The rate of transpiration, temperature of the leaves, and relative water content of leaves of pepper plants were measured in a small chamber in which the temperature, relative humidity, and carbon dioxide concentration of recirculated air were controlled and measured. The data reported were obtained by noting the response of pepper plants to all combinations of the following treatments: high light, 1.5 X 106 ergs per square centimeter per second; low light, 3.0 X 10' ergs per square centimeter per second; three levels of CO2: 50, 268, and 730 parts per million; nutrient solution osmotic potentials of -0.5, -5.0, -7.5, and -9.5 bars.The The flow of water from the root surface through the plant to the atmosphere encounters a number of barriers and resistances (6,7,14,15,19,21). These resistances are membranous, frictional, and diffusive. The magnitude of flow, transpiration, is governed by the behavior of these resistances to environmental conditions as well as by the energy gradient, both hydrostatic and osmotic, imposed across the conducting tissue. The greatest resistance to flow of water in the soil-plantatmosphere continuum is located at the leaf-air interface and is regulated by stomatal movement. A number of factors, particularly those associated with the availability of water at the root surface and the conductivity of the roots, influence water loss by bringing about changes in water potential in the leaf. A change in water potential in the leaf affects transpiration both by a change in water potential gradient from root to leaf and by its effect on stomatal aperture (19).The imposition of an osmotic stress on decapitated roots increases the resistance to flow of water (1, 9, 17). The data on the effect of osmotic potential of the nutrient solution on transpiration (5, 11, 12) indicate the possibility of a similar effect on roots of intact plants.Weatherley (21) has indicated that the resistance to flow of water through the castor bean plant increases as the flow of water through the plant decreases. Brouwer (4), using single roots in a series of potometers, also found a decrease in resistance with an increase in rate of flow of water through the root. Rawlins (18) has reported the significance of the resistance to flow of water within the plant and the possibility of its variation with rate of transpiration.The investigations reported here were undertaken to determine how changes in CO2 concentration, light intensity, and osmotic potential affect the water potential in the leaves and the rate of movement of water through the plant. These data were used to estimate the changes in resistance to flow of water through pepper plants. MATERIALS AND METHODSPepper plants (Capsicum annuum L. var California Wonder) were grown in Hoagland's nutrient solution in an environment controlled as follows: temperature 260, relative humidity 60 to 70%, illuminated for 18 hr/day with fluorescent and quartz iodine lights giving radiant energy of 1.05 X 105 ergs cm72 sec' at plant height. When the plants were 5 to 6 ...

Pepper plants Capsicum annuum L. var. California Wonder were grown in nutrient solutions of either -3.0 or -5.0 bars osmotic potential, using polyethylene glycol with molecular weights of 400, 600, 1000, 1540, or 4000 as osmotica. Polyethylene glycol with molecular weights of 1000 or 1540 proved most satisfactory as osmotica to decrease the water potential of nutrient solutions. temperature 26 C, relative humidity 60 to 70%, 18 hr per day of light from fluorescent and quartz iodine lights giving radiant energy of 1.05 X 105 ergs Cm-2 sec1 at plant height. Plants were approximately 4 weeks old at test maturity, with a leaf surface of 2 to 3 dMi2. The data were obtained from two different experiments. In the first experiment the plants were grown for 1, 3, or 7 days in solutions of either -3.0 or -5.0 bars osmotic potential using PEG with average molecular weights of 400, 600, 1000, 1540, or 4000 as osmotica. There were two plants per treatment (12 plants for each polymer). These plants were used to determine the rate of accumulation, distribution of PEG within the plant, and the relation between uptake of PEG and the amount of water transpired. In the second experiment the plants were grown for 7 days in solution with -5.0 bars OP using each polymer as the osmoticum. These plants were used to determine the effect of molecular size on transpiration, growth, leaf water potential, and selective absorption of molecular size. At the start of the treatment the plants were transferred to 1.8 liters of fresh nutrient solution containing the PEG polymer to be tested. The weights of the several PEG polymers added to 100 ml of nutrient solution to produce either -3.0 or -5.0 bars OP are given in Table I. Toxic impurities were removed from PEG 4000 by passing a concentrated solution through a column of standard Bantam demineralizing resin. The OP of the solutions was checked by determining the freezing point depression with a Precision Systems osmette.The loss of water and change in weight of plants was determined once a day. The entire culture was weighed, the plant and cover of the vessel were removed, roots were allowed to 226 www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

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, for his assistance and encouragement during the course' of this work. Paper from the Department of Botany at the University of Michigan. No. 777. 2 A caprified fig is one which has been pollinated by means of the insect Blastophaga, which grows in the caprifigs. These figs contain seeds. S The writer wishes to express his thanks to L. J. Alexander of the Ohio Agricultural Experiment Station for suggesting the use of the pollen of L. peruviawum to produce parthenocarpy in L. esculentum and also for the seeds of L. peruvianum.