SummaryThe effect of plant water status on the diffusion of C02 in the gas and liquid phase in leaves of cotton plants was studied in a single leaf chamber under conditions of constant light level and temperature and when photosynthesis was limited by the CO 2 supply. A controlled range of relative leaf water contents from 56 to 96% was obtained by varying root temperature from 6 to 30°C while the tops of the plants were -at a constant temperature. Decreasing water content resulted in an increase in the calculated leaf diffusive resistance and a decrease in CO 2 exchange. Under the environmental conditions used, plant water status primarily affects C02 exchange by regulating stomatal aperture. The mesophyll resistance, which was estimated in air and in an oxygen-free atmosphere, did not vary with the relative leaf water content down to 75% but increased progressively as relative water content dropped from 75 to 56%.

SummaryThe influence of plant water status and leaf temperature on the mesophyll resistance to C02 transfer for Deltapine cotton leaves was determined under conditions when the C02 supply was limiting photosynthesis. The mesophyll resistance was calculated from C02 response curves in normal air and oxygen-free air, under conditions when air was forced from the abaxial to adaxial side of the leaf to obtain a direct estimate of the CO 2 concentration at the mesophyll cell wall.The mesophyll resistance was about 25 % higher in normal air (rm ~ 4 sec em-I) than oxygen-free air (rm ~ 3 sec em-I), but neither variation in the relative leaf water content from 56 to 92% nor leaf temperature from 22·5 to 38°C affected the calculated mesophyll resistance in the oxygen-free air treatment.Photorespiration was substantially inhibited by the oxygen-free air and was approximately linearly related to leaf temperature in both oxygen-free and normal air treatments. The temperature dependence of the C02 compensation point was explained by the influence of temperature on photorespiration.

The ways in which transpiration and assimilation depend on stomatal aperture are compared. It is shown that transpiration and assimilation are equally sensitive to change of stomatal aperture when the internal resistance to assimilation is equal to an effective resistance to evaporation which exists because of the coupling of heat and vapour exchanges between leaf and atmosphere. Generally the ratio of transpiration to assimilation changes with stomatal aperture in a manner which is determined by the relative magnitude of these resistances and on temperature. Some possible implications in relation to the optimal behaviour of stomata are discussed.

SummaryExtracuticular wax and contact angles on wheat were studied because of their influence on the retention of chemical sprays and on disease resistance. Wax formed extensive deposits on wheat, irrespective of variety, stage of growth, or part of the plant, and these deposits overlaid or projected from the cuticle as platelets and rodlets. Platelets covered the adaxial and abaxial leaf surfaces of seedlings and some mature plants, while a net of rodlets covered the ear, culm, sheath, and flag leaf abaxial surface. Rods were occasionally present on the abaxial surface of mature vegetative leaves. Wax influenced the advancing contact angle of water droplets on wheat. Contact angles were all high, i.e. greater than 130° and generally about 150°. The contact angle on the adaxial leaf surface was higher than on the abaxial leaf surface, except on glasshouse-grown reproductive plants, where there was no difference between the two sides. Seedlings had higher contact angles than mature plants, but there was no trend in contact angle with tissue age within a leaf or within a mature plant. The contact angle on the flag leaf of glasshouse-grown reproductive Aotea plants was 24° higher than on a similar plant grown in the field.

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