Seasonal changes in gas exchange, water and macro-nutrient content differ between Citrus cultivars

Citrus is an evergreen broad-leaved fruit tree including numerous species and cultivars with an extensive global planting area. However, the relationship between physiological parameters and seasonal fruit ripening of different citrus cultivars has not been reported. With the aim to obtain information on this relationship, we selected two late-maturing (Newhall Navel and Ponkan) and two middle-maturing citrus cultivars (Fertile orange and Tarocco) and compared the seasonal patterns of foliar CO2 and H2O gas exchange, leaf water content and macro-nutrient concentrations in leaves and roots. Our results showed that seasonal changes of foliar CO2 and H2O gas exchange and water content greatly differed between cultivars at defined developmental stages, but especially during fruit ripening. For example, in December, the net rates of photosynthetic CO2 fixation (Pn) of the late-maturing cultivars were significantly higher than in middle-maturing cultivars. Seasonal fluctuations of total nitrogen (N), total phosphorus (P), and inorganic phosphorus (Pi), as well as C/N, C/P, and N/P ratios in leaves and roots differed between middle and late-maturing cultivars. These differences were most obvious at fruit ripening compared to other developmental stages and were significantly higher in leaves than in roots. We conclude that seasonal differences in foliar Pn and macro-nutrient contents between leaves of citrus cultivars are largely determined by seasonal fruit ripening. In addition, seasonal changes in root macro-nutrient contents were also affected by seasonal fruit ripening of citrus cultivars. However, this effect was much lower compared to the leaves, but still indicates particular shoot-root interactions during fruit ripening.