The foliar application of sunscreen formulations containing silicon dioxide (SiO₂) may be relevant to overcome heat burn in vegetable crops. The authors tested a Si-containing product on cucumber (Cucumis sativus) and pepper (Capsicum annum) potted plants to assess its protective properties against heat stress. Two-week-old plants were either sprayed (S+) or not sprayed (S-) with a 0.05% Si solution and were either exposed (H+) or not exposed (H-) to heat stress; greenhouse conditions were used and there was a total of four treatments (S-H-, S-H+, S+H-, and S+H+). The H+ treatment consisted of exposing plants, for 6 hours on two consecutive days, to mild heat stress (air temperature around 42ºC) and one week later to severe heat stress (air temperature around 48ºC). Also, H- plants received 25% less solar radiation than H+ plants.

Pepper plants exposed to heat burn (H+) showed a 1% decrease in their water percentage compared to H- plants. The leaf temperature was 7% lower (p < 0.01) in the S+H+ treatment group and around 5% lower in S+H- plants of both species, compared to plants not sprayed with Si (S-). In addition, plants sprayed with Si (S+H+ and S+H-) showed a 20% lower (p = 0.04) stomatal conductance (g_s). The chlorophyll content of leaves not exposed to heat-stress (H-) was almost 50% higher in S+ plants of either species, on average, of the two heat-stress treatments (mild and severe). Heat stress decreased by an average of 4% in relation to the quantum efficiency (Fv/Fm) of plants of either species. Also, Si improved the Fv/Fm of all plants that were not exposed to heat stress (i.e., S+H-) and plants after exposure to severe heat stress (48ºC), but not after mild heat stress (42ºC). The number of flowers produced by cucumber plants was 60% higher in Si+H+ than in Si-H+ plants.
These results suggest a beneficial effect of silica spraying but should be confirmed in the field.