Relationships among foliar idioblasts, leaf anatomy and leaf water relations: Using tropical Rhododendron species as exemplar

Background/Question/Methods

Idioblasts are large cells nested in tissues composed of smaller cells. These large cells can be found in many plant families and several different tissues. Some idioblasts have been shown to store secondary metabolites or aid herbivore defense. However, the functional significance of idioblasts to leaf physiology is unknown. In a survey performed at the Royal Botanical Garden, Edinburgh, on Rhododendron species, idioblasts were observed in leaves of tropical members of the genus. Following that discovery, a goal was set to determine the functional significance of idioblasts to leaf physiological traits in tropical Rhododendron species. This study focused on the relationship between idioblasts and leaf water relations. I posed that the large water holding capacity of idioblast cells could influence leaf water relations, particularly capacitance.  I hypothesized that the higher the proportional volume of idioblasts in the leaf lamina (furthermore called idioblast expression) the higher would be the bulk leaf capacitance. Also, I hypothesized that idioblast expression is positively correlated with high stomatal pore index (a proxy for maximum stomatal conductance). To assess these questions a survey of idioblast expression, stomatal pore index, and leaf capacitance was performed using 60 plants representing 25 tropical species of Rhododendron.

Results/Conclusions

All sampled leaves of tropical Rhododendron species had idioblasts. Most idioblasts were located in the adaxial palisades, although a few were in the abaxial mesophyll. The density of idioblasts ranged from 20/mm² to 90/mm². Idioblast expression ranged between 4% and 20% with an average of 9.85%. There was a significant regression (p<0.0001) between the density of idioblasts and their expression. Leaf capacitance, pre turgor-loss, was weakly positively correlated (0.0390) with idioblast expression and the water deficit at the turgor-loss point was positively correlated (0.3326). However, leaf capacitance post turgor-loss was negatively correlated (-0.4120) with idoblast expression. Therefore, hypothesis one was refuted because leaf capacitance did not increase significantly with an increase in idioblast expression. A negative correlation (-0.5055) between leaf succulence and idioblast expression weakened the relationship between idioblast expression and capacitance. For thin leaves (less than 0.5mm thick) idioblast expression had a stronger positive relationship with pre turgor-loss leaf capacitance (0.3178). There was no significant correlation (0.0262) between idioblast expression and stomatal pore index. Therefore, idioblast expression in tropical Rhododendron leaves increases capacitance and water deficit at the turgor-loss point of thin leaves, but high leaf succulence reduces the value of idioblast expression to leaf capacitance.