Abstract: The aim of this study was to monitor the effect of foliar application of humic acid on plant growth, photosynthetic metabolism and grain quality of durum wheat grown in a Mediterranean-type climate. Four fertilization treatments were applied: a non-fertilized control, a crop fertilized with foliar application of humic acid, a crop fertilized with mineral N on soil at sowing, tillering and stem elongation, and a crop fertilized with foliar application of N (ammonium-nitrate solution). Aboveground plant mass accumulation was measured throughout two growing seasons, and grain quality parameters were tested at harvest time. Gas exchange, leaf protein content and Rubisco activity were monitored at different stages of plant development. Differences between years were often relevant due to weather conditions. The foliar application of humic acid caused a transitional production of plant dry mass with respect to unfertilized control and split soil N application. This effect was also evident for grain yield, spike fertility and grain protein content during the two years of the study. Humic acid never affected photosynthesis or stomatal conductance, while Rubisco activity and leaf protein content showed intermediate responses between unfertilized control and split soil N application. We conclude that humic acid had limited promoting effects on plant growth, grain yield and quality, and photosynthetic metabolism of durum wheat crops grown in a typical Mediterranean-type agro-ecosystem of southern Italy, with respect to split soil N application.
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