Kaolin Particle Film Applications Can Increase Photosynthesis and Water Use Efficiency of `Ruby Red' Grapefruit Leaves

Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (T lf ), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees ( Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g·L -1 . Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (T lf ; ≈3 °C) and leaf to air vapor pressure differences (VPD; ≈20%) compared to water-sprayed control leaves. Midday reductions in T lf and VPD were accompanied by increased stomatal conductance (g s ) and net CO 2 assimilation rates (A CO2 ) of kaolin sprayed leaves, suggesting that g s might have limited A CO2 in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher A CO2 . Leaf intercellular CO 2 partial pressures (C i ) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced A CO2 . These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.