Growth and Development of Horticultural Crops (1+1)

Kinetics of growth – the course of growth (Grand period of growth) or sigmoid curve (Grand period curve)

• Usually under favorable conditions there is a characteristic increase in the plant’s growing parts. Growth is slow at first (Lag Phase), then gains speed (Log Phase) and eventually slows down (Decreasing Growth Rate) to come to a halt (Steady State). The total time during which this course of growth takes place is called as the Grand period of Growth. If this growth rate is plotted against time, a slanting S shaped curve is obtained which is called as Sigmoid Curve or Grand Period Curve.


Fig 2. Sigmoid growth curve
http://www.tutorvista.com

• The sigmoid curve represents the integrated sum of the curves for each growing organ and cell and presents the changing size of these parts. Similarly, when dry weight is measured as an index of growth before maturity, the curve takes the well known sigmoid form. Environmental conditions may alter growth rates but not the sigmoid form of the growth curve.
• In unicellular organisms such as Chlamydomonas or bacteria, growth is assessed by a count of number of cells per milliliter at increasing times after the cells are placed in a fresh nutrient medium and under environmental conditions (light, temperature, etc.) suitable for optimal growth. Here also, there is initial lag period during which cells activate their biochemical machinery for rapid growth by synthesizing necessary enzymes. This is followed by a time period during which there is exponential increase in cell number which is called as log period.
  
• This period of rapid growth does not continue indefinitely and due to depleted nutrient supply, accumulation of toxic products and other limiting factors ultimately leads to decreasing cell number until the population of cells reaches a steady state in which the number of cells remains constant (stationary) or even declines. If number of cells per milliliter is plotted against time (hours), again a sigmoid curve is obtained as described earlier.