Question

2. Imagine that you are conducting an experiment that compares the effects of drought on photosynthesis in two species in the family Ericaceae: Vaccinium parvifolium (red huckleberry) and Vaccinium oxycoccos (small cranberry), both native to Oregon.

1. What methods (name 2) could you use to measure water stress or availability in this experiment?
   
2. Why might you need to know if these plants have either C3 or C4 photosynthesis? Explain.
   
3. Design a greenhouse experiment that would allow you to address competition between these two species and the effects of drought. Be specific about your hypothesis, design and what variables you would measure.

Answer 1

a) We could measure water stress by these two paths:

1.- Measure atmosferic O2 concentration changes, because effective photosynthesis produces it and photorespiration depletes it

2.- Measuring atmosferic CO2 concentration changes, because effective photosynthesis depletes it and photorespiration produces it

b) Because C4 is a specialized photosynthetic metabolism that works to prevent harmful effects of drought to the plant. It fixes CO2 into a different molecule using a different enzyme (PEP carboxylase) that has more affinity for CO2 than RUBISCO, then this molecules can liberate the fixed CO2 again but now directly into the photosynthetic tissue that is undergoing calvin benson cycle with RUBISCO, in this way they manage to concentrate high CO2 concentrations for RUBISCO, even when the stomata close in order to prevent water loss (unabling the entrance of CO2 from the environment). In conclusion, C4 plants would greatly resist drought, so we would need to change the parameters to apply to reach any variation in these species.

c) Okay, we have one fact, V. oxycoccos is an species that grows in moist environments, they are actually used as indicators for certain types of moist soils. Thus, our hypothesis is: V. parvifolium can resist more intense drought conditions than V. oxycoccos.

For this we are going to have various treatments that will variate the water access for the plants from just a slightly lowered watering (certain ml/day) to treatments with just small quantities of watering (certain ml/day), and of course a control group that will provide the plants with a standar sufficient watering. Many plant individuals from both species will live in each treatment, it will last a month and at the end of the month we will register the number of plants that survived each treatment. If our hypothesis is correct we will be able to find a higher survival rate in V. parvifolium than V. oxycoccos along the treatments.