Effect of Various Citrus Rootstock-scion Combinations on Tree Growth, Fruit Quality, and Photosynthetic Characteristics under Red Loam Soil Conditions

[Objectives] To investigate the effects of 15 distinct citrus rootstock-scion combinations on tree growth, fruit quality, and photosynthetic characteristics under red loam soil conditions and provide a theoretical foundation for the selection of appropriate citrus rootstock-scion combinations in the Zhaoqing region. [Methods] A total of 15 citrus rootstock-scion combinations were utilized as test materials for a comprehensive analysis of their phenological periods (budding, flowering, and fruiting), tree growth indicators (tree height, crown diameter, and growth), and fruit quality (appearance quality and intrinsic quality). The photosynthetic characteristics of the test materials, including the net photosynthetic rate (Pn), transpiration rate (Tr), water use efficiency (WUE), apparent quantum yield (AQY), and carboxylation efficiency (CE), were analyzed to determine their significance. Additionally, the leaf photosynthetic physiological indicators, such as soluble protein, specific leaf weight, chlorophyll, and carotenoids, were evaluated. [Results] There were notable differences observed in the phenological period, growth potential of trees, fruit quality, and photosynthetic characteristics among various citrus rootstock-scion combinations. The phenological periods exhibited variation contingent on the grafting varieties. In terms of tree growth potential, the C. tangerina Tanaka ‘Hongju’ and C.haniana Hort ‘Suanju’ rootstocks demonstrated greater tree height, crown growth, and overall tree strength; however, they were also prone to excessive growth. Conversely, the C. limonia Osbeck ‘Hongningmeng’ and C. sinensis × P. trifoliata ‘Zhicheng’ rootstocks displayed medium growth potential, while the Poncirus trifoliate (L)Raf. ‘Zhike’ rootstock resulted in shorter trees. In terms of fruit quality, the single fruit weight of Citrus flamea Hort. ‘Shatangju’ ranged from 33 to 50 g, exhibiting a flat and round shape. The total soluble solids and titratable acid content of ‘Shatangju’ grafted onto the ‘Zhike’ rootstock were notably high. In contrast, the single fruit weight of C. haniana Hort. ‘Chuntianju’ varied between 65 and 81 g, characterized by a high flat round shape. The ‘Suanju’ rootstock demonstrated a higher sugar and acid content compared to other rootstocks. Additionally, the single fruit weight of Citrus nobilis Lour. ‘Gonggan’ ranged from 62 to 145 g, with the fruit shape being either round or oval. The soluble sugar and total soluble solids content associated with the ‘Zhike’ rootstock was also elevated. In relation to photosynthetic characteristics, the photosynthetic performance of the ‘Shatangju’ variety was superior when grafted onto the ‘Zhike’ and ‘Hongju’ rootstocks. Similarly, the ‘Chuntianju’ variety exhibited enhanced photosynthetic performance on the ‘Zhike’, ‘Zhicheng’, and ‘Hongju’ rootstocks. Furthermore, the ‘Gonggan’ variety demonstrated improved photosynthetic performance when grafted onto the ‘Zhike’ and ‘Suanju’ rootstocks. [Conclusions] Based on the characteristics of the red loam soil in the Zhaoqing region, the rootstocks ‘Zhike’ and ‘Hongju’ are conducive to the cultivation of the ‘Shatangju’ variety. Additionally, the rootstocks ‘Zhike’, ‘Zhicheng’, and ‘Hongju’ are optimal for the growth of the ‘Chuntianju’ variety, while the rootstocks ‘Zhike’ and ‘Suanju’ are appropriate for the growth of the ‘Gonggan’ variety.