Constructing normalized climate-quality index for Gannan navel orange

Climatic conditions can serve as the ecological influencing factors on the quality of agricultural products. The key meteorological elements can be determined to evaluate the climatic quality at the influencing stages on each quality factor. However, the current evaluation of Gannan navel orange has focused mainly on the single factor of climatic quality. The climatic evaluation on comprehensive quality (such as appearance, nutrition and flavor) has remained unknown. This study aims to construct the normalized climate-quality index for Gannan navel orange. The quality factors (single fruit weight, vitamin C and solid-acid ratio) were collected in 2008-2011 and 2021-2022, together with the meteorological data (including light-temperature variables (average temperature, the maximum temperature, the minimum temperature, diurnal temperature range, and sunshine hours), and water variable (precipitation and relative humidity)). The key meteorological factors were firstly identified to analyze their influencing periods in the 10-day evolution of correlation coefficients between the quality factors and the meteorological variables. Then, the meteorological relationship model of each quality factor was established by the stepwise regression. The "maximum-minimum" normalization was combined with the weighted summation to simulate the 1981 to 2020 Composite Quality Index (CQI). A correlation analysis was performed between CQI and meteorological elements. Finally, the Normalized Climate-quality Index (NCI) was developed to determine the grade thresholds by probability distribution fitting. The results showed that the correlation between quality factors and light-temperature variables from June to November was divided into three stages with the correlation direction as positive-negative-positive, indicating the different correlation with the water variables. The three stages varied among three quality factors. In the first stage, single fruit weight was from early June to mid-August, and the vitamin C and solid acid ratio were from early to late June, each of which was positive. The second stage was concentrated mainly in the midsummer from July to September, with the single fruit weight from late August to mid-September, vitamin C from early July to mid-August, and solid-acid ratio from early July to early September. Each of these correlation coefficients was negative. The third stage of each quality factor varied widely, with the single fruit weight from mid- to late November, vitamin C from late August to late October, and solid-acid ratio from early to late October. Each of them turned to be positive. The regression models were developed for each quality factor, where their relative errors and relative root mean square errors were within 10%. The meteorological factors for the Normalized Climate-quality Index (NCI) were the cumulative sunshine hours in June. There was the average maximum temperature from early July to mid-September, whereas, the average minimum temperature was found from late September to late October, respectively. NCI≤0.45 , 0.45<NCI≤0.52 , 0.52<NCI≤0.65 and NCI>0.65 , denoted as poor, good, better, and excellent, respectively. The quality data was measured from Ningdu and Xinfeng counties in 2021-2022, where most of the NCI deviations were within 0.07. Therefore, the NCI can be expected to carry out the climatic quality evaluation of Gannan navel orange.