Photosynthetic limitations by water deficit: Effect on fruit and olive oil yield, leaf area and trunk diameter and its potential use to control vegetative growth of super-high density olive orchards

Regulated deficit irrigation (RDI) reduces leaf area, which is advantageous for fruit tree orchards with high plant densities to increase their long-term productive life. However, RDI also decreases fruit yield. To establish an optimum irrigation level to control tree vegetative growth without severely penalizing fruit yield it is necessary to analyze the effect of the limited photosynthesis produced by RDI on the carbon allocation patterns between yield and tree vegetative growth, which are not fully established in olive. Thus, our main objective was to unravel the relationships between limited photosynthesis and tree growth as well as yield to establish an optimum level of deficit irrigation. We conducted the research during four irrigation seasons in a super-high density olive orchard using four irrigation treatments: a full irrigation treatment (control) and three RDI treatments with increasing levels of water reduction scaled to replacing 60%, 45% and 30% of the irrigation needs. The plant water stress produced by RDI reduced photosynthesis, which resulted in a significant decline of leaf area. In contrast, neither single fruit weight nor total fruit yield normalized by leaf area was adversely affected by RDI. We found significant and direct relationships between photosynthesis and leaf area (r2=0.90, p<0.0001) as well as between leaf area and yield (r2=0.55, p<0.05). Thus, we conclude that while leaf area is determined mainly by photosynthesis, fruit yield is largely determined by leaf area, and thus, photosynthesis and leaf area are the main variables to control tree growth without curtailing the yield. The lowest RDI levels (30% and 45%) lead to greater water savings than 60%, with a similar effect on leaf area and fruit yield, and thus, any of these lowest irrigation strategies is preferred to achieve the best balance between crop water consumption and fruit yield.