Net photosynthesis

The rate of photosynthesis is a gross measure of the rate at which a plant captures radiant energy and fixes it in organic carbon compounds. However, it is often more important to consider, and very much easier to measure, the net gain. Net photosynthesis is the increase (or decrease) in dry matter that results from the difference between gross photosynthesis and the losses due to respiration and the death of plant parts (Figure 3.8).

Net photosynthesis is negative in darkness, when respiration exceeds photosynthesis, and increases with the intensity of PAR. The compensation point is the intensity of PAR at which the gain from gross photosynthesis exactly balances the respiratory and other losses. The leaves of shade species tend to respire at lower rates than those of sun species. Thus, when both are growing in the shade the net photosynthesis of shade species is greater than that of sun species.

There is nearly a 100-fold variation in the photosynthetic capacity of leaves (Mooney & Gulmon, 1979). This is the rate of photosynthesis when incident radiation is saturating, temperature is optimal, relative humidity is high, and CO2 and oxygen concentrations are normal. When the leaves of different species are compared under these ideal conditions, the ones with the highest photosynthetic capacity are generally those from environments where nutrients, water and radiation are seldom limiting (at least during the growing season). These include many agricultural crops and their weeds. Species from resource-poor environments (e.g. shade plants, desert perennials, heathland species) usually have low photosynthetic capacity - even when abundant resources are provided. Such patterns can be understood by noting that photosynthetic capacity, like all capacity, must be 'built'; and the investment in building sun and shade leaves the compensation point photosynthetic capacity

(a) Chlorophyll a and b

Chlorophyll b

Chlorophyll b

Chlorophyll a

Chlorophyll a

ß-carotene

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ß-carotene

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400 450 500 550 600 650 700 750 Wavelength (nm)

(b) Chlorophyll c2 449

0.25

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(b) Chlorophyll c2 449

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R-phycocyanin a

400 500

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400 500

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Green algae

anc un

anc un

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400 450 500 550 600 650 700 750 Wavelength (nm)

Figure 3.7 (a) Absorption spectra of chlorophylls a and b. (b) Absorption spectrum of chlorophyll c2. (c) Absorption spectrum of ß-carotene. (d) Absorption spectrum of the biliprotein, R-phycocyanin. (e) Absorption spectrum of a piece of leaf of the freshwater macrophyte, Vallisneria spiralis, from Lake Ginnindera, Australia. (f) Absorption spectrum of the planktonic alga Chlorella pyrenoidos (green).

Figure 3.7 (continued) (g-h) Absorption spectra of the planktonic algae Navícula minima (diatom) and Synechocystis sp. (blue-green). (i) The numbers of species of benthic red, green and brown algae at various depths (and in various light regimes) off the west coast of Scotland (56-57°N). (After Kirk, 1994; data from various sources.)

Figure 3.7 (continued) (g-h) Absorption spectra of the planktonic algae Navícula minima (diatom) and Synechocystis sp. (blue-green). (i) The numbers of species of benthic red, green and brown algae at various depths (and in various light regimes) off the west coast of Scotland (56-57°N). (After Kirk, 1994; data from various sources.)

Figure 3.8 The annual course of events that determined the net photosynthetic rate of the foliage of maple (Acer campestre) in 1980. (a) Variations in the intensity of PAR (•), and changes in the photosynthetic capacity of the foliage (□) appearing in spring, rising to a plateau and then declining through late September and October. (b) The daily fixation of carbon dioxide (CO2) (o) and its loss through respiration during the night (•). The annual total gross photosynthesis was 1342 g CO2 m-2 and night respiration was 150 g CO2 m-2, giving a balance of 1192 g CO2 m-2 net photosynthesis. (After Pearcy et al., 1987.)

Figure 3.8 The annual course of events that determined the net photosynthetic rate of the foliage of maple (Acer campestre) in 1980. (a) Variations in the intensity of PAR (•), and changes in the photosynthetic capacity of the foliage (□) appearing in spring, rising to a plateau and then declining through late September and October. (b) The daily fixation of carbon dioxide (CO2) (o) and its loss through respiration during the night (•). The annual total gross photosynthesis was 1342 g CO2 m-2 and night respiration was 150 g CO2 m-2, giving a balance of 1192 g CO2 m-2 net photosynthesis. (After Pearcy et al., 1987.)

capacity is only likely to be repaid if ample opportunity exists for that capacity to be utilized.

Needless to say, ideal conditions in which plants may achieve their photosynthetic capacity are rarely present outside a physiologist's controlled environment chamber. In practice, the rate at which photosynthesis actually proceeds is limited by conditions (e.g. temperature) and by the availability of resources other than radiant energy. Leaves seem also to achieve their maximal photosynthetic rate only when the products are being actively withdrawn (to developing buds, tubers, etc.). In addition, the photosynthetic capacity of leaves is highly correlated with leaf nitrogen content, both between leaves on a single plant and between the leaves of different species (Woodward, 1994). Around 75% of leaf nitrogen is invested in chloroplasts. This suggests that the availability of nitrogen as a resource may place strict limits on the ability of plants to garner CO2 and energy in photosynthesis. The rate of photosynthesis also increases with the intensity of PAR, but in most species ('C3 plants' - see below) reaches a plateau at intensities of radiation well below that of full solar radiation.

The highest efficiency of utilization of radiation by green plants is 3-4.5%, obtained from cultured microalgae at low intensities of PAR. In tropical forests values fall within the range 1-3%, and in temperate forests 0.6-1.2%. The approximate efficiency of temperate crops is only about 0.6%. It is on such levels of efficiency that the energetics of all communities depend.

Figure 3.9 Computer reconstructions of stems of typical sun (a, c) and shade (b, d) plants of the evergreen shrub Heteromeles arbutifolia, viewed along the path of the sun's rays in the early morning (a, b) and at midday (c, d). Darker tones represent parts of leaves shaded by other leaves of the same plant. Bars = 4 cm. (After Valladares & Pearcy, 1998.)

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Responses

  • DAVID
    How is net photosynthesis different from gross photosynthesis?
    2 years ago

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