Rootstock selection for 'Swatow' Mandarin trees grown at different locations throughout the Brazilian subtropics.

Evaluating citrus rootstocks is of paramount importance in determining their suitability for a certain region and promoting resilience in orchards by increasing the genetic pool, thereby potentially contributing to a more strategic establishment of new plantings. This long-term field study (2000-2013) aimed to evaluate different rootstocks for 'Swatow' mandarin grown at two locations (Paranavaí and Londrina) in the Brazilian subtropics. Nine rootstocks were evaluated, including 'Rangpur' lime, 'Swingle' citrumelo, 'Volkamer' lemon, 'Caipira DAC' sweet orange, 'Cleopatra' and 'Sunki' mandarins, 'Trifoliate' orange, 'Carrizo', and 'Fepagro C-13' citranges. Trees were assessed for vegetative growth, yield, fruit quality, density, and yield estimates. The experimental design was a randomized block arranged in a 9 × 2 setting (rootstock × location) with 6 replicates and 4 trees per plot. 'Swatow' trees grew more vigorously in Londrina than Paranavaí, particularly for 'Cleopatra' and 'Sunki' pairings. Tree vigor was reduced with 'Trifoliate', resulting in higher tree density estimates and yield efficiency. This rootstock, along with 'Rangpur', 'Swingle', and 'Carrizo' provided superior yield to the scion. All tested rootstocks conferred good fruit quality. Fruits were larger and heavier in 'Sunki' pairings, showing higher soluble solids (SS) content, along with 'Caipira DAC', 'Trifoliate', 'Swingle', and 'Carrizo' at both locations. Our findings confirm the suitability of 'Trifoliate' orange, 'Carrizo' citrange, or 'Caipira DAC' orange rootstocks as promising candidates for 'Swatow' mandarin cultivation in humid subtropical and analogous regions. Further investigations are invoked to improve the horticultural performance of 'Swatow' mandarin trees grafted onto these rootstocks.

Revisiting Citrus Rootstocks Polyploidy as a Means to Improve Drought Resilience: Sometimes Less Is More.

Polyploid varieties have been suggested as an alternative approach to promote drought tolerance in citrus crops. In this study, we compared the responses of diploid and tetraploid Sunki 'Tropical' rootstocks to water deficit when grafted onto 'Valencia' sweet orange trees and subjected to water withholding in isolation or competition experiments under potted conditions. Our results revealed that, when grown in isolation, tetraploid rootstocks took longer to show drought symptoms, but this advantage disappeared when grown in competition under the same soil moisture conditions. The differences in drought responses were mainly associated with variations in endogenous leaf levels of abscisic acid (ABA), hydrogen peroxide (H2O2) and carbohydrates among treatments. Overall, tetraploids were more affected by drought in individual experiments, showing higher H2O2 production, and in competition experiments, rapidly increasing ABA production to regulate stomatal closure and reduce water loss through transpiration. Therefore, our results highlight the crucial importance of evaluating diploid and tetraploid rootstocks under the same soil moisture conditions to better simulate field conditions, providing important insights to improve selection strategies for more resilient citrus rootstocks.