Online Analyzing Data 5.1: How Does Acclimatization Affect Plant Energy Balance?


The supply of water to plants is a common constraint on photosynthesis in terrestrial plants in many different biomes. Potential ways that plants can acclimatize and/or adapt to low water supply are discussed in the text, including morphological and physiological changes which limit the exposure of the plants to water stress.

To investigate the influence of low soil water availability on photosynthesis, Klaus Winter and Joseph Holtum (2014) subjected a potted Clusia pratensis (now Clusia minor, common names Cupey de Monte or Tar Gum Tree) plants to an artificial drought by withholding water. Figure 1 shows the results of this experiment.

An area graph shows net C O 2 exchange per day over a period of 16 days from day 1 until day 16 during both daytime and night time. The horizontal axis shows day ranging from 1 to 16 in equal intervals of 1; the vertical axis shows net C O 2 exchange measured in micromole per meter square per second ranging from negative 2 to 10 in equal intervals of 2. Approximate data from the graph in the format, day, daytime net C O 2 exchange, night time net C O 2 exchange, are as follows: 1: 9.8, negative 0.2; 2: 10, negative 0.2; 3: 9.4, negative 0.2; 4: 8.4, 0.2; 5: 8, 1; 6: 8.6, 1.2; 7: 8.6, 1.6; 8: 8.6, 1.8; 9: 8.6, 2; 10: 8.8, 2; re-initiation of water; 11: 9, 2; 12: 9.2, 1; 13: 9.6, 0.2; 14: 10, negative 0.2; 15: 10, negative 0.2; 16: 10, negative 0.1.
Figure 1 Net CO2 Exchange in a Clusia Pratensis Plant over the Course of an Experimental Drought Water was withheld between days 1 and 10 (the arrow shows the re-initiation of watering). The off-white areas are the daytime period, and the orange areas are the nighttime dark period. (After K. Winter and J. A. M. Holtum. 2014. J Exp Bot 65: 3425–3441.)

Question 1. What would explain the observed changes in net CO2 exchange that occurred between days 3 and 4?

Question 2. How would you interpret the change in the daily patterns of CO2 exchange during the period between days 4 and 10? Hint: Note in particular the changes in net CO2 exchange during the night period, and see Concept 5.3 in your textbook.

Question 3. What do you think is happening to net CO2 exchange after the plant is again watered on day 10?

Literature Cited

Winter, K. and J. A. M. Holtum. 2014. Facultative crassulacean acid metabolism (CAM) plants: Powerful tools for unravelling the functional elements of CAM photosynthesis. Journal of Experimental Botany 65: 3425–3441.