4.0 The Radiant Energy Factor: Light as Information and Energy

4.1 The Nature and Importance of Solar Radiation

Radiant energy from the sun is the ultimate source of energy for nearly all ecosystems on Earth. Beyond its role as an energy source, it also acts as a critical environmental signal that governs numerous plant processes, from germination to flowering.

At the outer edge of Earth’s atmosphere, solar radiation has a virtually constant intensity of 1400 watts m⁻² (or 2 cal cm⁻² min⁻¹) (Johnson, 1954). About 98% of this energy falls within the 0.2-4.5 μ wavelength range, with 40-45% of it being in the visible spectrum. As this energy passes through the atmosphere, about one-third is reflected back to space, while smaller amounts are absorbed and scattered.

On average, about 50% of the initial solar radiation reaches the Earth’s surface. This amount varies significantly with climate, from over 70% in dry regions with few clouds to under 40% in tropical rainy areas (Slatyer, 1967). The visible portion of the light that reaches the surface remains about 40-45% of the total.

The chief plant processes affected by light are:

1. Chlorophyll synthesis
2. Photosynthesis
3. Phototropism (growth towards light)
4. Photoperiodism (response to daylength)
5. Transpiration

The following subsections will explore how variations in the intensity, quality (wavelength), and duration (daylength) of light affect these vital processes.

4.2 Light Intensity: Photosynthetic Adaptation in Sun and Shade

Light intensity is a key factor determining plant distribution. Species are often highly adapted to thrive in specific light environments, ranging from the deep shade of a forest floor to the full sun of an open field. These are often categorized as “shade plants” and “sun plants.”

Shade-tolerant species include many ferns, as well as the seedlings of climax forest species like redwood (Sequoia sempervirens), white oak (Quercus alba), red oak (Quercus rubra), beech (Fagus grandifolia), and sugar maple (Acer saccharum). Redwood seedlings, for example, can grow in light intensities as low as 100 ft-c. The physiological basis for their success in low light is that their photosynthetic machinery is highly efficient; the photosynthetic rate in oaks, for instance, reaches its maximum at one-third or less of full sunlight (Kramer and Decker, 1944).

In contrast, sun-requiring species like loblolly pine (Pinus taeda) and shortleaf pine (Pinus echinata) thrive in open areas but fail to survive under a dense forest canopy. Their photosynthetic rate continues to increase with light intensity all the way up to full sunlight. This explains why they are pioneer species in open fields but are later replaced by more shade-tolerant hardwoods in ecological succession.

A detailed comparative study by Björkman and Holmgren (1963) on sun and shade ecotypes of goldenrod (Solidago virgaurea) revealed the genetic basis of these adaptations. They found that each ecotype performed best when grown in the light environment to which it was adapted. The sun plants grown in strong light had a much higher maximum photosynthetic rate than shade plants grown in strong light. Conversely, when grown in weak light, the shade plants were photosynthetically superior. The authors concluded that this behavior was a result of genetic adaptation.

Further studies by Björkman (1966) reinforced these findings. As shown in the table below, plants adapted to shaded habitats are consistently more efficient at photosynthesis under low light conditions.

This research also delved into the underlying mechanism, finding that Photosystem II activity was impaired in plants grown outside their adapted light intensity, which was associated with a drastic reduction in the Emerson enhancement effect.

Elevation also influences a plant’s light intensity requirements. Studies on races of monkey-flower (Mimulus cardinalis) found that the saturating light intensity for photosynthesis increased with the elevation of the original habitat (Milner and Hiesey, 1964). Similarly, the high-elevation ecotypes of Oxyria digyna required higher light intensity for photosynthetic saturation than low-elevation ecotypes (Mooney and Billings, 1961). This corresponds to the fact that light intensity generally increases with elevation. We now shift from the quantity of light (intensity) to its quality (wavelength).

4.3 The Influence of Light Quality on Plant Processes

Different physiological processes in plants are triggered by specific wavelengths of light, demonstrating that light acts not only as a source of energy but also as a critical source of environmental information.

• Chlorophyll Synthesis: All visible light is effective in chlorophyll synthesis, with the exception of wavelengths above 680 mμ (Sayre, 1928).
• Photosynthesis: The absorption spectra for photosynthesis show distinct peaks. Chlorophyll a has maximum absorption at 430 mμ (blue-violet) and 660 mμ (red), while chlorophyll b peaks at 455 mμ (blue) and 640 mμ (orange-red). Classic curves of photosynthetic rate show corresponding peaks in the blue and red regions of the spectrum (Hoover, 1937; Warburg and Negelein, 1923). The modern understanding is that photosynthesis involves two distinct photosystems: Photosystem I with an absorption peak at 700 mμ and Photosystem II with a peak around 672 mμ (Salisbury and Ross, 1969).
• Phototropism: The bending of plants towards a light source is primarily caused by wavelengths in the blue-violet range (400–490 mμ), with a possible response to UV light as well (Johnston, 1934).
• Photoperiodic Responses: The most effective light quality for photoperiodic responses, such as interrupting the dark period in short-day plants to prevent flowering, is in the red range of 620–640 mμ.

We will now explore this last point—the effect of the duration of light and dark periods—in more detail.

4.4 The Role of Daylength: An In-Depth Look at Photoperiodism

Photoperiodism is formally defined as the response of organisms to the relative lengths of the light and dark periods. This phenomenon was discovered by Garner and Allard (1920) during their work with Maryland Mammoth Tobacco (Nicotiana tabacum), a variety that would grow vegetatively all summer but not flower.

Based on their flowering response to daylength, plants can be classified into four categories:

• Short-day plants: These plants flower only when the daylength is below a certain critical maximum.
• Long-day plants: These plants flower only when the daylength is above a certain critical minimum.
• Day-neutral plants: In these plants, flowering is not determined by daylength.
• Intermediate-day plants: These plants flower only when the daylength is between two critical daylengths. For example, strains of little bluestem (Andropogon scoparius) from south of 36°N latitude will not flower on 13-hour or 15-hour days but flower well on photoperiods in between (Larsen, 1947).

Photoperiodism regulates many other critical plant phenomena beyond flowering, including bulb formation, the type of vegetative growth, dormancy induction, leaf abscission, and even seed germination. This precise timing mechanism is controlled by a specific pigment within the plant.

4.5 Phytochrome: The Molecular Switch for Photoperiodic Responses

The pigment responsible for the timing mechanism in photoperiodism is phytochrome, which was discovered and isolated by Borthwick and his associates at the USDA in 1952. Phytochrome exists in two interconvertible forms that act as a biological light switch.

• Pr: This is the red-absorbing form, with an absorption peak at approximately 660 mμ.
• Pfr: This is the far-red-absorbing form, with an absorption peak at approximately 730 mμ.

These two forms are converted by light. When P_r absorbs red light (660 mμ), it is rapidly converted to P_fr. When P_fr absorbs far-red light (730 mμ), it is rapidly converted back to P_r. In addition to these light-driven conversions, there is a slow, spontaneous conversion of P_fr back to P_r that occurs in darkness.

The P_fr form appears to be inhibitory to the flowering process in short-day plants. This is why the uninterrupted dark period is so critical for these plants: it allows the inhibitory P_fr to slowly revert back to the non-inhibitory P_r form, permitting the flowering process to proceed. Salisbury (1958) also hypothesized that a postulated flowering hormone may be synthesized during the dark period. Borthwick and Hendricks (1960) hypothesized that the fundamental difference between long-day and short-day plants may lie in the specific ratio of P_r to P_fr that is required to initiate a response. This elegant molecular mechanism has acted as a powerful selective force, leading to the evolution of geographically distinct plant populations adapted to local daylength conditions.

4.6 Photoperiodic Ecotypes: Adaptation to Latitude and Altitude

Because daylength varies predictably with latitude and season, it has driven the evolution of photoperiodic ecotypes in many wide-ranging species.

The first demonstration of such ecotypes was Olmsted’s (1944) pioneering work on sideoats grama (Bouteloua curtipendula). He found that southern strains from Texas and Arizona were short-day or intermediate-day plants, while northern strains from North Dakota were long-day plants. Strains from mid-latitudes showed intermediate behavior.

Similarly, Larsen (1947) found that northern strains of little bluestem (Andropogon scoparius) were long-day plants, flowering on 15-hour days but not on shorter days. Southern strains, in contrast, were intermediate-day plants.

An extensive study by Vaartaja (1959) on tree seedlings from 82 origins across Europe and North America further solidified this concept. His key findings were:

• The farther north the origin, the longer the critical daylength required to maintain height growth.
• The more northerly the source, the shorter the critical dark period needed to induce dormancy.
• In some far-northern plants, photoperiods that inhibited elongation still permitted increases in stem and root weight, allowing cambial growth to continue later in the season.

Daylength adaptation can also occur along elevational gradients. Irgens-Moller (1957) studied Douglas-fir (Pseudotsuga menziesii) from different elevations in Oregon. He found that long days hastened bud bursting in high-altitude plants but had no effect on low-altitude plants. This is a crucial adaptation: if high-altitude seedlings break dormancy too early in the spring when days are still short, they risk being killed by low night temperatures.

In conclusion, light—in its intensity, quality, and duration—is a critical environmental factor that plants have adapted to both physiologically and genetically, profoundly shaping their growth, development, and geographic distribution. Our final section will examine the chemical interactions between plants.

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