Thursday, September 12, 2013

Dealing with Drought: How Do Plants Cope?

Have you noticed how often drought has been in the news lately? You don’t have to be a scientist to know that drought is bad. But, if you’re a plant, how bad is bad? I mean, you’re a plant; it isn't like you can pick up your roots and go looking for the nearest water source. You must have ways to cope, strategies that will let you survive until water arrives. A new paper in Tree Physiology caught my eye today that examines how plants handle drought in our changing climate.

We know that drought and elevated temperatures have all sorts of effects on ecosystems worldwide. The ecosystem level is a bit broad so let's narrow the scope a bit, just looking at plants. The average person tends to think about plants as just that: plants. They are green, they grow. Okay, but we really need to think of them as different species that have their own strengths, weaknesses, and strategies to cope with hard times. Different species of plants react to drought and elevated temperatures differently, some are better able to cope than others. It is known that drought and temperature influence seedling and sapling establishment and survivorship. To anthropomorphize a bit, some infant and toddler plants survive to grow up into adults and others...well...don't. The results of this survivorship will profoundly affect the composition of the community (collectively, which species are there) and how that community changes over time. Additionally, drought and elevated temperatures may affect our efforts to restore habitats. If newly planted saplings die within the first few years of planting then the restoration fails. If we get even more doom-and-gloom with it, extreme drought will cause massive forest dieback, releasing huge amounts of stored carbon and exacerbating the problem.

Let's narrow the scope a bit more. The survivorship of plants is related to their physiology and productivity. In general, growth is the most sensitive to drought. This is followed by photosynthesis and then respiration. The timing and extent of these declines is governed by changes and coping methods for water balance (water supply vs. water use), carbon balance, and strategies to balance the water loss and carbon gain. Most climate models predict that with increased atmospheric carbon dioxide (CO2) and warming the intensity and timing of droughts will go up. Now, carbon dioxide is good for plants because they use it like we use oxygen, to make energy. Give plants more CO2 and you see increased leaf area, productivity, photosynthesis, and carbon storage (in this case we are talking about non-structural carbohydrates (TNC)). Warming under well-watered conditions isn't too bad either, but under drought, warming will decrease photosynthesis and carbon storage while increasing water loss. But increasing CO2, warming, and drought are not solitary factors; you need to look at them in combination. Elevated CO2 will lessen drought stress, but warming worsens it. Based on what we know of the individual factors, the effects of elevated CO2 plus warming plus drought may vary depending on the trade-offs a plant makes. This new study in Tree Physiology looks at how drought alone and in combination with CO2 and warming affects carbon dynamics (growth, photosynthesis, respiration, TNC).

To do this they raised Blue Gum (Eucalyptus globulus) seedlings under ambient CO2 and temperature conditions. After one month they transplanted and separated them into their various treatment groups. These treatment groups were put in whole-tree growth chambers where the conditions could be very carefully controlled. There were four CO2 and temperature combinations: two CO2 levels (400 and 640 μl l−1) and two temperatures (28/17 and 32/21°C day/night, a.k.a.ambient and ambient + 3°C). Within each of these treatments, seedlings were designated to a drought regime: well-watered/control, sustained drought, rewatered drought (watering after sustained drought), and progressive drought (on rewatered and sustained treatments). They were able to maintain the "sustained drought" condition by adding just enough water to maintain leaf stomatal conduction (a measure of the rate of CO2 entering and water vapor exiting the stomata/pores of the leaf).

The researchers took gas exchange measurements, specifically looking at leaf net photosynthesis and leaf respiration rates. They also randomly selected individuals at various times during the experiment’s duration to be harvested for dry mass measurements (leaf, stems and roots). TNC concentration was calculated as the sum of starch and soluble sugar concentrations.

The results showed effects of progressive drought to be similar in rewatered and sustained drought plants. These plants were limited in growth, photosynthesis and respiration. However, there was not a decrease in TNC, although the drought plants did convert quite a bit of their starch into soluble sugar. This means that the plants consumed less TNC, and the soluble sugars likely serve some other function apart from a respiratory carbon source during drought (perhaps osmotic adjustment and/or hydraulic transport). They also found that elevated CO2 ameliorated the stress of their plants in the moderate drought treatments. These plants showed increased photosynthesis and TNC reserves. This suggests that the plants have the capacity to withstand drought, having sugar available for osmotic adjustment (think: better water movement, which is good for growth and photosynthesis). Conversely, elevated temperature exacerbated moderate drought stress by reducing photosynthesis, increasing leaf respiration and decreasing TNC reserves, and reducing the plants’ capacity to withstand drought. The combined effect of elevated CO2 and increased temperature is a little more complex. This study found only moderate benefits to plants, with similar/lower carbon uptake and greater carbon loss during the moderate drought treatment. TNC was found to be higher which suggests that there may be some carbon storage going on. However, these benefits went away when the plants were subjected to extreme drought. I suppose that is to be expected. They don’t call it extreme for nothin’ right?

Perhaps we can find a silver lining from this story. Plants are resilient organisms that have the capacity to withstand more than we had thought even when they are hit with multiple stressors. They can deal with moderate droughts, but extreme is still extreme and nothing survives everything. I didn’t say it would be a thick silver lining, but we can potentially use this knowledge to help us mitigate some of the effects of climate change and work to better restore lost habitats.

ResearchBlogging.orgDuan, Honglang, Jeffrey S. Amthor, Remko A. Duursma, Anthony P. O’Grady, Brendan Choat, & David T. Tissue (2013). Carbon dynamics of eucalypt seedlings exposed to progressive drought in elevated [CO2] and elevated temperature Tree Physiology, 33 (8), 779-792 DOI: 10.1093/treephys/tpt061

(image via TrendsUpdates)
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