Introduction
Citrus industry in Iran with 288 thousand hectares cultivated area, 5.5 million tons annual production and 20 tons per hectare Iran ranks sixth, seventh and fifth in the world, respectively. About 70% of orange production in Mazandaran province, the most important region for citrus production in Iran, is dedicated to the ‘Thomson navel’ variety and sour orange rootstock. So, it is very important to evaluate new varieties and suitable rootstocks to replace them, if necessary. In the present study, the effects of three trifoliate citrus rootstocks on vegetative growth, quantitative and qualitative yield of early maturing ‘Navelina’ orange was studied over a seven-year period in Mazandaran province in the North of Iran.
 
Materials and Methods
Research was performed in a complete randomized block design with ‘Navelina’ orange and three rootstocks (‘Carrizo citrange’; ‘Swingle citrumelo’ and ‘C-35 citrange’) with six replications and with 5 × 3 m planting distances during 2011 -2020. After planting in the main land, the young trees were trained for four years and then pruned annually in winter. Plant responses was included tree canopy volume, rootstock and scion morphological compatibility (affinity), fruit yield, yield efficiency, fruit characteristics in different time and on different rootstocks, appropriate time to start harvesting, and the theoretical space requirements of the trees for better land utilization.
 
Results and Discussion
Three trees on ‘C-35 citrange’ rootstock showed severe leaf chlorosis and leaf abscission in 2018 and 2019, but with nutrition and irrigation management, two of them returned to normal condition and one of them was completely declined. In the final year (2020), ‘Carrizo citrange’ rootstock showed the highest (0.765) and ‘Swingle citrumelo’ the lowest (0.655) morphological compatibility with ‘Navelina’ orange. The average tree canopy volume in 2020 ranged from a minimum of 4.48 m³ on ‘Swingle citrumelo’ to a maximum of 5.24 m³ on ‘C-35 citrange’ rootstock. So, the volume of Navolina orange trees on all three rootstocks were small. In early November of 2017-2020 some characteristics of fruits were evaluated. Results showed that ‘Navelina’ orange had elongated fruits (the ratio of fruit length to fruit width was higher than one), with a large size and weight (more than 280 grams), thick peel (about 4.4 mm), peel weight to the fruit weight was more than 46%, the juice content to fruit weight was more than 41%, and fruits in this time had a pale orange peel color. Average of harvest index in early November ranged from a minimum of 8.84 in ‘Swingle citrumelo’ to a maximum of 10.74 in ‘Carrizo citrange’ rootstock. ‘Carrizo citrange’ rootstock had the highest average four-year yield (16.61 ton.ha^-1) and the highest average and final-year yield efficiency (6.07 and 7.77 kg.m^-3, respectively). In the present study, of the seven-year field trials after planting on the main land, based on tree height and crown diameter and considering 2.5 m of space between rows of trees, the theoretical space required for each tree was about 7.5 m². Therefore, the number of trees can be increased to 1000 trees per hectare to make better use of the land in this region
 
Conclusion
Harvest time of ‘Navelina’ orange on ‘Carrizo’ and ‘C-35 citrange’ rootstocks begin in the third decade of October and on ‘Swingle citrumelo’ rootstock, starts at the first decade of November, at the same time as harvesting ‘Thomson navel’ orange (the common variety in the region). Due to the early discoloration of the fruit peel and the contamination of the region with the Mediterranean citrus fly, delay in harvesting causes high crop losses and pre-harvest fruit drop due to the bites of this insect pest. As well as, delay in harvest reasons the fruit to become larger and heavier, which reduces fruits marketability. Of the three rootstocks studied in this research, ‘Carrizo citrange’ was the best rootstock for ‘Navelina’ orange variety. In addition to showing no signs of decline, it had the highest average yield of four years and the highest average yield efficiency and final year. Also, the fruit harvest index reached the standard harvest level in late October, which is two weeks earlier than the common variety in the region.
 
Acknowledgement
We would like to express our sincere gratitude to the experts and colleagues of the Mazandaran Agricultural and Natural Resources Research and Education Center who sincerely cooperated in the field and laboratory work of this research.