Growth and Development: Stages, Hormones and Yield
What you must understand
Growth is an irreversible increase in size or mass; development is the sequence of stages. Learn the cereal sequence: germination, emergence, tillering (branching in grasses), stem elongation, booting, heading, flowering (anthesis), grain filling, physiological maturity. For legumes: germination, emergence, vegetative growth, flowering, pod set, pod fill, maturity. Recognise vegetative versus reproductive phases and know that stress at flowering and grain filling cuts yield most.
Know the growth curve (lag, log, stationary) and that dry matter accumulates through net assimilation. Understand photoperiodism (short-day crops such as soybean flower when nights are long; long-day crops flower when days are long), vernalisation (cold requirement), and apical dominance. Plant hormones: auxins (elongation, rooting), gibberellins (stem elongation, germination), cytokinins (cell division, delay of senescence), abscisic acid (stomatal closure, dormancy), ethylene (ripening, abscission).
Traps in questions
- Tillering is vegetative, not reproductive.
- Grain filling is when yield is set; flowering is when seed number is set.
- Abscisic acid closes stomata under drought; it is not a growth promoter.
- Ethylene ripens fruit; auxin at high concentration can act as a herbicide or root promoter depending on dose.
- "Physiological maturity" is not the same as harvest maturity.
Rule of thumb
Yield = plants per hectare × grains per plant × grain weight. Any stress at flowering attacks the middle term, so protect that stage.
Worked example
Two plots of sorghum: A has 60 000 plants/ha and 1 800 grains per plant; B has 90 000 plants/ha and 1 100 grains per plant. Assuming equal grain weight, A yields 108 million grains/ha and B 99 million, so A is higher. Dense planting can reduce yield per plant more than it gains in plant number.