Problems with photosynthesis (part 1): Maintaining water balance

As promised, a (slightly late) post about photosynthesis, and some of the problems that plants face when they try to do it. Because these can get complicated, I'll split it into 3 posts. Firstly we'll deal with maintaining water balance, and then photodamage caused by excess light (including the xanthophyll cycle - a really cool bit of evolution involving photosynthetic pigments). The last post will be an introduction to the actual process of photosynthesis at a molecular level, and the development of different kinds of photosynthesis to suit different environments (known as C3, C4 and CAM photosynthesis), and subsequently why higher CO2 levels are not necessarily a useful thing for most plants.

So, as every year 7 student can tell you, plants take up carbon dioxide from the atmosphere, water from the soil and absorb sunlight to magically produce sugars. The process is really a marvel of evolution, and involves complex chain-reactions at a molecular level. However for this to even begin, the basic ingredients (CO2, sunlight and water) need to be collected. Most plants use their leaves to obtain the first two, and roots to gather water (by the mechanisms discussed in the previous post!), although of course there are exceptions to this rule (some plants photosynthesise in only their stems, others are parasitic and get water from their hosts rather than root systems, but you get what I mean).
However one of the biggest problems plants have while trying to photosynthesise is maintaining a balance of all 3 requirements, and particularly maintaining enough water to survive (how well they do this is known as their Water Use Efficiency - WUE).

The means by which plants take-up CO2 is through the pores in their leaves known as stomata (singular: stoma). Plants have direct control over these pores, and can open and close them when they need. Opening stomata to uptake CO2 allows the absorption of carbon necessary for photosynthesis to occur, however it also allows water (and oxygen) to escape the leaf, and the plants have little control over this. Consequently in times of water-stress, plants must close their stomata and cease photosynthesis or they will lose valuable water. However allowing water to evaporate from the leaf in hot weather can also assist in cooling the leaf down, as excess light can heat the leaf and damage the photosynthetic organs within it (the other mechanisms to deal with this problem will be discussed in the next post).

Because this is a big problem, many adaptations to low-water environments have evolved (although usually perennial plants in low-water systems are deep-rooted as well). Three of the most common dry-climate adaptations include:

1. Reduced leaf area: This allows for fewer stomata to lose water per unit leaf area, and consequently a greater WUE, although as it does decrease the rate of absorption of CO2 these plants are usually slower-growing than plants without this adaptation. Often leaves are fleshy and succulent as well, and store fluids even in dry times. In some plants, such as cacti, the leaves are so reduced they became spines, and photosynthesis occurs in the stem, while the spines protect the valuable photosynthetic stem from herbivores.

2. Light colouring/hairs on leaves: Many species have a woolly coating on their leaves, intended to reflect excess light. If a leaf remains cooler, less water will be driven from it through evaporation, and the chances of damaging photosynthetic structures is lower. Saltbush and many other arid species are examples of plants that use this protective mechanism.

3. Sunken/hairy stomata: This is really quite cool - many plants have pits and/or hairs in the leaf surface that create a cooler, more moist microclimate around each stoma, reducing water loss but still allowing CO2 to enter the leaf. Banksia species often show this sort of adaptation, particularly those found in the drier regions of Western Australia.

Other plants have more simple mechanisms to deal with waterloss, such as members of the Eucalyptus genus that have leaves that hang downwards, rather than holding them horizontally. This ensures that during the morning and afternoon when the sunlight is cooler the leaves are exposed to it, but when it is overhead and hot at midday the thin edge of the leaf is the only part exposed. This simple anatomical re-arrangement dramatically reduces water loss and allows the trees to grow easily in hot, dry environments (now go find a gumtree and see what I mean!)

So to sum up, plants (like all living things) need to maintain their internal water levels to survive, but also have to balance collecting the ingredients required for photosynthesise. They have evolved many different ways to achieve this, while maintaining a capacity to collect sunlight for photosynthesis. Stay tuned for the next post on the nifty mechanisms used to deal with excess (hence potentially damaging) sunlight.

A word on water potential

So it's time for the first of the physiology posts.

Water is essential for all life on earth. Without a plentiful supply of it, we certainly wouldn't be here. Plants originated in aquatic environments, and one of the many challenges they had to overcome when they made their move onto dry land was how to obtain water. They cannot actively drink it like we can, so a way to passively move water from their roots to the stems evolved.

In order to regulate the water within their tissues, plants use pressure gradients. Because water will always flow from an area of high pressure to low pressure, by creating a pressure gradient in the tissues of the leaves water will be drawn into them from the stem, and into the stem from the roots. In this way, the whole plant becomes a pressure gradient - the roots always have the highest water potential, followed by the stems and finally the leaves. By opening and closing their stomata (pores on the leaf surface), water can leave the plant, and the gradient can continually function (otherwise the water would build up in the leaves, the pressure would increase and the gradient would eventually stop).
During drought of course the plant conserves it's water by closing the stomata, and usually the water potential of the whole plant decreases. If it continues without water for too long, leaves wilt and the plant eventually dies.

So, the underlying question about all this is how does the plant create different pressures in its tissues? There are two basic ways to achieve this. Firstly, you can change the volume of the plant cell, without altering it's size (think of a bottle of fizzy drink - if you shake it and the gas leaves the drink but you've not opened the bottletop, the bottle becomes highly pressurised), or you can pack the interior of the cell with solutes such as salt, and water will be drawn into it. Typically plants use solute loading to decrease and increase their water potential, taking up salts such as potassium from the soil and ground water.

In a nutshell, plants can use water thanks to basic physics. They can regulate their water use through a variety of mechanisms, all of which are vitally important for the plants photosynthesis and survival. An important component of why plants water regulate the way they do is a need to balance water loss with the uptake of the carbon dioxide necessary for photosynthesis (which occurs when stomata are open). So this will be the subject of the next post :)