This research included two independent yet build up objectives: (i) to examine the effects of Mn concentration in the irrigation water on the yield and fruit quality of greenhouse-grown bell pepper exposed to heat stress; and (ii) to investigate in a laboratory experiment the role of Mn addition on oxidative and anti-oxidative components of fruit apoplast. The field experiment included four concentrations of Mn in the irrigation water (0, 0.2, 0.6 and 1.0 mg L-1). The effect of Mn application rate on vegetative growth and total fruit yield was insignificant and despite low Mn concentrations in zero-Mn-fed plants, no visible foliar Mn symptoms were observed. A quadratic regression was obtained between high-quality fruit yield and Mn concentration in fruit. Based on the quadratic equation, maximum high-quality yield was obtained when fruit-Mn concentration approached 44 mg kg(-1) DW. The correlation also indicates that variations in high-quality yield could be explained by differences in fruit-Mn concentration. The combination of low-Mn nutrition and high air temperature which prevailed during the experiment (summer season) induced severe pale spots and blemished fruits; approximately 50% of fruits were infected in zero-Mn-fed plants. In a laboratory experiment with detached-fruit under high temperature the H2O2 concentration increased whereas ascorbic acid concentration decreased in the apoplast, In Mn-fed fruit the H2O2 concentration was attenuated and the activity of ascorbate oxidase was suppressed, with the result that apoplastic ascorbic acid concentration increased. In light of the known crucial role of Mn in enzyme activities and in detoxification of oxygen free-radicals, the pale spots might be related to Mn effect on oxidative stress-related apoplastic activities. The relationships between incidence of pale spots and plant-Mn concentration may support this hypothesis, but further research is needed to validate it. (C) 2009 Elsevier B.V. All rights reserved.