Day neutral plants
- Day-neutral plants do not flower in response to daylight changes.
- They flower when they reach a particular stage of maturity or because of some other cue like temperature or water, etc.
- This is the most common kind of flowering pattern.
- For example, rice, dandelions, tomatoes, etc.
Importance of photoperiodism
- Photoperiodism determines the season in which a particular plant shall come to flower. For example, short-day plants develop flowers in autumn-spring period (e.g., Dahlia, Xanthium) while long-day plants produce flowers in summer (e.g., Amaranthus).
- Knowledge of photoperiodic effect is useful in keeping some plants in vegetative phase (e.g., many vegetables) to obtain higher yield of tubers, rhizomes etc., or keep the plant in reproductive phase to yield more flowers and fruits.
- A plant can be made to flower throughout the year by providing favourable photoperiod.
- Helps the plant breeders in effective cross-breeding in plants.
- Enable a plant to flower in different seasons thus fruits can be produced during their offseason by controlling photoperiod.
Growth rate of arithematic and geometric growth
Growth rate can be defined as the increase in growth per unit time. Plants show two types of growth. They are
- Arithmetic growth - Only one daughter cell continues to divide while others differentiate or mature. For example, root elongating at a constant rate.
- Geometric growth - Initial growth is slow (lag phase), followed by a rapid increase in growth (log/exponential phase), and followed by a phase where growth slows down (stationary phase). For example, all cells, tissues and organs show this type of growth.
Effects of red light and far red light on plant growth
Effect of red light on plant growth
- Stimulates germination
- Induces formation of anthocyanins
- Stmulates flowering in long-day plants
- Induces increase in leaf area
- Inhibits germination
- Inhibits formation of anthocyanins
- Stmulates flowering in short-day plants
- Prevents increase in leaf area
Characteristics of growth regulators
- The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition.
- They could be indole compounds (indole-3-aceticacid); adenine derivatives (kinetin), derivatives of carotenoids (abscisic acid); terpenes (gibberellic acid,) or gases (ethylene).
Factors affecting the growth of a plant - Temperature
The degree or intensity of heat present in a substance is called temperature. Plants grow well only in a limited temperature range. Plants exposed to too high or too low-temperature experience abnormal growth.
Temperature directly affects the process of photosynthesis, respiration, transpiration (loss of water) and absorption of water and nutrients.
Warm-season vegetables and most flowers grow best between 60 and 75 F.
Cold-season vegetables (Example: Lettuce and spinach) should be grown between 50 and 70 F.
Soil temperature affects water and nutrient uptake.
Temperature directly affects the process of photosynthesis, respiration, transpiration (loss of water) and absorption of water and nutrients.
Warm-season vegetables and most flowers grow best between 60 and 75 F.
Cold-season vegetables (Example: Lettuce and spinach) should be grown between 50 and 70 F.
Soil temperature affects water and nutrient uptake.
Discovery of growth regulators
- Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.
- During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).
- Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.
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