Plant Evolution & Ecology (3): Seed Germination
Hui Yan March 14, 2011
Nobel, P.S. 1997. Root distribution and seasonal production in the northwestern Sonaran Desert for a C3 subshrub, a C4 bunchgrass, and a CAM leaf succulent. American Journal of Botany 84(8): 949-955.
The desert is an extreme environment in which water for plants’ growth is essential but extremely limited. To adapt to or tolerate this condition, plants have developed various growth forms. For example, cacti have special stems that have strong capacity to store water through long-term drought. Another plant, Atriplex. Confertifolia, has its roots deeper and deeper from spring through the summer to avoid being injured by high temperature, which changes with seasons and soil depth.
CAM plant Agave.deserti, C3 plant Encelia.farinosa, and C4 plant Pleuraphis.rigida are three perennial plants in the northwestern Sonoran Desert. They have different photosynthetic pathways, and are almost equally abundant through the vegetation ground cover. Interestingly, the P. rigida is like a nurse bed for A. deserti, i.e. most young A. deserti grow in clumps of P. rigida. The three plants also have different root systems. The E. farinosa has a tap root, i.e. many branch roots growing from a primary, central and strong root, whereas the other two have adventitious roots growing from their stems at the bases of the plants.
With the observation of the natural growth forms and distributions, the author tried to establish that whether the interaction among these plants affects their roots’ distribution, and whether seasonal changes in soil temperature affect the growth and distribution of new roots.
Materials and Methods
The author excavated two groups of roots for each of the three species through 4-cm-thick soil layers. One group were those growing closely together, indicating considerable root interaction, the other group was isolated plants growing at least 60 cm apart as control groups to compare data and analyses.
In order to investigate the situation of young roots, the excavation took place at two times of the year 1995, the end of the winter wet period and after summer rainfall and irrigation. The author measured the ability of roots to take up and transport water in the lab.
For each of the three species close to each other, root areas and depth were obviously different from those of their own isolated ones. Firstly, the total root areas of P. rigida became greater; secondly, the root depth of A. deserti was shallower, while the other two were deeper.
In terms of young roots, they 1) only took a small fraction among the overall root systems; 2) were deeper for E.forinosa than for the other two; 3) were deeper in summer time than in early spring; 4) mostly occured in the early spring for E.forinosa, mostly occurred in the summer time for P.rigida, and equally occurred in the two periods for A.deserti.
Firstly, the production of new roots indicates that the three species have their respective temperature tolerance ranges for growth. In the case of E.forinosa, the strongest growing ability of new roots occurs at the optimal air temperature of 25oC in wintertime; for P.rigida, it occurs at 37oC in summertime; for A.deserti, it occurs at 30oC in both winter and summer. (Fig.1) In addition, soil temperatue decreases as depth increases, so new roots production for the three species occurs in different layers. E.forinosa can tolerate the lowest temperature among the three, then its new roots were found at the lowest layers.
Secondly, plants growing close to each other causes competition for water. Obviously, light rainfalls are not enough to satisfy water requirements. For these competitors, one solution is to get permanent water from the soil. Therefore, the three species have developed different root growth forms in different layers. A.deserti has succulent leaves to store much water, which means higher leaf water potential*, and thus it only takes up water from upper layer with relatively higher soil water potential than deeper layers, whereas E.forinosa and P.rigida with lower leaf water potentials take up water from lower layers (Fig.2). Another solution is root proliferation, and so P.rigida growing close to other species has greater root area than isolated ones, and E.forinosa has a tap root with rich branch roots.
Thirdly, the roots’ competition for water appears separations not only in space (depth) but also in time. A.deserti only uses light rainfalls for a short period through a long-term drought, which is why it has developed succulent leaves for water storage.
*water potential is potential energy per unit volume relative to pure water. Naturally, water flows from high potential to low potential.
Application to the paper
- Precipitation: scant rainfalls cause long-term drought, and thus water becomes a major limited factor for plants’ growth.
- Temperature: changes with seasons
- Growing seasons: short-term growing seasons occur in early spring wet period and after summer light rainfalls
- Sandy soil: upper layers are more coarse, more spaced and drier, and lower layers finer, less spaced and wetter
- Soil temperature: seasonal changes with air temperature, and decreases as depth increases
- The three species growing close to each other inevitably have competition for limited water to survive.
Ø Allocation strategy
- In the special habitat, plants allocate energies mainly for competition or survival. First, they have developed unique root systems and forms to compete for water, and modified leaves to store water. Second, the three species are all short, i.e. most biomass (i.e. mass) close to the ground. In this case, less water is consumed and water transportation becomes easy. A.deserti is a rossete herb with a circular arrangement of leaves sitting close to the ground; E.forinosa is a subshrub, a short woody plant with ground-hugging stems; P.rigida is a bunchgrass.
vGenetic features of plants
Ø Life forms
§ Lifespan: they are all perennials, i.e. their life cycles are all more than two years.
§ Morphological modifications
- Upon drought stress, these plants appear unique in shapes. The roots of A.deserti grow upward into shallow soil layers, and the leaves are succulent with strong capacity to store water. The roots of E.forinosa and P.rigida are more spreading, and grow downward into deeper layers for permanent water.
§ Phenological adaptation (about timing)
- New roots occur in different seasons with respective optimal temperature, and plants catch the timing of short-term rainfalls to take up water.
§ Physiological adaptation
- E.forinosa is a C3 plant, and has the best growth timing in the early spring when the temperature is relatively low. This is consistent with that C3 plant photosynthetic pathway is limited by high temperatures, under which a critical substance, RuBP, prefers to bind to O2 over CO2, the key material for photosynthesis. The process is called photorespiration, which inhibits photosynthesis to produce sugar, the final food for plant growth. (Fig. 3)
- P.rigida is a C4 plant. C4 photosynthetic pathway has overcome the limitation of C3 pathway, i.e. photosynthesis can and prefer to occur under high temperature because of unique cellular structure, in which RuBp is isolated to bundle sheath cells so that does not have chance to bind to O2. However, the pathway is limited by cold temperature. (Fig.4)
- A.deserti is one of CAM plants, which have developed another pathway to overcome the limitation of C3 pathway. During the photosynthesis of CAM plants, CO2 uptake and storage occur during nights. During days when photosynthetic cycles happen, stomata for gas exchange get close, so RuBP also lose chance to bind to O2. A.deserti can equally have same photosynthetic rates in both wintertime and summertime to supply energy for growth. (Fig.5)
Ø The three species have their respective habitat niche, i.e. taking up resources, including space, water, and light. Those isolated plants take resources from the habitat without interactions with other organisms, which is a relatively ideal state and called fundamental niche; those plants with competition have realized niche, i.e. their capacities to occupy resources are limited by other species.
Ø Phenological niches arise from their temporal growth, i.e. the development and distribution of roots are greatly influenced by seasonal changes.
Ø Life form niches: subshrub E.forinosa, bunchgrass P.rigida and rossete herb A.deserti have a common character in shapes, i.e. their meristems (tissues in shoots making growth happen) are all close to the ground. This life form (generally short) benefits to water conductivity and absorption, which is one of adaptations to the water-limited environment.
v Life history
- According to Grime, plant life history can be classified into three groups: R-ruderals have abundant resources, very short lifespan, fast growth rate, high reproduction and mortality; S-stress tolerators can tolerate extreme environment with rare resources. They have very long lifespan, slow growth rate, late and few reproduction, and energy allocation focusing on survival; C-competitors are general plants with long lifespan, fast growth rate, late and few reproduction, competition for resources, and energy allocation focusing on growth. Among the three plant species, A.deserti should belong to S group. Although being one of competitors for water, it actually gets water from different resource, i.e. rainfalls, compared with the other two that obtain water from permanent resource. A.deserti in fact avoids the competition, and is a real tolerator to an extreme environment. E.forinosa and P.rigida should belong to S-C mixed group. They are both tolerators to the desert habitat and competitors for limited water resource.
v Study approach
Ø The author first observed the situation of the three plant species in such an extreme habitat, then put forward questions about competition for limited water resource and production of new roots with different photosynthetic pathways, i.e. came into questions (or hypotheses) from a general observation, which is an inductive reasoning; to solve the question, the author carried on experiments, collected and analyzed data, then made conclusions, which is a deductive reasoning, i.e. a general conclusion derived from testing hypotheses.
ØThe experiments are manipulative (controlled). Data and measurements from treated subjects, those plants growing close to other species, were compared with those from controlled subjects, those isolated plants.
Three plant species with different photosynthetic pathways in the northwestern Sonoran Desert appear to compete for water, which is a limited but essential resource. The author wondered if ability to co-exist is related to their root distributions in time (season) or space. He excavated roots from different soil layers and different seasons measuring root area, root depth, leaf area, water uptake ability, etc. to determine distributional and developmental forms of these roots. He found that the roots of A.deserti (a CAM plant) distribute in upper layer of soil and grow upward, while the roots of the other two species grow more deeply and downward, and that the production of new roots for E.forinosa (a C3 plant) is greater during the winter than during the summer, for P.rigida (a C4 plant) is greater during the summer than during the winter, and for A deserti is the same rate during the two seasons. He concluded that the root distributions of the three species reduce competition by using water in different parts of the soil, and that growth rates of new roots are consistent with their respect photosynthetic pathways: C3 pathway limited by high temperature, C4 pathway limited by cold temperature, and CAM pathway with no difference between high and low temperatures.
The study investigated how and to what degree competition influence the roots' distribution of three co-exist plant species in desert habitat, and the action played by temperature on production of new roots. Although the author did not mention their photosynthetic activities, the results completely embodied their different photosynthetic pathways. Photosynthetic activity supplies plants fundamental energy and food for growth. C3 pathway takes place in a relatively low temperature range: E.forinosa has its photosynthetic optima at 25oC, under which most production of new roots occurs. On the contrary, C4 pathway has a relatively high temperature range: P.rigida has its photosynthetic optima at 37 oC. CAM pathway has an intermediate range: A.deserti has its photosynthetic optima at 30 oC.
The three photosynthetic pathways are results that plants keep modifying their physiological behaviour to adapt various environmental stresses along the evolutionary time. It should be believed that such modification for adaptation is still going on. For example, global warming is influencing various habitats. The increasing temperature seems to be able to cause more and more C4 plants, while to push C3 plants to a dangerous edge. However, under warming climate, CO2 concentration is becoming higher, which obviously benefits C3 photosynthetic pathway. This seems to be a dilemma! Plants will have new evolutionary adaptation to changing climate!
Lopez, F.B. and Nobel, P.S. 1991. Root Hydraulic Conductivity of Two Cactus Species
in Relation to Root Age, Temperature, and Soil Water Status. Journal of Experimen-
tal Botany 42: 143-149.