Screening Evaluation and Physiological Difference Analysis of Kiwifruit Germplasm Resources for Tolerance to Combined Stress of Iron Deficiency and Waterlogging

To screen kiwifruit germplasm with tolerance to combined stress of iron (Fe) deficiency and waterlogging, and to clarify physiological and biochemical differences under single or combined stresses of the two factors, so as to provide a scientific basis and genetic resources for the breeding of kiwifruit germplasm tolerant to Fe deficiency and waterlogging.

Seventeen tissue-cultured kiwifruit genotypes belonging to four species, namely Actinidia chinensis (Ac), A. macrosperma (Am), A. polygama (Ap) and A. valvata (Av), were used to determine plant growth parameters, leaf net photosynthetic rate (Pn), and biomass in various parts for assessing the tolerance differences under combined stress of Fe deficiency and waterlogging. Moreover, two tolerance-contrasting genotypes (Av1 and Av5) from the same kiwifruit species were analyzed for the similarities and differences in physiological responses to combined stress.

Using the weight-based membership function method and cluster analysis, the combined tolerance of the seventeen kiwifruit genotypes was classified into three groups: High tolerant, including Av1, Av2, Am1, Av3, Av4, Am2, Ap1, Am3, and Av5; Medium tolerant, including Acd1, XX, Ap2, YX, Acd2, and HWD; And low tolerant, including QH and QM. Across kiwifruit species, the greatest tolerance was observed in Av and Am, followed by Ap, and then Ac. The superior tolerance of Av and Am was related to their excellent aboveground performance, including plant height, new leaf number, leaf area, leaf length and width, fresh weight of aboveground parts and whole plant; and that of Ap was associated with its good root performance, including root volume and root fresh and dry weight. Combined stress enhanced proton extrusion, ferric chelate reductase activity and Fe concentration in roots of both genotypes. However, in new leaves, combined stress decreased Fe concentration in Av5 but increased that in Av1, which contributed to more positive root physiological regulation in Av5. Both waterlogging and combined stresses increased root porosity of both genotypes. In Av1, combined stress increased cortex proportion and decreased the ratio of the endodermis to phloem of roots and stems. Additionally, combined stress increased C/N in various plant organs, particularly for Av1.

The tolerance in combined Fe deficiency and waterlogging stress differed substantially among four kiwifruit species or different genotypes within the same species. Compared with Av5, Av1 is better able to uptake and transport Fe, to adjust anatomical structures of roots and stems, and to raise C/N in various plant organs, thereby performing better in tolerance to combined stress of Fe deficiency and waterlogging.