The mechanisms of photosynthetic inhibition and the development of scorch in tomato plants treated with spray oils

Photosynthetic inhibition and the development of scorch in tomatoes (cv. Minibelle) were investigated after spraying 3-wk-old plants with oils of viscosities 32-00 and 2–37 cSt (at 22 °C) in controlled conditions in a glasshouse. Environmental conditions (15–25 ± 2 °C, 60–90 ±10% r.h., 100–6000 lux) during spraying did not affect the degree of photosynthetic inhibition but were important in determining the development of scorch. Increasing the spray volume from 0–7 to 14.0 μl/plant (c. 13–260 nl/cm2 leaf surface area) increased the severity of scorch in the environmental conditions in which it occurred. The major cause of photosynthetic inhibition by oil appeared to be interstomatal penetration of the cuticle and a mechanism is proposed for scorch involving direct penetration of open stomata by oil droplets.