Temperature is an abiotic environmental factor that significantly affects life processes in all organisms by modifying membrane properties, enzyme activity levels, the rate of chemical reactions and diffusion, viscosity of vacuole
A. Zröbek-Sokolnik (H) Department of Botany and Nature Protection, University of Warmia and Mazury in Olsztyn, Plac tödzki 1, 10-727 Olsztyn, Poland e-mail: [email protected] uwm.edu.pl solution and the cytoplasm, phloem, and xylem solutions in plants (Sung et al. 2003). Living organisms can be classified into three groups, subject to the preferred temperature of growth (Fig. 5.1) . This chapter analyzes the impact of temperature on plant growth with emphasis on plant response to temperature stress.
)t is believed that land plants evolved in a tropical climate. This evolution process was spurred not so much by a warm climate, but by the stability of ambient temperature. Plants gradually migrated into temperate regions both north and south of the equator as they developed mechanisms that allowed them to accommodate
P. Ahmad and M.N.V. Prasad (eds.), Environmental Adaptations and Stress Tolerance of Plants in the Era of Climate Change, DOI 10.1007/978-1-4614-0815-4_5, © Springer Science+Business Media, LLC 2012
wider variations in temperature on both a daily and a seasonal basis (Fitter and Hay 2002). The growth and development of plants involves a countless number of biochemical reactions that are sensitive to temperature. Plant life is generally limited by the freezing point of water at the low end of the temperature scale and the irreversible denaturation of proteins at the high end. Temperature is a critical factor in the plant environment, and it may play a significant role in growth and development. Growth is defined as an increase in dry weight, while development is the increase in the number and/or dimension of organs by cell division and/or expansion: leaves, branches, spikelets, florets, root apices, etc., including those present in seed embryos. It also seems that the rate of plant development tends to be controlled primarily by temperature, and it is less sensitive to other environmental factors. The development of vegetation is determined by a broad variety of environmental factors that exert combined effects. Plant organisms are rarely affected by individual factors, and temperature stress is usually accompanied by water stress and, in consequence, oxidative stress (Fitter and Hay 2002) . Temperature can also play a part in controlling the pattern and timing of plant development, and this accounts for the below phenomena.
In some plant species, a period of low temperatures is required to induce flowering, while in other plants, low temperatures only accelerate flowering or have no effect at all. Plants with a vernalization requirement experience a period of low temperatures in late fall and/or winter at the stage of seed imbibition or young seedlings (annual winter crops) or upon reaching vegetative maturity (biennial and perennial plants) (Kim et al. 2009) . Flowering is induced in the temperature range of 0 to +10°C. The duration of the vernalization period, that is, the required number of days with low temperatures, varies subject to species, and it usually reaches from 2 weeks to several weeks (Dennis et al. 1996; Amasino 2006). In seeds, temperature stimuli are perceived by the embryo, while in seedlings and matured plants, this signal is sensed by apical meristems. A vernalized meristem retains competence following the reception of the inductive signal. When the signal is absent for a longer period of time, the plant is de-vernalized, and a similar effect can be achieved by exposing the plant to higher temperatures (around 40°C for 1-2 days) (Tretyn et al. 2003). The mechanisms underlying vernalization have not yet been fully explained. It is believed that low temperatures lead to changes in the permeability of cell membranes and/or the level of expression of "vernalization" genes. Phytohormones, in particular gibberellin, significantly contribute to this process (Sheldon et al. 2000; Amasino 2005).
Stratification is a popular method of breaking seed dormancy that has been used for centuries. This technique involves the storage of seeds in a moist and well-ventilated environment at relatively low temperatures in the range of 1-10°C. Stratification is generally defined as the process of subjecting seeds to cold or warm and cold conditions in a moist and ventilated environment to break the dormancy stage. Low temperature, high moisture content, and oxygen supply during the treatment induce deep physiological and biochemical changes in seeds. Stratification leads to the decomposition of germination inhibitors in seeds, and it induces the production of growth stimulators: cytokinin, gibberellin, and auxin. At various stages of the dormancy breaking period, changes are noted in the quantitative ratio of various stimulators which modify the seeds' sensitivity to light and temperature and support dormancy breaking in various dormancy mechanisms (e.g., Baskin and Baskin 1998; Opik and Rolfe 2005; Wrobel et al. 2005).
1.3 The Effect of Temperature on Membranes, Enzymes, and Metabolic Processes
An increase or a decrease in temperature changes the kinetic energy of particles, accelerating their motion and weakening hydrogen bonds in mac-romolecules. All of the reactions contributing to growth are catalyzed by enzymes whose activity depends on their precise, three-dimensional, tertiary structures, to which the reacting molecules must bind exactly for each reaction to proceed. As the temperature rises, tertiary structures are damaged, reducing enzyme activity and reaction rates (Price and Stevens 1999). The asymmetry of response curves, such as Fig. 5.2a, b, is the net result of an exponential increase in the reaction rate, caused by increased collision frequency, and increasingly modified by the thermal denatur-ation of macromolecules (Fitter and Hay 2002).
The effect of temperature on enzyme activity is not a simple correlation. Activity levels rise with an increase in temperature, but only within a temperature range that guarantees the enzyme's stability (Cornish-Bowden 2004). When the critical temperature is exceeded, enzymes undergo thermal denaturation, and their activity drops rapidly. The average rate of enzymatic reactions increases twofold with every 10°C increase in temperature within the range that does not cause enzyme denaturation (Fig. 5.3). The correlation between temperature and the increase in enzymatic activity is described by temperature coefficient Q10 which illustrates changes in reaction rate when the temperature increases by 10°C:
Parameter Q10 applies only in a nondenaturing range of temperatures, it is enzyme specific and determined by the activation energy of the catalyzed reaction. Enzyme activity reaches the highest level at optimal temperature. The representative values of temperature coefficients (Q10) for selected plant processes measured at varying intervals within the range 0-30°C are determined at 1-2.3 (e.g., light reactions of photosynthesis ~1; diffusion of small molecules in water: 1.2-1.5; water flow through seed coat: 1.3-1.6; water flow into germinating seeds: 1.5-1.8; hydrolysis reactions catalyzed by enzymes: 1.5-2.3; root axis extension: 2.3). Coefficient value reaches 2-3 for dark reactions of photosynthesis, 0.8-3 for phosphate ion uptake into storage tissue, and 2-5 for potassium ion uptake into seedlings. Grass leaf extension is characterized by Q10 of 3.2, and the relative growth rate is marked by coefficient value of 7.2 (Fitter and Hay 2002). The observed optimal temperature is the product of two processes: an increase in the reaction rate related to an increase in kinetic energy and an increase in the rate of thermal denaturation of an enzyme above a critical temperature point. When the
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