NITROGEN AVAILABILITY AND FRUIT YIELD GENERATION IN CACTUS PEAR (OPUNTIA FICUS-INDICA): III. EFFECTS ON FRUIT YIELD AND DRY MATTER ALLOCATION TO REPRODUCTIVE SINKS

The effect of N availability on fruit yield and dry matter partitioning to reproductive sinks in cactus pear is not well known. The objectives of this work were to determine the effects of N shortage and excess on fruit yield and its components, and on aerial dry matter allocation to reproductive sinks. The trial was carried out under irrigation at Santiago del Estero, Argentina, during 1998-1999 and 1999-2000 growing seasons, using ‘Amarilla sin Espinas’ cactus pear. Treatments were: N_low (16 t ha^-1 sucrose to immobilize N), N_medium ( 100 and 150 kg ha^-1 N) and N_high (200 and 300 kg ha^-1 N) the higher rates was applied in 1999-2000. N content of soil, cladodes and fruit, number of vegetative and reproductive buds, dry matter accumulation to cladodes, fruits and aerial biomass, and aerial dry matter allocation to reproductive sinks, harvest index (HI) and HI of the current year (HIY), were determined. N fertilization increased fruit yield, and fruit yield standardized by cladode number except during the second year for fruit yield standardized by cladode number at the end of first year. Fruit load was the most important component to explain fruit yield variability, whereas fruit weight remained stable among N treatments. These results indicate a strong fruit yield limitation by reproductive sinks. There were significant differences between years in aerial dry matter partitioning, with highest HI and HIY values for the second year. A significant Year x N interaction was detected. N fertilization treatments showed the highest values of dry matter partitioning. When aerial dry matter and dry matter partitioning increases due to N fertilization were compared (as a relative percentage of Nlow) it was found that dry matter increments predominated during the first year whereas the inverse occurred during the second year. The results may help to optimize N fertilization and to improve simulation models.