Dinâmica de carboidratos na dormência de macieiras cultivadas em diferentes ambientes de inverno ameno

Abstract

Apple cultivation in regions with mild winters faces increasing challenges, particularly due to the difficulty in understanding the responses associated with the onset and release of dormancy, which affect both reproductive and vegetative physiology across different crop cycles, as a result of the higher frequency of warm spells and thermal fluctuations during the winter period. Insufficient chill accumulation and the occurrence of temperature fluctuations throughout the winter alter biochemical processes. In this context, non-structural carbohydrates play a central role in plant resilience during winter and in supplying energy and carbon for growth resumption in spring; however, their dynamics under mild winter conditions remain poorly understood. Thus, the objective of this study was to evaluate carbohydrate dynamics and their relationship with dormancy and cold hardiness at two locations characterized by mild winters. From April to October, shoots (current-season shoots) and spurs of the apple cultivars Galaxy and Fuji Suprema were collected in two commercial orchards located in the municipality of Palmas, Paraná State, Brazil. The first, Lovo orchard, had an average accumulation of 56.4 chill portions, whereas the Horizonte orchard presented an average of 63.72 chill portions. Tissues were fractionated into bud, bark, wood, spur base, and spur bud, and analyzed for glucose, fructose, sorbitol, sucrose, raffinose, and starch contents. The observed carbohydrate dynamics differed from the classical pattern described for temperate climates, showing a smaller relative decrease in starch content, lower amplitude of variation in soluble carbohydrates in some years, and the absence of robust starch resynthesis at the final stage of dormancy. There was a progressive reduction in starch throughout winter, accompanied by the accumulation of soluble carbohydrates, especially raffinose, sucrose, and sorbitol, followed by a decrease near budbreak, with distinct magnitudes and temporal patterns among tissues, cultivars, environments, and years. Consistent relationships were also identified between mean temperatures prior to sampling and carbohydrate dynamics. Hexoses remained at low levels during most of the dormancy period and did not show a consistent relationship with cold hardiness under mild winter conditions, being mainly associated with the pre-budbreak phase. Significant negative correlations were observed between raffinose, sucrose, and sorbitol contents and cold hardiness (TL50), with greater strength and consistency for raffinose across all evaluated tissues. Maximum cold hardiness occurred after the peak accumulation of raffinose, whereas sucrose reached its maximum near the minimum TL50, indicating complementary roles of these sugars in the acquisition and maintenance of cold tolerance. Conversely, during winter heat waves, changes in carbohydrate levels were less pronounced and slower compared to the rapid loss of cold hardiness, indicating that under mild winter conditions, acclimation and deacclimation are more responsive to temperature than to the immediate dynamics of non-structural carbohydrates (NSC), and that these processes are not always tightly synchronized.