Specific Leaf Area and Forage Analysis

1.      Experiment Objectives

The main objectives is to calculate the Specific Leaf Area (SLA) of plants in three different growth ages (young, medium and adult). This enables the proper understanding of the relationships between SLA, LA (leaf area), growth rate and biomass production. In addition, another objective is to prepare samples of plants under NH₄NO₃ and NaCl treatments in order to realise a forage analysis (FA) in a future experiment.

2.      Theoretical Background

The specific leaf area (SLA, ratio of leaf area per leaf dry weight) is an important leaf trait in plant ecology because it is related to many critical aspects of plant growth. Together with the LDMC (LDMC, ratio of leaf dry mass to fresh mass), the SLA is an indicator trait of resource-use strategies (Li, 2005). According to Garnier et al. (2001), SLA and LDMC reflect a fundamental trade-off in plant functioning between a rapid production of biomass (high SLA, low LDMC species) and an efficient conservation of nutrients (low SLA, high LDMC species). The important roles that these two traits play in explaining variation in potential relative growth rate (RGR) and ecological behavior in plants have been demonstrated in recent studies under controlled environments (Li, 2005).

3.      Description of Experimental Work

·         Selection and harvest of plants in the GH1

For the SLA, five plants in different growth stages were to be selected, i.e. one adult, two mediums and two young. All plants were cut at ground level and put into water right after cutting to prevent them from wilting to fast.

For the FA, five plants under different treatments were selected, i.e. two plants with a NH₄NO₃ treatment (one from a single plant pot and the other from a multiple plant pot), two plants with a NaCl treatment (one from a single plant pot and the other from a multiple plant pot), and finally a control plant under no special treatment for comparison purpose. All plants were cut at ground level and put into water right after cutting to prevent them from wilting to fast. The plants were then labeled with the name of the participant and the work group. For SLA plants with additionally indicated the growth age (adult/medium/young) and for FA plants we indicated the treatment of the plant (NH₄NO₃/ NaCl; single/multiple). The plants were then taken from GH1 to GH2 for processing reasons.

·         Processing of plants in the GH2

SLA: Each leaf of each plant had to be identified, measured and kept in a paper bag to be afterwards dried in an oven.

The group decided to start counting (and then cutting) the leaves of the plants from the first green leaf at the bottom, to the last smallest leaf at the top of the plant. This methodology appeared to us as the easiest way to proceed due to the practical reason of cutting the older, outer leaves at first and the newer, inner leaves in the end.

We labeled paper bags for each one of the leaves of each plant with the name of the participant, the work group, the growth age of the plant (to be able to identify the plant, e.g. Medium2) and the leaf number. Each bag was labeled with a pencil in order to be sure not to lose the labeling in the oven (possible evaporation of ink due to alcohol).

We then proceed with the measuring of their LA. Each leaf was cut, measured and placed in its respective labeled paper bag. The LA in cm² of each leaf was then written on the respective bags. The rest of the plants (e.g. stems and dried leaves) were thrown away.

4.      Original Measurement Protocol

5.      Sketch of Experimental Setup

6.      Results

6.1  SLA curves for the different growth stages

The graph above shows an irregular trend in tha SLA of young, mature and old leaves. In general, we can apreciate that the SLA of the young plants is higher the medium and the adult plants.

 

 

 

 

Result

6.2   Total LA and SLA curves for the different growth stages

From the previous graph, it can be clearly seen that the total LA is increasing as a plant is growing. This results in an increase of dry weight biomass, which therefore leads to a decreasing total SLA for the plant.

7.      Interpretation of Results

7.1  SLA analysis

7.2  Effects of growth stage on SLA

The older the plant is, the more SLA decreases.

Young plant have less dry weight compared to its leaf blade area. As the plant grow older, its dry weight increases and this result in lower value for adult plant. Plant having high SLA produce a leaf area more rapidly than those having low SLA.

7.3  Leaf position and SLA

No clear relationship

Increasing SLA in plants result in increasing leaf area index with increase in the biomass. The plastic response of the specific leaf area within plants species substantially increases the aboveground competition for light along the nitrogen gradient.

8.      Discussion of errors

• The curling of the edges of cut leaves forces the person doing the LA measuring to put its patience into practice. This apparently minor challenge of trying to flatten the leaves reveals being time consuming and annoying. The LA measuring results all in all being a meticulous work. The measuring of a great number of plants turns into a monotonous activity prompting to make errors.

• A part of a hand was calculated together with the leaves during the measuring of the LA. As it was not possible to “erase” this specific data and do once again the measuring, we had to measure approximately how much cm² of light do this part of the hand covers in order to subtract this value in the end. This error showed us how easy it is to loose concentration while doing repetitively the measuring again and again. This applies for all when several persons work simultaneously on the same LA measuring device with time pressure.

• We also were at some extent skeptical with the precision of data from the LA measuring device. This happened as some senescence and young leaves (yellowish, not anymore or not yet photosynthetic active) were given different values in different trials.

• Another discussion within the group about the accuracy of the data given was regarding the light intensity of the place the device is measuring. Do the light intensity from “outside” has an impact on the measurement of the LA?

9.      References

Garnier, E., Shipley, B., Roumet, C. and Laurent, G. (2001), A standardized protocol for the determination of specific leaf area and leaf dry matter content. Functional Ecology, 15: 688–689. doi: 10.1046/j.0269-8463.2001.00563.x

Li, Y., (2005) Specific leaf area and leaf dry matter content of plants growing in sand dunes. Botanical Bulletin of Academia Sinica, Vol. 46: 127-128.