PLASTOCHRON INDEX and LEAF PLASTOCHRON INDEX

 

I.  Introduction

 

            The plastochron index (plant age) and leaf plastochron (leaf age) index are ways of measuring the age of plants and leaves on a morphological, rather than a temporal time scale.  Why is this useful for botanical research?

 

            Problems with a time-based scale include:  different germination times;  genetic variation in growth rate;  exponential (not linear) growth with respect to time.

 

            By “ageing” plants by their morphology rather than chronological time,  P.I. eliminates the variation due to germination time and growth rate and creates a linear scale to measure development based on morphology.  Also it is non-destructive.

 

Requirements for the plant system:   

 

1. Plastochron (leaf production rate) must be constant over study period.

 

2.  Early leaf growth exponential up to initiation of next leaf.

 

3.  Early growth rate for leaves is the same (plots of log length vs. time creates a series of parallel lines)

 

II.  Formulas

 

                                    P.I.  =   n         +           log Ln   -  log l                       

                                                                       ----------------------                                  L.P.I. = P.I. - i

                                                                        log Ln  -  log Ln+1

 

 

                                       l       =          reference value leaf length (e.g. 10 mm)

 

                                        Ln     =           length of leaf n

 

                                        n      =          the number of the leaf equal to or just

                                                            longer than the reference leaf

 

                                         i      =           number of the i th leaf

III.  Figures (from Maksymowych 1973)

 

                                   
IV.  Examples

 

                                    P.I.  =   n    +   log Ln   -  log l                       

                                                             ----------------------

                                                             log Ln  -  log Ln+1

 

 

 

           

                        Example 1:  if  L6  = 10 mm and L7  = 5 mm;  then n = 6

                                               

                                                P.I. = 6  +     log 10   -  log 10            

                                                                  ------------------------    =  6 + 0 = 6

                                                                    log 10   -  log 5

 

                                               

 

                        Example 2:  (the same plant later)  L6  = 12 mm and L7  = 7 mm;

                                                 then n = 6

                                               

                                                P.I. =   6  +       log 12   -  log 10                    

                                                                       -------------------------  =  6 + .34 = 6.34

                                                                          log 12  -  log 7

 

 

 

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                                                L.P.I. = P.I. - i

 

Example 1                                                                  Example 2

 

L.P.I. for L7 = 6 - 7  =  -1                                             L.P.I. for L7 = 6.34 - 7  =   - 0.66

L.P.I. for L6 = 6 - 6  =   0                                             L.P.I. for L6 = 6.34 - 6  =  + 0.34

L.P.I. for L5 = 6 - 5  =   1                                             L.P.I. for L5 = 6.34 - 5  =  + 1.34

 

V.  Lab Excercise

 

Each pair of students will work with one Xanthium  (sunflower) plant.

 

Count the number of leaves starting above the cotyledons (or scars) and continuing to the smallest leaves that you will be able to measure.  Identify the leaf just larger than the index length (20 mm).

 

Calculate the P.I. for the plant and the L.P.I. for each leaf on the plant. 

 

Is there a consistent mathematical relationship (difference) between the calculated values for P.I. and L.P.I and the L.P.I. values of the successive leaves? 

 

Which leaves have a negative L.P.I.?

 

If you have time you can measure a second plant and make the calculations.

 

 

VI.  Literature

 

Dickson,  R.E. and P.R.  Larson.  1981.  C14 Fixation,  metabolic labelling patterns and translocation profiles during leaf development in Populus deltoides   Planta  152: 461-470.

 

Erickson,  R.O. and F.J.  Michelini.  1957.  The plastochron index.  Amer. J. Bot.  44: 297-304.

 

Larson, P.R. and J.G.  Isebrands.  1971.  The plastochron index as applied to developmental studies of cottonwood.  Can. J. Forest Res. 1: 1-11.

 

Maksymowych,  R. and R.O. Erickson.  1960.  Development of the lamina in Xanthium italicum represented by the plastochron index.  Amer. J. Bot.  47: 451-459.

 

Maksymowych,  R.  1973.  Analysis of Leaf Development.  Cambridge University Press,  Cambridge.