This article will guide you about how to determine the plastochron index.
Plastochron is defined as the time interval between the formation of one leaf primordium and the initiation of next leaf primordium.
Plastochron is a developmental measure and is expressed as ‘plastochron index’. The index indicates the developmental status of each leaf and relates the observation to time. The developmental index is the plastochron age of a shoot. Plastochron Index (P. I.) can also be defined as a measure of plant growth and is used to determine the rate of growth based upon the initiation of successive leaf primordia.
Plastochron Index provides a morphological time scale rather than chronological age in studies relating to morphological and physiological development of a whole plant or plant organ. Similarly Leaf Plastochron Index (L. P. I.) provides a morphological time scale rather than chronological in studies relating to morphological and physiological development of a leaf.
In developmental studies plastochron is used as the unit of developmental scale. It is observed that morphologically similar leaves may be of quite different chronological age, while leaves with similar chronological age may also have quite different stages of development. Leaves of same plastochron age reveal similar morphology and development.
In developmental studies it is to relate an aspect of developing leaf directly to time. When the aspect is leaf development the length and breadth are measured in relation to time and the leaf remains intact. The aspect may be the dry weight, development of mesophyll tissue etc. and in this case the leaf is to be sacrificed to obtain data.
In this type of study one has to deal with many parallel samples when each is sacrificed at a given time for the required data. These parallel samples must be of same plastochron because P. I. eliminates the variation due to growth rate and creates a linear scale to measure development based on morphology.
For plastochron study the plants must be genetically uniform, grown under exactly controlled conditions and in vegetative growth. The Xanthium leaf is one of the most thoroughly investigated leaves among dicotyledons using P. I. & L. P. I. The estimation of P. I. fails when flowering is induced in Xanthium.
Plastochron Index applies to shoot. So it is very useful in developmental studies in which interest centres on characteristic of the shoot. The characteristic may be the rate of metabolic process, content of a biochemical constituent, total fresh weight or dry weight of shoot etc. and these can be determined at various stages of development.
The development of shoot apex in Zea was studied using P. I. Abbe et al. (1951) demonstrated the progressive increase in mass of the apical meristem of Zea during growing season. In Hypericum Zimmermann in 1928 demonstrated cyclical changes in the area of apical meristem and these changes are correlated with the initiation of leaf using P. I.
During one plastochron the shoot apex undergoes changes in its morphology and this change is referred to as plastochronic change (Fig. 25.1). The shoot apex is in the form of a small rounded mound before the initiation of a new leaf primordium. As growth continues the shoot apex widens. Later leaf buttresses are initiated on the sides of apical meristem.
A leaf buttress constitutes the base of a leaf and it forms a lateral prominence on the sides of apical meristem. Leaf primordium grows upward from leaf buttress. After the formation of new leaf primordium the shoot apex again becomes in the form of a small rounded mound. During plastochronic change the volume and surface area of the shoot apices also change.
The changes are expressed as minimal-area phases and maximal-area phases. Later these phases are respectively referred to as minimal phases and maximal phases or minimal area and maximal area. The following plants are suitable for the study of plastochronic changes in the shoot apices of dicotyledons with opposite leaves – Coleus, Lonicera, Syringa etc.
L. P. I. is used to study the development of leaf. L. P. I. is also used to determine the oxygen uptake, chlorophyll content, and fresh weight of Xanthium leaves at various stages of development. Maksymowych (1956) studied the histogenesis of Xanthium leaf applying L. P. I. The study revealed the relationship between marginal meristem activity and mesophyll development.
The plastochron index is estimated in the following way:
P. I. = Plastochron Index; n = it is the serial number of that leaf, which is 10 mm or just exceeds 10 mm length (counted from the base of the shoot); n + 1 = it is the succeeding leaf with a length that is less than 10 mm; Ln = length of n leaf; log 10 = it is the predetermined reference value of leaf length.
Specifically a leaf is n plastochrons old when the leaf is 10 mm in length. The n + 1 leaf is n + 1 plastochrons old when it is 10 mm long and so on. But it is seldom observed that a leaf is exactly as long as 10 mm. For instance leaf 5, n, is 18 mm in length and leaf 6, i.e. n + 1, is 5 mm long at a given time.
This indicates that the plant is more than 5 plastochrons but less than 6 plastochrons old. In this case the equation (1) is used to estimate the plastochron index of a plant in relation to leaf 5 at a given time. Examples 1, 2 and 3 illustrate the procedure to estimate P. I. of a shoot putting the length value of two successive leaves in equation (1).
The derivation of plastochron index for shoot development in Xanthium is described below.
The experimental plant was Xanthium italicum. Seeds were collected and allowed to germinate in pots. Xanthium is a short-day plant, i.e. the plant needs 8 hours of light and 16 hours of dark to flower. The short-day plants can be kept in vegetative growth by treating them as long-day/short-night plants only.
Xanthium plants were kept in lighted green house. The long-day plants need 16hrs of illumination per day. Light was provided to Xanthium before dawn and dusk in such a way that the plants got 16 hrs of illumination per day. Thus the plants were kept strictly in vegetative growth. About 20-30 plants were selected and each leaf of plants was measured daily from July 8 to August 26, 1952.
The length of a leaf with petiole was measured only. Measurement was taken daily at a fixed time. The length of leaf was plotted logarithmically against time. The graph shows sigmoid growth curves of the leaves. This signifies that the leaf growth was exponential in the early portion. From the curve it also appears that the relationship between logarithm of leaf length and time for one plant was linear up to approximately 70 mm of leaf length (Fig. 25.2).
From the above data Erickson et al. developed the equation:
(1) Using Xanthium. Consult Erickson et al. and Maksymowych for details of derivation of the equation 1. In the equation (1) The reference value of leaf length is 10 mm, i.e. Log 10 at a given time.
Ten millimeter length is considered for:
(1) The 10 mm length of a leaf is large enough to be measured accurately;
(2) The leaf can be measured without injuring the shoot apex and
(3) The leaf of Xanthium of this order grows exponentially.
In Fig. 25.2 the reference value of leaf length, i.e. 10 mm is designated with dashed horizontal line. In the reference value of leaf length logarithm of any base may be used.
So the equation (1) may be rewritten as:
P. I. = Plastochron Index; n = serial number of leaf that is just the predetermined value of leaf length or exceeds the value; logLn = natural logarithm of length of n leaf; logLn+1 = natural logarithm of the succeeding leaf with a length that is less than the predetermined value of leaf length and logLλ = the predetermined reference value of leaf length.
L. P. I. is estimated according to the following formula: L. P. I. = P. I. – i where i = serial number of i-th leaf. Example 4 illustrates the procedure to estimate the L. P. I. from the data obtained from examples 1 to 3.
In the examples the plastochron age of the entire plant is respectively 6,6.338 and 5.458. In the example 2 the age of leaf 5 in plastochron unit can be estimated in the following way: plastochron index of the entire plant minus the leaf number 5, i.e. LPI- = PI – 5 = 6.338-5 = 1.338.
L. P. I. has negative value in leaf primordia that originate next to the reference leaf. In other words when a leaf primordium is shorter than the reference length L. P. I. has negative value and when the leaf primordium is longer than the reference leaf L. P. I. has positive value.
In example 1 the reference value of leaf length (L6) was 10 mm. So the L. P. I. is zero. In example 2 L7 is shorter than L6 and L5 is longer than L6. Accordingly the L. P. I. for L7 has negative value and L5 has positive value. A few data recorded by Erickson et al. to formulate P. I. and L. P. I. is represented in Table 25.1.
Conventionally a plastochron is defined as the period between initiations of two successive leaves. In a broad sense it might be the period between the successive stages of initiation, development, maturity or intermediate stage of development of two leaves as the stage of reference.
The main purpose of studying plastochron is to obtain many leaves having same morphology and development. When one is sacrificed to obtain data, the other leaf is ready to provide data if required. This is because leaves of same plastochron age grow exponentially.