Photosynthesis

Photosynthesis is performed differently by different species. The process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other coloured) pigments. In the light-harvesting reactions, chlorophyll (a light absorbing pigment) fixes energy from visible light, producing sugars which are distributed to the plant for storage and to support growth of roots, stems, trunks and leaves

Energy fixed by photosynthesis directly supports plant growth and produces organic matter that is consumed by animals and soil microbes. The carbon derived from photosynthesis makes up about half the organic matter on earth. Hydrogen and oxygen account for most of the rest. Plant photosynthesis provides the carbon and energy that drive most biological processes in ecosystems.

Photorespiration

Respiration is the process by which nutrients (sugars) are converted to energy in a plant or animal’s cells. About half of gross primary production (GPP) of plants (the sugars produced by a plant) is respired by the plant, producing CO2. Plant respiration provides the energy plants need to support their growth and maintenance. Net photosynthesis (called NPP or Net Primary Production) is the net rate of carbon gain and is measured by determining the biomass created by plants.

Factors that affect the productivity of a plant

Photosynthesis depends on:

• the availability of soil resources (water and nutrients)
• climate and time since disturbance
• the carbon-fixation capacity of the plant (or light-independent reactions) to convert CO2 into sugars, a more permanent form of chemical energy that can be stored, transported and metabolized and
• the location of the plant’s leaves in a forest or plant canopy. The availability of soil resources, especially water and nutrient supply is critical.

Vascular plant species differ 10 to 50-fold in their photosynthetic capacity.

Plants in high resource environments produce a large amount of leaf biomass.

Water Limitation

Water limitation reduces the capacity of individual leaves to match CO2 supply with light availability. Lack of water or extreme low temperatures can, prevent even evergreen plants from gaining carbon. Water stress is often associated with high light because sunny conditions correlate with low precipitation (low water supply) and with low humidity (high rate of water loss). When water supply is abundant, leaves typically open their stomata in response to high light, despite the associated high rate of water loss.

Adaptation to dry habitat

High radiation absorption is a disadvantage in dry environments because it increases leaf temperature, which increases respiratory carbon loss and transpirational water loss. Plants in dry habitat typically have thicker leaves and similar leaf-nitrogen per unit leaf area compared to plants in moist habitats. Plants in dry areas minimize water stress by reducing leaf area, by shedding leaves or by producing fewer new leaves. Some drought-adapted plants produce leaves that minimize radiation absorption, their leaves reflect most incoming radiation or are steeply inclined towards the sun.

Disturbances

Disturbances, herbivory (eating of plants) and pathogens can reduce leaf area below levels that a plant needs.

The impact of the seasons

In midsummer, when plants of most ecosystems are photosynthetically active, the daily input of visible light is nearly as great in in higher latitudes (Arctic and Antarctic) as in the tropics but is spread over more hours and is more diffuse. The length of the photosynthetic season accounts for much of the ecosystem differences in biomass production. During winter in cold climates and during times with negligible soil water in dry climates, plants either die (annuals), loose their leaves or become physiologically dormant. During these times there is negligible carbon absorption.

The impact of canopy processes

Photosynthetic capacity of individual leaves decreases exponentially in the forest canopy in parallel with the exponential decline in irradiance. In dense canopies, more leaves are shaded and operate in the linear portion of the light response curve, increasing light use efficiency (LUE) of the canopy as a whole. Canopy properties extend the range of light availability over which the light-use efficiency of the canopy remains constant. Clumped distribution of leaves in shoots, branches and crowns increases light penetration. Conifer canopies are particularly effective in distributing light through the canopy due to the clumping of needles around stems, explaining why coniferous forests support a higher Leaf Area Index (LAI) than deciduous forests.

The net effect of wind on photosynthesis is generally positive at moderate wind speeds and adequate moisture supply, enhancing photosynthesis at the top of the canopy where wind speed is highest. Rough canopies, characteristic of many forests, create more friction and turbulence increasing the vertical mixing of air within the canopy.

Pelagic Photosynthesis in open water ecosystems

Photosynthesis is less often carbon-limited in aquatic than in terrestrial ecosystems. Nutrients limit phytoplankton photosynthesis.