Water stress and cool temperatures modify the flower type and fruit load of 'Orri' mandarin grown in an arid region

Climate change and high temperatures that occur due to the El Niño phenomenon affect the flowering and fruit production of citrus orchards on the Pacific Coast. However, there is little information about the mandarin flowering and fruit load in these regions, close to the equator. An experiment was carried out with adult 'Orri' mandarin trees (10-year-old) that were subjected to four levels of water regimes: a commercial control (27-day deficit irrigation with 1.25 mm applied twice a week) and three intensified stress treatments with zero irrigation for 20, 40 and 60 days. Additionally, branches were defoliated and girdled to quantify the effect of cool temperatures (number of hours below 20°C) on flowering. There was no interaction between water stress and cool temperatures on flowering intensity and flower type (p>0.05). Trees subjected to 60 days of water stress had a greater number of single-flowered leafless shoots. The control branches, which received 2578 h of cool temperatures, had a lower number of single-terminal-flowered leafy shoots and a higher number of multiflowered leafless shoots. Excessive water stress and cold caused a higher proportion of leafless flowering shoots, which are correlated with lower fruit set. Commercial control trees maintained significantly higher stem water potential and transpiration than all stress treatments. While photosynthetic rates showed no significant difference between control and 20-day stress treatments, stomatal conductance was significantly reduced after 40 and 60 days of stress (p<0.05). The number of fruits tree^-1 was greater when the trees received 20 and 40 days of water stress (p<0.05). The lowest fruit load was observed in trees that received 60 days of water stress, accompanied by leaf yellowing and slight defoliation.