Responding to the Environment

1. Responding to the Environment

2. Plants don’t have highly specialized receptors like animals do. • A sensitive cell in plants responds to a stimulus and produces a hormone which travels relatively slowly to reach a responsive tissue. • Plants respond to physical and chemical factors. • Physical factors include: direction and wavelength of light, day/night length (photoperiod), gravity, temp and touch. • Chemical factors include: water, CO2, and specific chemicals such as ethylene.

3. Sensing Light • Phototropism is a response to blue light (350-500nm). The pigment carotene (orange-yellow) is likely to be involved. • Other growth responses are stimulated by red-orange light (620-670nm) and reversed by far-red light (710-750nm). These responses include germination of seeds, expansion of leaves, and induction and breaking of dormancy. • While flowering and elongation are stimulated by far-red light and inhibited by red-orange light.

4. Photoperiod and Flowering • Flowering in most plants is unaffected by phototperiod. • Some plants however are grouped as short-day or long-day plants according to the affect of photoperiod on flowering. It is a bit of a misnomer as it is now known that is actually the length of the night which is the important stimulus. • Short-day plants require longer nights to flower and are often late-summer, autumn or winter flowering plants. • Short-day plant flower numbers may be affected by the length of the light period even though it is the length of the dark period which stimulates the blooming of flowers. • Long-day plants flower when the nights are short or if the plants are continuously illuminated, typically late spring and early summer flowering plants. • Neutral plants are largely unaffected by the photoperiod in regards to flowering.

5. The Receptor is a Leaf Pigment • The receptor stimulated by photoperiod appears to be a pigment in leaves that responds to red light. • When the dark period is of the right length, a hormone is produced in the leaves that passes to shoot tips and other parts of the plant where it stimulates flower development. • Sensitivity to the photoperiod explains varying times of the year when different species of plants flower. • This also explains why flowers of the same species always bloom around the same time. Also why some plants cannot bloom in certain regions where the night never reaches the proper length to stimulate flowering.

6. Sensing Gravity • Shoots are negatively geotropic and grow opposite the pull of gravity, roots are positively geotropic and grow towards the pull of gravity. • The root of a germinating seed penetrates the soil, which provides it with water, nutrients and anchorage. • The root cap is sensitive the force of gravity. • Roots grow downwards because the upper side outgrows the lower side, bending the tip downwards. The hormonal mechanism of this is not fully understood but appears to be connected to the particular location of starch-containing organelles called amyloplasts. • A mechanism involving amyloplasts and asymetrical distribution of auxin is believed to be involved in shoot negative geotropism prior to their response to phototropism.

7. Sensing Touch • Thigmotropism is growth responses as a result of contact with a surface. • The coiling and bending of tendrils around objects (ie. Pea plants and vines) is a result if thigmotropism. • When they appear to climb up and around trees it is a combination of thigmotropism and phototropism.

8. Movements and Plant Rhythms Sleep Movements • The response where plants close their flower petals or fold their leaves at night. • Stomata usually close as a result of a combination of stimuli: light, humidity and CO2. • These responses are typically due to changes in turgor (internal fluid pressure) of particular cells or in the case of stomata the change in turgor of guard cells. • If water leaves, the cells become less rigid (lose turgidity)

9. Solar Tracking • When leaves twist on their petiole (stalk) in response to light. • Particularly cells at the junction of the leaf blade and the petiole control the moment. • As with sleep movements, this is controlled by turgor changes, loss of turgor makes the petiole bend.

10. Nodding and Contraction Movements • Ethylene production results in a thickening and bending of the young pea shoot during seed germination. This causes the shoot to nod as it grows and helps the plant push its way up through the soil. • Lillies have contractile roots. Temp fluctuations at the soil surface appear to stimulate the contraction. This allows a young bulb that has formed after germination on the surface to bury itself deeper in the soil.

11. Rapid Responses • Plants typically are incapable of rapid movements. A few however show quite rapid movements. • The venus flytrap, a carnivorous plant which snaps shut in response to the touch of an insect. • The trigger plant which has a special structure called the column where both male anthers and female stigma are located. When triggered the column is flung against a bee. Aiding in pollination and reproduction for the plant.

12. Temperature • Temperatures effects on plants is typically related to its effects on enzyme function. • There is usually a minimum temperature range below which plants may survive but will not grow (0-5°C), an optimal range for growth (28-35°C) and an upper temperature range above which plants will not survive (45-52°C). • Some plants thrive in climates where temps vary greatly between day and night. In seasonal climates where winters can be harsh, bud and seed domancy allow plants to survive successfully.

13. Seed Dormancy • As seeds mature, the development of the embryo is arrested and the seeds enter a dormant stage. They remain dormant until conditions are right for successful germination. • Many seeds require cold for a number of months to break dormancy. This allows seeds to typically germinate in spring when conditions are ideal for growth. • In deserts some species of plants will not germinate until they have been leached by considerable amounts of water. Abscisic Acidprevents the seed from developing until there is sufficient water to allow the seedling to become established.

14. Bud Dormancy • Some plants in temperate regions become dormant prior to winter. The lower temps and shortening of days trigger changes that involve the loss of chlorophyll from leaves and withdrawal of nutrients from leaves into stems and roots. • The leaves fall, the buds enter a period of dormancy, this is largely triggered by an increase in abscisic Acidproduced in leaves due to the environmental changes.

15. Vernalisation • Is the period of winter cold that stimulates flowering. • Vernalisation can occur at various stages of plant development. But an exposure to cold is necessary at some point to induce flowering in bulbs. • Some plants respond to the cold after their first year of growth. These plants grow vegetatively for their first year and are stimulated to flower the following year by the cold winter (these flowers are known as biennials because they grow over two years).