Chapter 28: External Factors and Plant Growth

Phototropism involves shoots bending toward light sources through the lateral redistribution of the hormone auxin toward shaded regions, facilitated by blue-light photoreceptors called phototropins. Gravitropism enables roots to grow downward and shoots upward through the action of statocytes containing sedimenting amyloplasts (statoliths) that trigger auxin redistribution; because roots are more sensitive to auxin than shoots, higher auxin concentrations on the lower root side inhibit elongation while stimulating it in shoots. Additional tropistic responses include hydrotropism, which guides roots toward moisture sources, and thigmotropism, which allows roots and tendrils to navigate obstacles and coil around supports. The chapter then addresses circadian rhythms, endogenous approximately 24-hour cycles that regulate leaf positioning, stomatal function, photosynthetic rates, fragrance emission, and gene expression patterns. These rhythms depend on oscillating gene networks such as TOC1, LHY, and CCA1, which are synchronized to daily environmental cycles through blue-light receptors (cryptochromes) and red/far-red light receptors (phytochromes). Photoperiodism represents the flowering response to daylength, with short-day plants flowering when nights exceed a critical length, long-day plants flowering under longer days, and day-neutral plants flowering independently of photoperiod. Phytochrome, a reversible pigment interconverting between red-absorbing (Pr) and far-red-absorbing (Pfr) forms, functions as a biological light sensor for this response. The floral stimulus, identified as the FT protein, is synthesized in leaves and transported through the phloem to the shoot apical meristem where it associates with the FD protein to initiate flowering. Vernalization, prolonged cold exposure, renders biennials and winter annuals competent to flower while preventing premature fall flowering through the action of genes like FLC and FRI and long noncoding RNAs such as COLDAIR. The chapter also covers dormancy as a survival mechanism in seeds and buds, with seeds requiring conditions such as cold stratification, scarification, leaching, or light exposure to germinate. Nastic movements, including nyctinastic sleep movements and thigmonastic responses in sensitive plants and Venus flytraps, represent rapid turgor-driven responses regulated by circadian and phytochrome signals. Finally, thigmomorphogenesis and heliotropism demonstrate how mechanical stress and solar position optimize plant growth and light capture across diverse environmental contexts.