3 minute read
33 Plant Rhythms
Key Idea: Plant hormones play crucial roles in the timing of activities including fruit ripening and breaking dormancy. Many of the responses of plants to cues in the environment, such as low temperatures and daylength are mediated by hormones. Low temperature stimulation of flowering (vernalisation) and seed germination (stratification) are common in many species. They are examples of responses mediated by plant hormones, and enable the plant to track and respond to seasonal changes. Plant hormones are chemicals that act as signal molecules to regulate plant growth and responses. Alone or together, plant hormones target specific parts of a plant and produce a specific effect. Many have roles in coordinating timing responses in plants including promoting and breaking bud dormancy, seed germination, and fruit ripening. In addition these rhythms are linked to temperature.
Hormones, plant growth, and fruiting
Bud burst and flowering follow exposure to a cold period in many plants, including bulbs and many perennials. This process is called vernalisation and it ensures that reproduction occurs in spring and summer, not autumn. Gibberellins are important in breaking bud dormancy. Auxins and gibberellins are important in promoting the growth and development of shoots.
November October Lea f an d flower development
September December
January
February
March
USDA
F rui t d e v e l o p men t
Ethylene is a gaseous plant hormone with an important role in the ripening process of many fruits. Auxin and ethylene are believed to work together to promote fruit fall.
August
July Dormancy period
June
Dormancy is a condition of arrested growth. The plant, or its seeds or buds, do not resume growth until increasing daylength and temperatures provide favourable growing conditions in spring. Abscisic acid (ABA) promotes dormancy, preventing development of the leaf and flower bud under unfavourable conditions
April
May
Deciduous plants shed their leaves every autumn in a process called abscission. A decline in auxin (IAA) and an increase in ethylene work together to bring about leaf drop. Losing leaves conserves resources at a time when there is not enough light for photosynthesis and the cold weather may damage the delicate leaf structures.
Seed germination and hormones
The seeds of many cold-climate plants will not germinate until they have been exposed to a period of wet, cold (5°C) conditions. This is called cold stratification. ABA accumulates in seeds during fruit production and is important in seed dormancy. A high level of ABA in the seed embryo promotes dormancy. Gibberellins are required for seed germination. They stimulate cell division and cell elongation, allowing the root to penetrate the seed coat.
Daily rhythm in tulips
Many flowers, including tulips, show sleep movements. In most species, these are triggered by daylength, but in tulips the environmental cue is temperature. This series of photographs shows the sleep movements of a single tulip flower over one 12 hour period during spring. Sleep movements may prevent flower damage, stop the entry of non-pollinating insects, or stop the pollen becoming wet with dew.
7.00 am 8.30 am 11.00 am 5.00 pm 7.00 pm
All photos: RA
1. (a) Describe the adaptive value of dormancy in plants:
(b) What cues are likely to be involved in breaking dormancy?
2. How does vernalisation ensure a plant will not flower in autumn?
4. Why is it likely that the same hormones are responsible for both leaf fall (abscission) and fruit fall in plants?
5. Describe is the advantage of cold stratification in plant seeds:
6. (a) Describe the sleep movements of tulips in response to temperature:
(b) How might these movements be adaptive?
(c) What is the name of this type of response?
