Photosynthesis & transpiration of banana leaves as affected by severing the vascular system

Abstract

A temporary increase in transpiration following an interruption of the vascular supply to the leaf under measurement has been observed in a number of plants (1, 2, 3, 6, 7, 9). The explanation given for this phenomenon is that originally formulated by Iwanoff (8). He proposed that the sudden release of tension in the vascular elements caused a sudden temporary increase in the availability of water to the leaf, resulting in a further stomatal opening and consequent increase in transpiration. Allerup (2) has indeed demonstrated a temporary increase in stomatal opening in decapitated barley seedlings corresponding to the increase in transpiration. The following study was undertaken to determine if this phenomenon occurs in the banana, and if so to determine if it also reflects itself in the photosyn-thetic activity of the leaf. If the banana behaves in this manner, it would necessitate a certain amount of caution in interpreting data from photosynthesis and transpiration experiments involving excised portions of banana leaves. MATERIALS & METHODS The banana plants used in this study were of two varieties: Musa acuminata L. var. Hort. Gros Michel and var. Hort. Dwarf Cavendish. The Gros Michel plants were grown from rhizome seed pieces (button seeds) in 9 inch pots. These plants were about 50 cm tall (from soil line to leaf crotch) and about four months old when used; they had a pseudostem diameter of 3 cm, measured 5 cm above the soil line. In the experiments involving root cutting, 4 month old Gros Michel plants, grown in aerated Hoagland's solution, were used. These were 70 cm tall and had a pseudo-stem diameter of 5.5 cm. The Dwarf Cavendish plants used in some of the experiments were essentially mature plants rooted in soil in 12 inch pots. They were about 110 cm tall (from soil line to leaf crotch) and had a pseudostem diameter of approximately 13 cm. Measurements of photosynthesis and transpiration were carried out essentially as before (5). The humidity of an air stream, containing 300 ppm C02, was adjusted to a dew point of 10 C by passing it through a series of three bubbling towers containing 1 Received March 6, 1961. sulfuric acid with a specific gravity of 1.435. At 30 C this corresponds to a relative humidity of 29 %. The air stream was passed through a flow meter to the leaf chamber and thence to a lithium chloride type hygrometer, a concentrated sulfuric acid drying tower, and an infrared CO2 analyzer. The only other deviation from the previously described arrangement of the apparatus (5) was that the leaf chamber was equipped with a molded silicone rubber edge (Dow's Silastic RTV 501). This permitted the leaf chamber to be clamped in place rather than fastened with modeling clay. Continuous measurements of the temperatures at the lower leaf surface were made with copper-constantan thermocouples (30 AWG) connected to a recording potentiometer (Bristol's Dynamaster). The measuring junction was placed inside the leaf chamber so that it touched the lower leaf surface. The reference junction was placed in a slurry of ice and water. Light intensities were measured with a Weston Illumination Meter on the light incident upon the FIG. 1. Diagram of potted young banana plant showing the positions in which the vascular supply to the leaf was interrupted by cutting. 577