PS 15-198 - Effects of defoliation on stomatal properties

Abstract: Stomata regulate CO₂ intake and water loss in plants. Stomatal density and size are therefore important determinants of maximum photosynthetic rate as well as maximum and minimum transpiration rates. These properties are known to be affected by environmental conditions during leaf development such as temperature, water availability, and atmospheric CO2. However, the effects of defoliation on stomatal density and size are currently unknown. In response to reduced carbon availability, trees may increase stomatal density and/or size in reflush leaves to increase leaf-level photosynthesis. However, because reflush leaves are often smaller, a higher density of smaller stomata may result simply from the reduced cellular expansion of epidermal cells, including guard cells.

In this study, we measured stomatal properties of leaves from two defoliation experiments to address the following questions: (1) How does defoliation stress affect stomatal size and density? (2) Does heat stress alter defoliation effects on stomatal properties? And (3) are defoliation effects driven by changes in leaf size or carbon availability? In the first study, we investigated how defoliation and girdling impacted stomatal traits in mature, field-grown White Ash (Fraxinus americana). In the second study, we investigated the effects of a heatwave (+10°C above ambient) on defoliation recovery in potted Pin Oak (Quercus palustris) saplings. For both studies, leaf stomatal impressions were made via the nail-polish impression technique, and stomatal length, width, pore length, and density were measured. In addition, we tested for treatment differences and correlations between stomatal properties, nonstructural carbohydrate concentrations, leaf size and gas exchange measurements.

In Ash, stomatal density was significantly higher in reflush leaves of defoliated trees. While there was no correlation between leaflet size and stomatal density, a significant negative correlation between initial bark starch concentration and stomatal density was found among defoliated trees. The results suggest that increased stomatal density is therefore not a function of reduced leaf expansion, but rather a response to reduced carbon availability. Increasing stomatal density may help defoliated plants increase photosynthetic rate, but it may come at the cost of increased drought intolerance. In contrast to Ash, defoliation and heatwave treatments had no significant effect on stomatal density in Oak saplings, though both treatments significantly increased minimum water conductance and net photosynthetic rate. The differences between experiments therefore suggest that the effects of defoliation may vary across species and/or developmental stages.