Stomatal Movements, Frequencies, and Resistances in Two Maize Varieties Differing in Photosynthetic Capacity

The Stomatal characteristics of two maize varieties previouslyfound to differ in rates of net photosynthesis were examinedin a controlled environment. Measurements with a ventilateddiffusion porometer showed that one variety exhibited a pronouncedand the other a weak periodicity in stomatal resistance of theadaxial epidermis. At equal illumination the stomatal resistanceof the adaxial epidermis decreased from upper to lower leaves,while the resistance of the abaxial epidermis changed in theopposite manner. Stomata on the adaxial and abaxial surfacesof maize leaves exhibited random not compensatory, movementsin a constant environment. The variety with the lesser stomatalfrequency and higher total leaf resistance to water loss hadnevertheless faster net photosynthesis than the variety withthe greater stomatal frequency, demonstrating the importanceof the so-called mesophyll resistance.     

Physiological responses of dogwood (Cornus florida) to infestation by the dogwood borer (Thamnosphecia scitula)

Studies of a healthy dogwood tree ( Cornus florida ) and one suffering from a chronic infestation of Thamnosphecia scitula revealed some physiological responses of dogwood to insect damage. In high light the stomatal resistances, measured with a diffusion porometer, of leaves from the infested tree were more than double those of leaves from the healthy tree, indicating that the stomata were not open as wide in the infested tree. The greater stomatal resistance and the curling of leaves from the infested tree implied a deficiency of water, but measurements with a pressure chamber revealed only slight differences in dehydration between the two trees. Furthermore, the leaf curling was not relieved by allowing infested leaves to absorb water and attain full turgor. Leaves from the infested tree contained 35 % more reducing sugars and 63 % more sucrose, but only about half the nitrogen and ash of the healthy counterparts. Photosynthesis was significantly depressed, but dark respiration was not modified in leaves of infested compared with healthy trees. Leaves from the infested tree were 85 % as large and exhibited a 35 % greater specific leaf weight than those from the healthy tree. The conductance of water by stems from the infested tree was only 61–78 % of that by the healthy tree. It is suggested that the inferred disruption of the vascular system by insect activity alters the distribution of minerals and metabolites, hastens senescence and stomatal closure, and modifies growth by diminishing stem conductance, slowing photosynthesis, reducing leaf area, and changing leaf morphology.