In the majority of middle and late maturing citrus species and cultivars, the relationships between climatic parameters including temperature, precipitation, sunshine and solar radiation, and morphological characteristics and inherent quality of the fruit, for which data were collected from many locations in Japanese citrus growing areas during several years, were analyzed by single and multiple regression analysis. The objective of the investigation was to determine the range of variations in temperature, precipitation, sunshine and solar radiation in the citrus growing areas where citrus fruits of distinctive quality are produced.
Temperature is the climatic factor which was found to have greatest effect on fruit quality characteristics, in particular the citric acid content, Brix/acidity ratio, juice pH, rind thickness, flesh percentage, shape index of fruit and fruit weight. The parameters related to temperature included the annual mean temperature, mean temperature from Apr. to Nov. and annual effective cumulative temperature.
Rainfall was also found to significantly affect several fruit quality characteristics. Contradictoly results were obtained for relationships between the precipitation and the citric acid content in juice. These disagreements may be ascrived to the interaction of factors including the duration of rainfall, the rainfall intensity in each period and the development and maturation period of fruit.
The changes observed in the fruit quality characteristics related to the climatic parameters did not correspond to variations in the Brix values but corresponded to variations in the citric acid content and the Brix/acidity ratio.
Five multiple regressions among the fruit quality characteristics (Dependent variable) and the climatic parameters (Independent variable) were obtained. The climatic conditions associated with a high fruit quality in 8 citrus species and cultivars, based on the relationships obtained from the regression lines, were tabulated.
The relationships between mean monthly temperature, monthly duration of sunshine and monthly solar radiation were highly significant. In further investigations, the annual cumulative solar radiation should be determined to assess the adaptability of citrus species.