Photosynthetic water-use efficiency (WUEₚ), which represents the ratio of photosynthetic carbon assimilation to transpiration rate, can be considered a useful indicator of a plant’s water-use efficiency at the leaf level. However, it does not fully represent the plant’s overall water-use efficiency at the whole-plant or ecosystem scale, because total WUE is also influenced by factors such as biomass partitioning, respiration losses, canopy structure, soil evaporation, and seasonal water availability. Therefore, while WUEₚ provides valuable insight into instantaneous gas exchange performance, it should be complemented with integrated measurements to accurately assess a plant’s overall water-use efficiency

This is actually a pretty tricky question that gets to the heart of plant physiology. The short answer is: not always, and your hormone example perfectly illustrates why.

Think about what's happening when you spray a hormone on a plant. Let's say it's something like cytokinin or auxin that makes both photosynthesis and stomatal conductance go up. Now, if stomatal conductance increases more than photosynthesis does, your calculated photosynthetic WUE (photosynthesis ÷ stomatal conductance) actually drops. But here's the kicker—the plant might actually be doing better overall! It's taking in more CO₂, growing faster, and potentially using water more effectively at the whole-plant level.

This disconnect happens because photosynthetic WUE is just a snapshot at the leaf level, while whole-plant water-use efficiency considers the bigger picture—like how much biomass the plant produces per unit of total water used. These two measures can tell completely different stories.

The research shows this problem isn't just theoretical. Studies on crops like wheat found that leaf-level and whole-plant WUE sometimes correlate well, but other times they don't, especially under stress conditions. In grapevines, researchers found that daily integrals of leaf WUE often had poor correlation with midday measurements, and the correlation got even worse under water stress. This suggests that relying on single-leaf measurements can be misleading when trying to understand how efficiently a plant uses water overall.

What's really interesting is that the relationship between photosynthesis and stomatal conductance isn't fixed. Under well-watered conditions, stomatal conductance often limits photosynthesis, so opening stomata more can boost carbon uptake significantly. But under stress, the relationship changes, and other factors like leaf hydraulic conductance and metabolic limitations become important.

The hormone example also highlights another issue: timing and integration matter. Photosynthetic WUE gives you an instantaneous reading, but plants don't live in instantaneous moments. They have to deal with changing light throughout the day, varying water availability, and different growth stages. A plant might have lower instantaneous WUE but still be more water-efficient overall because it's better at capturing carbon during peak conditions or conserving water when it's scarce.

So when researchers want to understand a plant's true water-use strategy, they often look at multiple measures: instantaneous leaf-level WUE, integrated daily measurements, whole-plant biomass-to-water ratios, and sometimes even carbon isotope discrimination patterns that reflect long-term WUE. Each tells part of the story, but none tells the complete story alone.

Your hormone example is actually a great case study for why plant physiologists have to be careful about drawing conclusions from single measurements. It shows how a treatment that appears to decrease water-use efficiency at the leaf level might actually be improving the plant's overall performance and water-use strategy.

Dear Dr. Shweta Dwivedi and Dr. Aqeela Saleh

Many thanks for your comprehensive answers. They have greatly helped me in understanding the topic and advancing my research.

Photosynthetic water-use efficiency (WUE), defined as the ratio of carbon assimilation (photosynthesis) to stomatal conductance or transpiration, is an important indicator of how effectively a plant uses water for carbon gain. However, it does not fully represent a plant’s overall water-use efficiency, which also depends on whole-plant factors such as biomass allocation, root water uptake, leaf area, growth rate, and long-term water balance. In short, photosynthetic WUE provides valuable insight at the leaf or physiological level, but it is only one component of the broader concept of overall plant water-use efficiency.