SECTION 3: Water Use Efficiency (WUE) & Growth Trade-offs

Q: What is Water-Use Efficiency (WUE)?

A: The ratio of carbon gained (growth) to water lost; measures how efficiently a plant uses water during photosynthesis.

Q: What is Relative Growth Rate (RGR)?

A: The increase in plant biomass over time; a measure of growth speed.

Q: Describe the trade-off between WUE and RGR.

A: High RGR = fast growth but high water loss (low WUE); High WUE = slow growth but better drought tolerance.

Q: What analogy is used to describe WUE vs RGR trade-offs?

A: The Prius vs. Porsche analogy — Prius = high efficiency (WUE), Porsche = high speed (RGR).

Q: Why can’t a plant have both high WUE and high RGR?

A: Because maximizing photosynthesis (RGR) requires open stomata, increasing water loss and lowering WUE.

Q: What conditions favor high-WUE plants?

A: Dry conditions with infrequent rainfall (need to conserve water).

Q: What conditions favor high-RGR plants?

A: Wet conditions or frequent rain pulses (can grow rapidly before drying out).

Q: How does leaf area relate to photosynthesis and water loss?

A: Larger leaf area = higher photosynthesis and growth potential but also higher transpiration and water loss.

Q: What leaf area adaptations are common in Sonoran Desert plants?

A: Small leaves or reduced leaf area to minimize water loss.

Q: What two strategies do Sonoran winter annuals use to survive?

A: (1) Grow fast after rain and reproduce quickly, or (2) grow slowly and conserve water.

Q: How does rainfall variability promote biodiversity?

A: Different rainfall patterns favor different strategies (high-WUE vs. high-RGR), allowing many species to coexist.

Q: 🔴How might climate change alter these trade-offs?

A: 🔴More variable rainfall and higher temperatures may favor high-WUE (slow-growing) species, reducing diversity of fast growers.