15. PLANT GROWTH AND DEVELOPMENT
It is the response of plants to periods of day/night.
Some plants require light to induce flowering.
Based on light duration, plants are 3 groups:
- Long day plants: They require the exposure to light for a period exceeding a well-defined critical duration.
- Short day plants: They require the exposure to light for a period less than the critical duration before the flowering is initiated in them.
- Day-neutral plants: They have no correlation between exposure to light duration and induction of flowering.
While shoot apices modify into flowering apices, they by themselves cannot perceive photoperiods. The site of perception of light/dark duration is the leaves.
It has been hypothesised that there is hormone(s) for flowering. When plants get enough photoperiod, the hormone migrates from leaves to shoot apices to induce flowering.
It is the phenomenon in which some plants depend quantitatively or qualitatively on exposure to low temperature for flowering.
It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity.
Examples for vernalisation:
1. Some food plants, wheat, barley & rye have two varieties:
- Spring varieties: These are normally planted in the spring and come to flower and produce grain before the end of the growing season.
- Winter varieties: Winter varieties if planted in spring would normally fail to flower or produce mature grain within a span of a flowering season. Hence, they are planted in autumn. They germinate, and over winter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.
Biennials are monocarpic plants that normally flower and die in second season.
E.g. Sugar beet, cabbages, carrots etc. Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.
Certain seeds fail to germinate even under favourable external conditions. Such seeds are in dormancy.
Dormancy is caused by endogenous conditions within the seed. E.g. Hard seed coat; chemical inhibitors such as ABA, phenolic acids, para-ascorbic acid; and immature embryos.
Dormancy can be overcome naturally and artificially. E.g.
- Breaking of seed coat barrier: By mechanical abrasions using knives, sandpaper etc. or vigorous shaking. In nature, abrasions are caused by microbial action, and passage through digestive tract of animals.
- Removing inhibitory substances: By subjecting the seeds to chilling conditions or by application of certain chemicals like gibberellic acid and nitrates.
- Changing the environmental conditions such as light and temperature.