3.0 The Temperature Factor: A Primary Regulator of Biological Processes

3.1 Cardinal Temperatures and Their Influence on Plant Functions

Temperature is a primary driver of all biochemical reaction rates in plants and therefore exerts a powerful influence on growth, development, and survival. To understand thermal limits on plant life, we use the framework of cardinal temperatures: the minimum, optimum, and maximum temperatures for a given biological process.

It is crucial to understand that these cardinal temperatures are not fixed values. They are highly variable and can differ for:

• The same function in different plants.
• Different functions (e.g., photosynthesis vs. respiration) within the same plant.
• The same function at different developmental stages of a plant.
• Different organs (e.g., roots vs. shoots) of the same plant.

Furthermore, these cardinal temperatures can be altered by the plant’s general physiological condition, the duration of the temperature exposure, and simultaneous changes in other environmental factors like water or light availability.

A few examples illustrate these principles clearly:

• Growth Minimums: The minimum temperature for growth in melons (Citrullus) and sorghums (Sorghum) is around 15–18°C, while for cool-season crops like peas (Pisum), rye (Secale), and wheat (Triticum), it is as low as 2–5°C (Daubenmire, 1959).
• Organ-Specific Minimums: Roots typically have a lower minimum temperature for growth than the shoots of the same plant (Daubenmire, 1959).
• Process-Specific Optimums: In all known cases, the optimum temperature for photosynthesis is lower than the optimum temperature for respiration. This has significant ecological consequences. In low latitudes, high night temperatures can lead to excessively high rates of respiration. This creates a negative carbon balance, where the plant expends more of its stored energy on maintenance than it can accumulate during the day, effectively starving it of the resources needed for growth and reproduction. This is a likely reason why plants like peaches and potatoes do not accumulate normal food reserves in such climates.

One of the most critical developmental processes regulated by temperature is dormancy, a key survival strategy in seasonal climates.

3.2 Low-Temperature Requirements: Dormancy in Seeds and Buds

Dormancy is a crucial survival strategy for perennial plants and seeds in temperate and colder regions. It is an evolved mechanism that prevents germination or bud break during brief warm spells in winter, which would expose the vulnerable new growth to subsequent lethal cold.

Many seeds require a period of moist, cold treatment to break dormancy and enable germination, a process known as stratification.

• Research in my own laboratory has shown that common ragweed (Ambrosia artemisiifolia), western ragweed (A. psilostachya), and drop seed (Sporobolus pyramidatus) require 6 to 10 weeks at 4–7°C in moist conditions to germinate.
• Seeds of the annual sunflower (Helianthus annuus) need 90 days at 4°C in moist sand to completely break dormancy (Lane, 1965).
• Elberta peach (Prunus persica) seeds require 60 to 100 days at 7°C or below (Donoho and Walker, 1957).

A parallel requirement exists for the overwintering buds of many perennial plants.

• Elberta peach buds need 950 hours at or below 7°C to break dormancy (Weinberger, 1956).
• Other peaches require over 400 hours, blueberries (Vaccinium spp.) need 800 hours, and apples (Pyrus malus) require even more (Daubenmire, 1959).

Observing this phenomenon naturally leads to an investigation of its underlying chemical mechanisms.

3.3 The Biochemical Basis of Breaking Dormancy

While the precise chemical changes stimulated by low temperatures are not yet fully understood, growing evidence indicates that breaking dormancy involves a shift in the internal balance between growth-inhibiting and growth-stimulating hormones.

Abscisic acid (ABA) has been identified as a key growth inhibitor. Research by Sondheimer, Tzou, and Galson (1968) demonstrated that ABA inhibits the germination of non-dormant embryos, and this inhibition can be reversed by a combination of a growth stimulator, gibberellic acid (GA), and kinetin. Crucially, after chilling treatment, the amount of ABA in ash (Fraxinus americana) seeds decreased by 68%, supporting its regulatory role in dormancy.

Gibberellic acid (GA) has been shown to be a primary growth stimulator that can often substitute for a cold treatment. Junttila (1970) found that GA could effectively break seed dormancy in birch (Betula nana) in place of low temperatures. In a striking experiment with dormant lettuce seeds, 84% germinated when treated with GA, compared to just 1.5% when treated with a combination of GA and the inhibitor ABA, and 0% in water alone (Haber, Foard, and Perdue, 1969). This hormonal mechanism also applies to bud dormancy. Donoho and Walker (1957) were able to break dormancy in Elberta peach buds after only 164 hours of chilling (far short of the required 950 hours) by spraying them with a 4000 ppm solution of GA.

Further research on tulip bulbs (Tulipa gesneriana) suggests a dynamic relationship between free (active) and bound (inactive) forms of gibberellins. At 18°C, a temperature that accelerated development, free GA increased while bound GA decreased. At 13°C, the opposite occurred (Aung, Hertogh, and Staby, 1969).

While the hormonal balance of GA and ABA governs the timing of growth resumption, plants must also possess robust physiological mechanisms to survive the low temperatures that induce dormancy in the first place. This leads us to the process of cold hardening.

3.4 Adaptation to Cold: The Process of Cold Hardening

Cold hardening is a physiological process that improves a plant’s ability to withstand low temperatures without severe damage or death. This process is critically important for the survival of plants in regions with freezing winters.

Interestingly, the mechanisms of cold hardening appear to be linked to those of dormancy. Research on box elder (Acer negundo) found that an inhibitor similar or identical to ABA builds up during the cold hardening period. Applying ABA or this inhibitor actually increased the hardiness of branches after an initial hardening period (Irving, 1969).

According to Klages (1947), cold hardening is associated with a specific set of cellular chemical changes:

• Increase in bound water
• Decrease in total water
• Increase in osmotic pressure
• Change of starch to sugars
• Increase in pentosans
• Change of certain proteins to amino acids

Remarkably, this list has remained the definitive summary of these changes since its publication in 1947. These survival mechanisms at extreme temperatures are complemented by more subtle adaptations that optimize metabolic rates at different temperatures, leading to the evolution of distinct temperature ecotypes.

3.5 Genetic Adaptation and Acclimation: Temperature Ecotypes and Gas Exchange

A plant’s response to temperature is not fixed; it is a product of both its long-term genetic heritage (adaptation) and its recent environmental history (acclimation). This distinction is fundamental to understanding plant distribution and performance.

A classic case study of temperature ecotypes comes from the research of Mooney and Billings (1961) on alpine sorrel (Oxyria digyna), a species with a very wide geographic distribution. They studied populations from a range of latitudes and elevations and found that plants from northern populations had higher respiration rates at all temperatures. Furthermore, these northern plants had higher photosynthetic rates at lower temperatures and reached their maximum photosynthetic rate at a lower temperature than did the southern alpine populations. Their conclusion was clear: definite, genetically-based temperature ecotypes exist within this species.

Plants also exhibit significant environmentally conditioned plasticity. In a study in the White Mountains of California, field measurements showed that the ratio of photosynthesis to respiration decreased with altitude, primarily due to higher respiration rates (Mooney, Wright, and Strain, 1964). However, when seeds from different elevations were grown under uniform greenhouse conditions, there were no differences in respiration rates, indicating a high degree of plasticity. A persistent genetic component remained, however, as the high-elevation plants still achieved peak photosynthesis at lower temperatures.

A detailed study of photosynthetic acclimation by Mooney and West (1964) further illustrates this interplay. They used five species native to the White Mountains, collected from a range of elevations.

Seedlings grown in a greenhouse were moved to acclimation stations at different elevations, with different temperature regimes but identical soil and water conditions.

After acclimating, the plants were tested. While dark respiration rates were similar across all treatments, photosynthetic efficiency was strongly influenced by the acclimation environment. Plants acclimated to the colder subalpine environment were more efficient at colder temperatures, while plants acclimated to the warmer desert environment were more efficient at higher temperatures. They also found that the two species with the widest natural elevation ranges showed the greatest plasticity in their photosynthetic response. This finding is ecologically significant because it suggests that high physiological plasticity is a key trait that enables a species to successfully colonize and persist across a broad range of environmental conditions.

A final, contrasting example comes from Miller (1960), who compared cool-adapted creeping bent grass (Agrostis palustris) with warm-adapted bermuda grass (Cynodon dactylon). Creeping bent achieved its optimal photosynthetic rate at 25°C, while bermuda grass peaked at 35°C and still maintained 97.7% of its maximum rate at a scorching 40°C.

In summary, plant responses to temperature are a complex interplay of genetic pre-disposition (adaptation) and physiological adjustment (acclimation). Together, these processes determine a species’ ability to survive and thrive in a given thermal environment. We now turn to our next major environmental factor: light.

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