Plant of the week #4

Cassytha pubescens
(Downy Snotty-Gobble) 

A brief description: Also known as dodder-laurel, devil's twine, jungle string or love vine, this interesting plant is actually a parasite. All species in the genus are hemiparasites, meaning they still photosynthesise and tap into the host plant's xylem (taking water and the associated dissolved nutrients) but not sugars. It is rootless as an adult (although the seedlings have roots) and is completely dependent on its host for survival. Flowering and fruiting in summer, the picture above shows some flowers in bud and opening, as well as some immature fruit. The host in this case is an Acacia pycnantha, although it is a generalist parasite and will grow on pretty much anything (even barbed-wire fences!).


Taxonomy: The taxonomy of this genus is not well researched. There are approximately 23 species, 16 of which are endemic to Australia. The genus is placed within Lauraceae, often within it's own subfamily as it's so weird. *It is unrelated to the Dodder species in the Cuscuta genus. They're a completely different family (Convolvulaceae) and are an excellent example of convergent evolution as they look very similar but are completely unrelated, and physiologically very different*

 Distribution: Eastern states; Coastal SA, Vic, Tas, NSW and QLD. Also found in New Zealand and Japan (introduced).

Conservation status: Locally common, not considered at risk in the wild. Distribution is typically patchy


Interesting things about downy snotty-gobble: There are many, where should I start?! (this is the plant I'm studying for my PhD, so I'm perhaps a bit biased :P). It has been seen to damage some weedy species in South Australia, even killing Gorse (Ulex europaeus) and Scotch Broom (Cytisus scoparius) but not the native species. This is thought to be due to the native species co-evolving with Snotty-Gobble.
The other thing I find rather interesting, is the seedling stages of this parasite. It germinates and grows for the first 6-8 weeks with a short root (usually no more than a couple of cm long) which dies back after it's found a host and wrapped itself securely around it. This means if there's a disturbance such as a fire, the adults are all toasted and it's up to the seeds left in the soil to re-establish the population. Interesting, yes?

How does photosynthesis work? (part 3): Photorespiration, C4 and CAM

So the final installment of this thread of posts is about one of the big problems of C3 photosynthesis, and also how alternative methods of photosynthesis help plants overcome this problem.

As previously mentioned, the enzyme responsible for the whole shebang is Rubisco - it captures CO2 in the first place, and allows the chloroplasts to do their thing. The problem with this system is that Rubisco also has an affinity to Oxygen (particularly at higher temperatures or low CO2 levels), meaning when the temperature of a leaf increases less photosynthesis is achieved. The process of Rubisco fixing O2, rather than CO2, is called photorespiration and can slow plant growth by reducing the rate of photosynthesis by a significant amount.

Photorespiration is a particular problem in the tropics, as the temperature is usually high (and this also causes problems with waterloss). This has lead to the evolution of a different form of photosynthesis, that allows the plant to separate the absorption of CO2 either temporally or spatially, hence reducing photorespiration by limiting Rubisco's access to O2.

The first alternative to C3 photosynthesis is called C4. This is because it involves the CO2 being fixed into a 4-Carbon sugar at the first stage. For the initial absorption of CO2, C4 plants use a different enzyme called PEP Carboxylase. This then delivers the CO2 to the photosynthetic part of the leaf by the 4-C sugar breaking down and releasing the CO2 directly to the Rubisco.
The whole process works well because C4 plants have a slightly different leaf anatomy to C3 - they have specialised cells that surround the veins in the leaves, called bundle-sheath cells (this is known as Kranz anatomy). These are packed with chloroplasts, and photosynthesis takes place here rather than in the mesophyll. So by spatially separating the initial capture of CO2 and the Rubisco, photorespiration is significantly reduced. However this form of photosynthesis is also more energetically costly, requiring more ATP than C3. This means it is less efficient that C3, and only occurs in areas where the benefit of reducing photoresipration and waterloss outweigh the larger energy cost, such as the tropics and some arid zones.

The second alternative to C3 photosynthesis is CAM photosynthesis. CAM photosynthesis (Crassulacean Acid Metabolism photosynthesis) was named after the family of plants in which it was first discovered, Crassulaceae (a family of succulents). It tends to occur in arid plants, like cacti, and works by separating the CO2 capture and photosynthesis temporally. This again reduces photorspiration, however perhaps more substantially reduces waterloss. It works in two stages, at night when it is cooler and waterloss is less severe, the plant opens its stomata. It stores the captured CO2 as a 4-Carbon acid, Malate, and then breaks this down back to CO2 during the day to feed back to the Rubisco so photosynthesis can occur. This allows the plant to keep its stomata closed during the day, when it is hot and dry, helping it avoid waterloss and photorespiration.
I also have a healthy admiration for CAM plants, as they can also use this system to do what other plants cannot. During times of extreme stress, such as a severe drought, they can 'CAM-idle'. This means they can keep their stomata closed during the day and the night, and use the CO2 released through respiration at night for photosynthesis during the day and the O2 released from the photosynthesis during the day for respiration at night. Which is pretty cool. But they can't do this indefinitely, and will eventually need to open their stomata to start the photosynthetic process up again. It does, however, allow them to survive long periods of water stress, and also to recover quickly when water again becomes available.

So that, in 3 long posts, is how photosynthesis works. Perhaps next time I'll discuss talking trees, or some other thing that isn't physiology related ;)

How does photosynthesis work? (part 2): Light and dark reactions and the Calvin cycle

The basic reaction of water + CO2  (catalyze with sunlight) ------> H2O + O2 is explained in more depth below, mostly because I get sick of the 'add CO2 and water and sunlight and.... magic happens!' explanation ;)
Photosynthesis is a rather complicated process, and I'll try to keep this simple.

Within a leaf there are cells that specialise and become functionally different from each other. As previously discussed, in leaves one of the major cell types are known as mesophyll, and this again differentiates into 2 types with different functions.
However it's not just plant tissues that specialise, they do it at the cellular level too, and even the sub-cellular level! (I told you it gets complicated).
Within the cells that make up mesophyll tissue, there are organelles that are called chloroplasts (these are what make leaves appear green). This is where the photosynthetic process actually happens.

So the mechanisms of photosynthesis are best discussed if you split them along the conventional lines and talk about them in two parts. The first part is known as the 'Light reactions' as it takes place in direct sunlight, and the second part is the 'Dark reactions' which do not need direct sunlight (although they usually take place during the day).

Light reactions occur in the membrane of the chloroplasts (which remember are within the plant cells). When light is available, it causes a chain reaction that converts chemicals in the chloroplast to ATP (adenosine triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate), two molecules that are basically the universal fuels of cells. The ATP acts to move energy to where it's needed, and the NADPH allows phosphorous and more importantly Hydrogen ions to move to where they're needed for photosynthetic reactions.
So once these molecules are generated in light reactions the ATP and NADPH can be used to make sugars in the Dark reactions.

The Dark reactions are where the carbon dioxide is converted into sugars by the plant, in a process known as the Calvin cycle (because it was discovered by Melvin Calvin, James Bassham, and Andrew Benson). For a diagram of this cycle, use your favourite search engine - wikipedia is more than a little complex for this one. Or for a written explanation, keep reading below ;)
So the ATP and NADPH generated in the Light reactions are essential to the turning of this cycle, which is a rather complicated piece of molecular-energy transfer. It occurs in 3 stages, known as Carbon fixation, Reduction and Regeneration.

In Carbon fixation, CO2 is incorporated into a 5-carbon sugar, called ribulose biphosphate, via the enzyme Ribulose-1,5-bisphosphate carboxylase oxygenase (aka Rubisco). Rubsico is an important enzyme, not only as it allows CO2 to be captured and converted into a sugar, but also as it has an affinity to Oxygen, which can cause problems with something called photorespiration (I'll discuss why photorespiration is a problem and how plants deal with it in the next post). So at this point if the Rubisco has fixed CO2 as it should (and not O2), the sugar has 6 Carbon units in it. However this is only an intermediate product and it is then split into two 3-carbon sugars.

The next stage, Reduction, uses the ATP and NADPH from the light reactions to alter the structure of the 3-carbon sugars (this is where the chemistry will take over this post, so I'm keeping it simple here!) and turns them into the precursor to glucose. This can then be used to generate the 6-carbon sugars for the plant. The remaining 3-C sugars continue within the cycle, and enters the next stage, Regeneration. For this, more ATP is used to convert the 3-C sugar from this pool back into the original 5-C sugar that is used to capture the CO2, and the cycle can begin again.

So in summary, while a complicated process, each turn of the Calvin cycle will actually only capture just a single additional Carbon, so for a 6-C glucose molecule to be generated it will require 6 turns of the cycle. This involves 12 photons hitting the chloroplasts and generating 18 ATP and 12 NADPH molecules in the light reactions, which can then be used for 6 turns of the Calvin cycle in the Dark reactions to fix 6 CO2 molecules to end up with a single 6-Carbon sugar. In a nutshell, the light reactions provide the energy required for the dark reactions to fix carbon, and this is how basic C3 photosynthesis works (so called C3 as it's based on 3-Carbon sugars).

However some plants run into trouble with maintaining water balance, or with excessive light or temperatures. As such they've evolved to cope with this by photosynthesising in a different way. There are 2 main alternatives to C3 photosynthesis: C4 or CAM photosythesis, and I'll discuss these (along with photorespiration) in the next post...