also use practices like vernalization, pruning, and chemical applications to induce desired flowering responses.
Further research continues to unravel the intricate genetic and molecular networks that underlie flowering transitions. This knowledge will improve the ability to manage flowering in plants to maximize productivity, quality and aesthetic value.
