We are analyzing the data from the experiment Monica conducted with mayapple from 1990-1992. Here is a brief summary of the experiment, and the questions/hypotheses we are addressing/testing with these analyses.
1.1 General problem
Mayapple is a common rhizomatous perennial of eastern deciduous forests. It has a sympodial rhizome system in which an annual shoot is borne above ground only on the terminal (youngest) rhizome segment. At the same time, it maintains older rhizome segments (up to 10 or more) that do not bear shoots. While roots are maintained on these older segments and do contribute to nutrient and water uptake, the question is why maintain such a large number of rhizome segments, because there is likely to be some (resource) cost to maintaining older rhizomes.
There are two potential roles of the older rhizome segments: 1) as a storage organ for carbon fixed by the current shoot and stored for the production of future shoots (there is a well-defined seasonal pattern of carbon transport to and from the current shoot and the rhizome, as well as into new rhizome segments (Landa et al)); 2) dormant buds reside at locations along the rhizome system where earlier shoots arose, and these dormant buds allow a plant to recover from damage to the forward part of the rhizome system.
Early in the season (April), when the new shoot is expanding, resources (carbon) flows from storage in the rhizome into the expanding shoot. Once the shoot has fully expanded, new carbon fixed by the current shoot through photosynthesis, flows backward into the old rhizome for storage and maintenance, and forward into the new rhizome segments that are being formed.
The roles of the rhizome as a storage organ and bud bank were assessed through an experiment that consisted of severing the rhizome at different positions along the rhizome system and at different times in the growing season in 1990 – and thus interrupting the flow of carbon between rhizome and shoots, and also mimicking the type of damage that can occur to the rhizome from stem boring moths. Half of the systems were picked specifically because they had sexual shoots and half had vegetative shoots in 1990, the year of severing.
1.3 Data collection
The effect of severing and time of severing on the performance of shoots produced in 1) the year of severing, and 2) in each of two years following severing was measured. The measures of performance are: size (leaf area) of shoot, type of shoot, number of shoots, reproductive success in the case of sexual shoots, and timing of senescence.
In the year of severance, severing could not affect the number or the type of shoot produced because the shoots had already been pre-formed in the previous year.
For shoots produced in the years following severing, shoot production was assessed both on the front part of the rhizome system (ahead of the point of severing) and on the back part of the rhizome system (behind the point of severing).
1.4 Q1: shoot performance in 1990
To what extent does timing and position of rhizome severing affect the leaf area of the shoot that expands in the year that severing was conducted?
Background: Severing the rhizome separates the front and back of the rhizome system, preventing the front part of the system from gaining access to any resources stored in the back of the system. Thus, any activity in the front part of the system that depends on stored resources (e.g., expansion of current shoot) could be affected by severing. The closer the severing is to the current shoot, the more resources are left in the back part of the system and are made inaccessible to the current shoot.
The timing of severing matters to resource access. If severing occurs after resources in the rhizome have already been accessed, then it won’t affect the current shoot.
Hypothesis 1: Newly expanding shoots are dependent on rhizome stores to support the costs of shoot expansion and reproduction.
Hypothesis 2: Early in the season, expanding shoots draw on rhizome stores, whereas late in the season, the current shoot already has acquired the necessary resources for expansion and reproduction.
Prediction 1: The closer that severing occurs to the expanding shoot segment, the greater the reduction in leaf area, because a greater portion of the rhizome system (and a greater portion of stored resources) is rendered inaccessible to the expanding shoot.
Prediction 2: Rhizomes severed in April will experience a greater reduction in leaf area of the current shoot than rhizomes severed in May and June, because it is in April that the expanding shoot is drawing on resources stored in the rhizome. By May and June, the shoot will have already fully expanded and drawn on whatever resources it needed from the rhizome before severing.
1.5 Q2: front shoot performance in 1991
To what extent does rhizome severance in 1990 affect metrics of performance of the front shoot(s) in 1991?
Background: The production of new shoot(s) in the front part of the system for expansion in 1991, and the performance of those shoots, are likely to depend 1) on the amount of resources acquired by the 1990 shoot through photosynthesis, 2) on the amount of resources stored in older rhizome segments, and 3) on how much of 1990 resources are allocated to the maintenance of the older rhizome.
- The amount of resources acquired by the 1990 for shoot production in 1991 will be a function of the size of the 1990 shoot. The bigger the 1990 shoot, the more/bigger the 1991 shoot(s). To the extent that severing affected the size of the 1990 shoot, it will also affect 1991 shoot production and performance.
- The amount of resources stored in the old rhizome and available for the production of new shoot(s) for 1991 will depend on how many rhizome segments remain attached to the front part of the system after severing. The fewer rhizome segments remain attached (i.e. the closer severing is to the 1990) shoot, the fewer resources are available from the old rhizome.
- At the same time, severing prevents resources acquired by the 1990 shoot to be moved into the back of the system for storage and rhizome maintenance, and forces all of those new resources to be concentrated in the front part of the system. The earlier in the 1990 season that severing is done, the sooner 1990 resources are prevented from being used for storage rhizome maintenance.
- The actual balance between the cost of being deprived of old resources vs. the benefit of not having to maintain the old rhizome is difficult to judge a priori.
1.6 Q3: Number and type of front shoots in 1991
How does 1990 sex (or leaf area), severing position and time of severing affect the number and type of new front shoots produced in 1991?
The number of new shoots varies from 0 (front system dies) to 3. We can analyze the data by asking 1) whether or not the front system survives (new shoot number >=1) and then 2) for systems that survive, we can ask what shoot types were made.
Survival of 1991 front system Hypothesis: The likelihood that the front system in 1991 survives (shoot number >=1) depends on the vigor/size on the 1990 shoot, which in turn, is affected by the position and timing of severing 1990. Prediction: Likelihood of death of the front system in 1991 is negatively correlated with 1990 leaf area. Prediction: Systems that were severed near the 1990 shoot (S1) and early in the season (T1) are most likely to die in 1991.
Does rhizome severance in 1990 affect the probability of branching in 1991? Hypothesis: Severance affects the resources available to a plant to invest in an expanding shoot the following year. Prediction: Plants severed closer to the expanding shoot and early in the season the year before will have the greatest deaths and the lowest probability of branching the following year. Plants severed close to the expanding shoot but late in the season will incur less of a cost the following year.
Hypothesis: In severed rhizome segments all carbon is concentrated in the front of the system and can be invested in a subsequent expanding shoot. Prediction: Leaf area will increase in severed rhizome segments one year post-severance when compared to control rhizomes, with the largest increase in the S1 treatment.
Alternate Hypothesis: In severed rhizome segments substantially less carbon is available for shoot production even one year post severance, limiting the production of expanding shoots. Prediction: Leaf area will be reduced in severed rhizomes in 1991 when compared to control rhizomes, with the greatest reduction in the S1 treatment.