Biomass partitioning is the process by which

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

s divide their energy among their

leaves A leaf (plural, : leaves) is any of the principal appendages of a vascular plant plant stem, stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", wh ...

, stems,

root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s, and reproductive parts. These four main components of the plant have important morphological roles: leaves take in CO₂ and energy from the sun to create

carbon compounds Carbon compounds are defined as chemical substances containing carbon. More compounds of carbon exist than any other chemical element except for hydrogen. Organic carbon compounds are far more numerous than inorganic carbon compounds. In general ...

, stems grow above competitors to reach sunlight, roots absorb water and mineral nutrients from the soil while anchoring the plant, and reproductive parts facilitate the continuation of species. Plants partition

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...

in response to limits or excesses in resources like sunlight, carbon dioxide, mineral nutrients, and water and growth is regulated by a constant balance between the partitioning of biomass between plant parts. An equilibrium between root and shoot growth occurs because roots need carbon compounds from photosynthesis in the shoot and shoots need nitrogen absorbed from the soil by roots. Allocation of biomass is put towards the limit to growth; a limit below ground will focus biomass to the roots and a limit above ground will favor more growth in the shoot. Plants

photosynthesize Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

to create carbon compounds for growth and energy storage. Sugars created through photosynthesis are then transported by

phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This tran ...

using the pressure flow system and are used for growth or stored for later use. Biomass partitioning causes this sugar to be divided in a way that maximizes growth, provides the most fitness, and allows for successful reproduction.

Plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...

s play a large part in biomass partitioning since they affect differentiation and growth of cells and tissues by changing the expression of genes and altering

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

. By responding to environmental

stimuli A stimulus is something that causes a physiological response. It may refer to: * Stimulation ** Stimulus (physiology), something external that influences an activity ** Stimulus (psychology), a concept in behaviorism and perception * Stimulus (eco ...

and partitioning biomass accordingly, plants are better able to take in resources from their environmental and maximize growth.

Abiotic Factors of Partitioning

It is important for plants to be able to balance their absorption and utilization of available resources and they adjust their growth in order to acquire more of the scarce, growth-limiting resources: sunlight, carbon dioxide, mineral nutrients, and water. The equilibrium of biomass partitioning can be explained by

Liebig's Law of the Minimum Liebig's law of the minimum, often simply called Liebig's law or the law of the minimum, is a principle developed in agricultural science by Carl Sprengel (1840) and later popularized by Justus von Liebig. It states that growth is dictated not by t ...

and modeled through the metaphor of Liebig's Barrel in which the limiting resource of plant growth is like the shortest slat on a barrel full of water. The barrel can only hold water up to the level of the shortest slat and, likewise, plants can only grow at a rate allowed by the limiting resource. To continue growing, biomass must be partitioned to help sequester these resources.

Sunlight

The main light-sensing mechanism for plants is the

phytochrome Phytochromes are a class of photoreceptor in plants, bacteria and fungi used to detect light. They are sensitive to light in the red and far-red region of the visible spectrum and can be classed as either Type I, which are activated by far-re ...

system with pigments located throughout the plant to detect changes in red and far-red light. The phytochromes' detection of light quality is what helps trigger changes in biomass partitioning. Plants grown in low light conditions have been shown to allocate more biomass to shoots (mainly leaves). By measuring leaf area of plants at different levels of irradiance or sunlight, it has been determined that lower levels of light cause total plant leaf surface to increase. If sunlight is limiting the plant increases growth to the shoot and decreases the energy partitioned to the roots because the lower rates of photosynthesis lessen need for water and mineral nutrients. As low light causes greater allocation to the shoot, the same correlation is made for plants in high light; higher irradiance causes smaller total lower surface area because less surface area is needed to absorb sunlight. When a plant has more than enough photosynthetic capacity, it will instead prioritize growth in the roots to absorb water and nutrients.

Carbon Dioxide

In a situation where carbon is the limiting resource, increasing the levels of CO₂ increases photosynthetic rates. This will also cause increases in nutrient uptake and water use, focusing more growth towards the roots. In low CO₂ concentrations, plants create larger and more numerous leaves to bring in more CO₂. The impacts of atmospheric carbon dioxide concentration on biomass partitioning is important to understanding in impacts on plants in the face of climate change. Plant cells have increased carbon to nitrogen ratios when CO₂ concentrations are higher, which decreases levels of decomposition. The decrease in decomposition caused by high carbon dioxide concentrations has the effect of decreasing nitrogen availability for plants.

Mineral Nutrients

Overall, nutrient availability has a strong effect on partitioning, with plants growing in poor nutrient areas partitioning most of their biomass to underground structures. Soil nitrogen availability, for example, is a strong determinant of biomass allocation. For example, in low productivity systems (low levels of soil nitrogen) such as

boreal forests Taiga (; rus, тайга́, p=tɐjˈɡa; relates to Mongolic and Turkic languages), generally referred to in North America as a boreal forest or snow forest, is a biome characterized by coniferous forests consisting mostly of pines, spruces ...

, trees devote a large portion of their biomass to roots. But as soil productivity increases, biomass is primarily allocated to aboveground structures such as leaves and stems. As an overall pattern, the lengths of roots decrease as nutrient concentrations increase. High concentrations of a mineral nutrient that lead to

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

can have a strong effect on growth and partitioning as well. For instance, in toxic concentrations of cadmium and lead,

Fagus sylvatica ''Fagus sylvatica'', the European beech or common beech is a deciduous tree belonging to the beech family Fagaceae. Description ''Fagus sylvatica'' is a large tree, capable of reaching heights of up to tall and trunk diameter, though mor ...

was shown to develop a more compact and less branching root system while establishing few root hairs. This altered structure functions to reduce surface area of the roots and the effects of toxic nutrients while also conserving biomass for parts of the plant where growth would be more beneficial.

Water

Plants grown in dry conditions often have a decreased total biomass production but they also contribute more of their biomass to the roots and develop a higher root to shoot ratio. When plants allocate more of their biomass to their roots, they are able to enhance water absorption by tapping further down into the

water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...

and extending root mass further laterally. Increased

root hair Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root ...

s also aid in increasing absorption. When there is an extreme soil drought, there is not an increase in root to shoot biomass, because a state of dormancy is adopted. Water availability also impacts leaf surface area as too much surface area could allow for excessive transpiration in low water conditions.

Biotic Factors of Partitioning

Interactions between organisms can also alter partitioning due to competition for resources, sharing of resources, or reduction of plant biomass.

Competition

Interspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organi ...

and

intraspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...

competition can cause a decrease in the available resources for an individual plant and alter how it partitions biomass. For instance, competition between plants causes decreased radial growth in branches and the stem while increasing growth of the roots and height of the stem. This provides reasoning for the importance of phenotypic

plasticity Plasticity may refer to: Science * Plasticity (physics), in engineering and physics, the propensity of a solid material to undergo permanent deformation under load * Neuroplasticity, in neuroscience, how entire brain structures, and the brain it ...

in maintaining fitness in a competitive environment; the better a plant is at altering its morphology, the more competitive it will be. Mutualistic mycorrhiza en

Mutualism With Mycorrhizae

The relationship between plants and

mycorrhizal fungi A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...

is an example of mutualism because plants provides fungi with carbohydrates and mycorrhizal fungi help plants absorb more water and nutrients. Since mycorrhizal fungi increase plants' uptake of below-ground resources, plants who form a mutualistic relationship with fungi have stimulated shoot growth and a higher shoot to root ratio.

Herbivory

Herbivory A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

causes a short-term reduction in leaf mass and/or stem mass that shifts stability and proportions of plant parts. To restore the balance between uptake of nutrients/water and photosynthetic rates, plants send more energy to the stems and leaves. Herbivory causing high levels of defoliation (greater than 25% of leaf area) increases growth to the shoot, seeking to achieve the same shoot to root ratio as before defoliation. Therefore, defoliation also reduces root growth and nutrient uptake until the before-defoliation root to shoot ratio is restored.

Plant Type

As different plants have different structures and growth forms, their partitioning of biomass is not identical either. For example,

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...

trees have higher leaf mass fractions (LMFs) compared to

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...

trees. Additionally, the stem to root ratio varies between species more greatly in

herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

plants than woody ones as the plasticity for roots in herbaceous plants is higher. Furthermore, herbaceous

monocots Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of t ...

graminoid In botany and ecology, graminoid refers to a herbaceous plant with a grass-like morphology, i.e. elongated culms with long, blade-like leaves. They are contrasted to forbs, herbaceous plants without grass-like features. The plants most ofte ...

s compared with herbaceous dicots have larger root to total mass ratios (RMFs). The difference in herbaceous monocots and dicots may be explained by differences in efficiency of nutrient uptake or by the need for graminoids to store more starch and nutrients to regrow after fire or grazing. Biomass partitioning can also be affected by a plant's method of carbon fixation: C3 or C4. For herbaceous monocots, the type of photosynthesis does not affect the percentile LMF or RMF, but in herbaceous dicots, C4 species have lower RMFs. The life cycle of plants can also cause different allocation strategies and ratios for leaves, roots, and stems; compared to same-size eudicotelydonous

perennials A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widel ...

, annuals put more energy into growing leaves and stems than perennial species.

Age

Biomass partitioning patterns can also change as plants age. In

pinus strobus ''Pinus strobus'', commonly called the eastern white pine, northern white pine, white pine, Weymouth pine (British), and soft pine is a large pine native to eastern North America. It occurs from Newfoundland, Canada west through the Great Lake ...

stands of 2, 15, 30, and 65 years old, the root to shoot ratio was 0.32, 0.24, 0.16, and 0.22 respectively showing a decrease in root mass compared to shoot mass in the first few decades of growth. Age also affects how trees partition biomass to different components of the stem. Growth of branches and leaf surface area decreases with age as partitioning to the trunk increases.

Methods/ Measurement

Many biomass partitioning studies consist of manipulative experiments that increase or reduce levels of growth-limiting resources and observe the effects. Often-times these studies use potted plants grown in a greenhouse to measure effects of density, excess nutrients, low light etc. Other studies may focus on observation and analysis of naturally occurring plants or use data analysis of previous measurements. The methods and measurements for determining biomass partitioning can be quite difficult due to the weight and volume of larger plants. Furthermore, measuring size and weight of roots entails washing and careful removal of soil to get accurate measurements. Plant biomass is often measured in terms of leaf mass fraction (LMF), stem mass fraction (SMF), and root mass fraction (RMF) where the dry mass of the plant part is set over the total dry mass of the plant. Lateral, radial, and height increases are used to quantify rates of growth.

References

{{reflist Plants Botany Agriculture