Acclimation of stomatal responses to light: environmental cues, strategies and mechanisms

abax_edited-8People

  • Pedro J. Aphalo, PhD, Docent (Senior Lecturer, Principal Investigator, project leader)
  • Krõõt Aasamaa, PhD (Postdoctoral Resaercher, 2011-2014)
  • Luis O. Morales, PhD (Postdoctoral Resaercher, 2014)
  • Jorge Casal, PhD (Senior Researcher, Professor).
  • Students
  • Fang Wang, MSc, (PhD student)
  • Collaborators
  • Ariel Novoplansky, PhD (Professor)
  • Hannes Kollist, PhD (Group Leader)
  • Mikael Brosché, PhD (Senior Lecturer, Principal Investigator)
  • Tarja Lehto, PhD, Docent (Senior Lecturer, Acting Professor)
  • Otmar Urban, PhD (Scientist)
  • Titta Kotilainen, PhD (Research manager)
  • T. Matthew Robson, PhD (Academy Resaerch Fellow)

Aim and objectives

The aim of this project is to describe the environmental variables involved and the mechanisms behind the acclimation of stomatal responses to light. We will characterize the reversible acclimation to growth conditions, which happens on a given leaf on a relatively short time of between one day and some weeks.

The objective questions are:

  • Which variable(s) is(are) causing the acclimation?
  • How is the acclimation reflected in the sensitivity of stomata to light of different colours?
  • The responses mediated by which photoreceptors are up of down-regulated during acclimation?
  • Are there species- and ecotype-specific differences in acclimation?

Background

Guard cells are probably the most sophisticated sensory systems in plants. They regulate the diffusion of CO2 into leaves and of water vapour to the atmosphere. They are crucial for the survival and success of land plants. They establish a delicate balance between the benefit of carbon assimilation and the cost of transpiration. This balance has to be fine tuned to the prevailing internal and external environment of the plant. If stomata are too widely open, water use for transpiration will exceed the supply and the plant will in the worse case die. If stomata are not as widely open as water supply would allow, then carbon assimilation will be less than the maximum possible and the plant will grow more slowly, and possibly be outcompeted by its neighbours.

Stomata are sensitive to light, intercellular CO2 concentration, vapour pressure deficit at the leaf surface, plant hormones, and leaf water potential. Hormones can relay information about roots and hence soil to stomata. Stomata are themselves sensitive to light (ultraviolet, blue, green, red and far-red) through different photoreceptors. The solutes accumulated in guard cells are different during the morning and afternoon. The responses to light of different colours (dependent on different photoreceptors) is at least in some conditions independently modulated in response to other environmental variables.

What it is known is, for example, that transferring plants from controlled environment conditions to a greenhouse may have a rather fast and marked effect on the sensitivity of stomata to light. A recently published paper reports increased sensitivity to light in a greenhouse, while my personal observations of 20 years ago on a different species support the opposite effect. Also responses to CO2 acclimate.

Conditions in different controlled environment rooms and chambers can be quite different, and differences among greenhouses at different locations or in different seasons also exist. The point is that it is not known which ones of the many possible environmental variables are driving acclimation of stomatal responses.

Implementation

We are doing experiments to test acclimation in relation to several environmental variables that are likely to have differed between chambers, greenhouses, and outdoors testing them one by one. The variables being tested in different experiments are: photosynthetically active irradiance during acclimation period, red to far-red photon ratio during acclimation period, ultraviolet radiation (A and B) during acclimation period, air temperature and mild water stress during acclimation period. The response variable tested will be stomatal conductance responsiveness (quantitative assessment by gas-exchange) to red, green and blue light. Green light antagonizes the opening effect of blue light, so it will be tested under a background of blue light. To avoid the confounding effects of photosynthesis the effect of blue light will in most cases be studied under a background of saturating red light.

Experiments will be done on birch (Betula pendula), beeches (Fagus sylvatica and Nothofagus spp.), ivy (Hedera helix), Petunia axillaris or another solanacea, and Arabidopsis. Ivy has been chosen because is very suitable for gas exchange measurements, and in our experience at least some clones have stomata very sensitive to blue light. In addition clones with variegated leaves are readily available. Petunia, as well as some other solanacea have stomata whose readiness to open has a significant component dependent on circadian rhythm. Arabidopsis has the problem of having small leaves, which makes gas exchange measurements more difficult, but on the other hand the availability of well characterized photoreceptor mutants makes this species a very useful tool, especially for a detailed description of the mechanism of acclimation.

Partners and links with other research groups

Our main partner in this research is Dr. Jorge Casal at the University of Buenos Aires. He is a leading plant photobiologist with whom I have collaborated for many years. The idea for this project started from discussions with him, and he has already done some experiments on the subject.

Status and funding

Ms. Fang Wang has obtained in 2009 a schollarship from CSC (China) to do her PhD thesis within this project. Dr. Matthew Robson had from January 2010 to December 2012 a position as postdoctoral researcher from our university for working on this project. We have been awarded a four-years research grant by the Academy of Finland starting on 1 September 2011. If you are interested in doing a MSc thesis or having a practice period just contact us.

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