Photosynthesis and nutrients

Potassium is required for enzyme activity in a few special cases, the most widely studied example of which is the enzyme pymvate kinase. In plants it is required for protein and starch synthesis. Potassium is also involved in water and nutrient transport within and into the plant, and has a role in photosynthesis. Although sodium and potassium are similar in their inorganic chemical behavior, these ions are different in their physiological activities. In fact, their functions are often mutually antagonistic. For example, increases both the respiration rate in muscle tissue and the rate of protein synthesis, whereas inhibits both processes (42). 

Copper is one of the twenty-seven elements known to be essential to humans (69—72) (see Mineral nutrients). The daily recommended requirement for humans is 2.5—5.0 mg (73). Copper is probably second only to iron as an oxidation catalyst and oxygen carrier in humans (74). It is present in many proteins, such as hemocyanin [9013-32-3] galactose oxidase [9028-79-9] ceruloplasmin [9031 -37-2] dopamine -hydroxylase, monoamine oxidase [9001-66-5] superoxide dismutase [9054-89-17, and phenolase (75,76). Copper aids in photosynthesis and other oxidative processes in plants. 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

In certain plant habitats or niches, access to resources depends crucially upon rapid growth under conditions of climatic stress. Examples of this phenomenon are particularly obvious on shallow soils in continental climates where the growth window between winter cold and summer desiccation may be extremely short. In deciduous woodlands in the cool temperate zone an essentially similar niche arises in the period between snow melt and closure of the tree canopy. Both circumstances provide opportunities for high rates of photosynthesis and mineral nutrient capture in the late spring but depend upon rapid expansion of roots and shoots in the low-temperature conditions of the late winter and early spring. 

The effects of photosynthesis are clearly seen in the low TDIC and nutrient concentrations of the surface water. The O2 concentrations are high because of contact with the sea surfece and production by phytoplankton. The temperature and O2 concentration data have been used to compute the percent saturation with respect to O2. The high degree of supersaturation in the surfece water suggests that the rate of O2 supply via photosynthesis is exceeding its removal via the dual processes of aerobic respiration and degassing across the air-sea interface. 

In principle, the alkalinity of the water will also be affected by the balance of nutrient ions consumed and released by organisms in the water. But in practice these have a minor affect compared with CO2. The average composition of the algal biomass in natural waters is given by the Redfield formula (Redfield, 1934) as C106H263O110N16P. Therefore for the complete stoichiometry of algal photosynthesis and respiration, we have with NO3 as the source of N... 

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

The phenolic acids of interest here [caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (p-hydroxycinnamic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), sinapic acid (3,5-dimethoxy-4-hydroxyxinnamic acid), p-hydroxybenzoic acid, syringic acid (4-hydroxy-3,5-methoxybenzoic acid), and vanillic acid (4-hydroxy-3-methoxybenzoic acid)] (Fig. 3.1) all have been identified as potential allelopathic agents.8,32,34 The primary allelopathic effects of these phenolic acids on plant processes are phytotoxic (i.e., inhibitory) they reduce hydraulic conductivity and net nutrient uptake by roots.1 Reduced rates of photosynthesis and carbon allocation to roots, increased abscisic acid levels, and reduced rates of transpiration and leaf expansion appear to be secondary effects. Most of these effects, however, are readily reversible once phenolic acids have been depleted from the rhizosphere and rhizoplane.4,6 Finally, soil solution concentrations of... 

Redox intensity or electron activity in natural waters is usually determined by the balance between those processes which introduce oxygen (e.g. dissolution of atmospheric oxygen, photosynthesis) and those which remove oxygen (e.g. microbial decomposition of organic matter). Often these processes are controlled by the availability of inorganic nutrients such as phosphate and nitrate, e.g. as utilized in the formation of organic matter during photosynthesis (see Section 3.3.4). 

Roots anchor plants in the ground as well as absorb water and nutrients from the soil and then conduct these substances upward to the stem. Approximately half of the products of photosynthesis are allocated to roots for many plants. To help understand uptake of substances into a plant, we will examine the cell types and the functional zones that occur along the length of a root. 

In Equation 8.27, Vmax and, to some extent, Kcch depend on the photosynthetic photon flux (PPF), temperature, and nutrient status. For instance, Vmax is zero in the dark because photosynthesis ceases then, and it is directly proportional to PPF up to about 50 jimol m-2 s-1. If we continually increase the PPF, Fmax can reach an upper limit, its value for light saturation. This usually occurs at about 600 junol m-2 s-1 for most C3 plants, whereas photosynthesis for C4 plants is generally not light saturated even at full sunlight, 2000 pmol m-2 s-1 (see Chapter 6, Section 6.3D for comments on C3 and C4 plants also see Fig. 8-20 for responses of leaves of C3 plants and a C4 plant to PPF). Photosynthesis is maximal at certain temperatures, often from 30°C to 40° C. We note that Vmax increases as the leaf temperature is raised to the optimum and then decreases with a further increase in temperature. 

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