BOTANY VOLUME 4 - ECOLOGY - 2007

13. PLANTS IN THEIR HABITAT SPACE

13.2. Light as a Signal

This section focuses on the qualitative, specifically the signaling effects of light (the fundamentals are covered in Chapter 7, while quantitative effects of light are discussed in 13.7.1).

13.2.1. Photoperiodism and Seasonality

Plants in seasonal climates receive highly precise calendar information via their Phytochrome system (see 7.7.2), which governs the key stages of their development (photoperiodism). The ecological implications of this are diverse. In regions prone to frost, photoperiodic sensitivity protects plants from late and early freezes by shifting the timing of bud burst and senescence to safer periods, independently of current temperatures. In regions with pronounced seasonal precipitation patterns (such as monsoon areas), photoperiodic sensitivity ensures flowering and fruiting even if delayed or scant rainfall stunts plant growth (induction of flowering despite suboptimal vegetative development). This phenomenon accounts for the renowned yield stability of photoperiodically sensitive Indian rice varieties compared to photoperiodically insensitive Japanese varieties.

Photoperiodism is strongly ecotypically (genetically) differentiated, as demonstrated by transplant experiments involving shifts in latitude or altitude. Arctic plants struggle to flower in temperate zones, whereas temperate plants in high-latitude regions flower earlier and shed their leaves later, effectively "waiting" for shorter days. Tree seedlings sourced from the timberline and replanted in valleys remain in winter dormancy until June despite more favorable temperatures; conversely, transplanting forest plants from low altitudes to the timberline is equally unsuccessful because their photoperiodism is misaligned with local thermal conditions. These factors represent major barriers to utilizing extended growing seasons driven by climate warming.

Observations of herbaceous neophytes (alien species) indicate that at least 6 generations must pass before new genotypes (ecotypes) adapted to the local photoperiod are established; for trees, this timeframe is substantially longer. Alongside well-documented effects on flowering (see 7.7.2.2), numerous impacts on Morphology and metabolic physiology have also been observed. For instance, according to O. M. Heide, when *Poa pratensis* from northern Scandinavia is subjected to a simulated long-day Treatment via red light irradiation, it produces more leaves, yet these leaves are thinner (resulting in a higher SLA, see 13.7.3). This promotes greater growth and biomass production compared to plants receiving the identical daily light dose without red light photoperiod extension, even though photosynthetic capacity per unit of leaf area actually decreases.

It is generally established that photoperiodic regulation during the transition to autumn senescence and increased stress resistance proceeds more precisely and is less

dependent on weather fluctuations than the resumption of growth in spring. Many alpine plants act as "opportunists" in spring and "stubbornly fixed" in autumn, which ensures the retroactive withdrawal of mobile resources from leaves before frost damage occurs.

13.2.2. The Red Light Signal in Plant Communities

Radiation that penetrates green leaves or is reflected by foliage is enriched in the far-red spectral region (700–800 nm) and depleted in the red region (620–680 nm). In other words, The ratio of shorter red wavelengths to longer ones (e.g., I₆₆₆/I₇₃₀) decreases (known as the red/far-red ratio, or R/FR). The ability of plants to assess their position relative to neighbors using red light sensitivity (phytochrome, see 7.7.2.4) has profound implications for plant community assembly and interspecific competition. Model plant experiments have demonstrated that growing shoots actively avoid already occupied "green" microsites, optimizing resource allocation into newly formed branches.

A. Novoplansky and coworkers placed green and gray cards around creeping, multidirectionally growing young shoots of *Portulaca oleracea*, effectively fencing in the plants. SHOOT apices navigated toward the gray cards and away from the green ones, which absorb red light. Similarly, the upward elongation of competing seedlings—prior to the onset of mutual shading—is stimulated by the subtle transmission of red wavelengths emanating from neighboring plants. C. Ballaré and colleagues shielded the shoots of *Datura ferox* and *Sinapis alba* with red-light filters, rendering them "blind" to their neighbors. As a result, these neighboring plants were no longer perceived, and the shielded shoots failed to exhibit elongation responses compared to control plants. It is hypothesized that such mechanisms Complement "standard" phototropism in the sensory repertoire of many species.

In dense plant stands, the spectral shift from red to far-red exerts such a strong influence on seed regeneration beneath closed canopies that germination—despite prior seed imbibition—only occurs when a canopy gap (signaling red light availability) indicates the prospect of photosynthetic activity and thus survival. Consequently, forest regeneration via seeds is heavily dependent on gap dynamics. Dense canopies reduce the R/FR ratio regardless of radiation intensity, dropping from 1.2 in direct sunlight to 0.2 beneath the forest understory (Fig. 13.5). Thus, determining the R/FR ratio serves as an additional indirect method for quantifying canopy closure (LAI, see 13.1.3).

Class="center">Fig. 13.5. Daily course of R/FR values in the understory of a subtropical forest in northeastern Australia (after S. Turnbull and D. J. Yates, from R. Chazdon et al.). The R/FR ratio on the forest floor reaches its minimum when the residual light filtering through the canopy is further obstructed by broad understory leaves, and peaks when a sunfleck (a direct ray of sunlight) passing through a canopy opening strikes the sensor. The ratio above the canopy as a whole is approximately 1.2.

The red-light spectral shift caused by chlorophyll is also reflected in the radiative flux of the plant canopy as a whole, which can be utilized to determine fractional vegetation cover via remote sensing. The Normalized Difference Vegetation Index (NDVI) can be estimated using aerial or satellite imagery. NDVI is based on reflectance measurements in the red (I_R, 660 nm) and infrared (I_FR, 730 nm) spectral bands:

NDVI = (I_FR - I_R) / (I_FR + I_R).

The denser the ground cover of photosynthetically active structures, the greater the reduction in red-band reflectance and the increase in far-red (infrared) reflectance. Such data enable the early estimation of potential crop yields over vast geographic areas (for instance, poor overwintering of winter crops in spring can be detected from space). Furthermore, data from this thematic mapper scanning can be used to track broad-scale changes in land use or natural vegetation. Remarkably, even ecosystem productivity can be reliably assessed from space.