Optimized differential thermal analysis sheds light on the effect of temperature on peach floral bud cold hardiness and transition from endo- to ecodormancy.

The greatest threat to profitable peach production is cold damage to reproductive tissues. To better understand and mitigate cold damage in peach accurate and efficient assessment of floral bud cold hardiness (Hc) is critical. Differential thermal analysis (DTA) was optimized for efficient and precise detection of low-temperature exotherms (LTE) created by the freezing of supercooled intracellular water in peach floral primordia to determine Hc weekly during the dormant season. DTA-estimated lethal temperatures (LT) were validated against the standard oxidative browning method (OB) and in situ field damage following three freezing events. Chilling (0-7.2 °C) accumulation tracked throughout the dormant season to determine DTA-related changes across dormancy phase transitions. LTEs showed rapid acclimation of 'Redhaven' peach floral buds following the first frost of the dormant season (Tmin=-6.8 °C on November 18, 2016) and maintained similar Hc levels for 45 days through maximum Hc (LT50 =-23.9 °C recorded on January 9, 2017) and until the accumulation of 868 chilling hours was reached. Following this milestone, a significant 55% loss of LTEs upon the accumulation of the first growing degree day (Tbase=7 °C) was recoded on February 7, 2017. An LTE recovery approach, pre-exposing buds to a non-freezing low temperature (-2°C) for a period of 12 h, more than doubled the number of LTEs detected for another 27 days extending DTA use for LT prediction. The results presented herein confirm that the use of DTA is efficient and accurate to determine Hc in peach floral buds, and suggest that the LTE loss in early spring may be a signature response related to the shift from endo- into ecodormancy following two environmental temperature cues, chilling satisfaction and the first heat accumulation post chilling satisfaction.

Total soil water content accounts for augmented ABA leaf concentration and stomatal regulation of split-rooted apple trees during heterogeneous soil drying.

A split-rooted containerized system was developed by approach grafting two, 1-year-old apple (Malus×domestica Borkh. cv 'Gala') trees to investigate the effect of soil moisture heterogeneity and total soil moisture content (θ(v)) on tree water relations, gas exchange, and leaf abscisic acid (ABA) concentration [ABA(leaf)]. Four irrigation treatments comprising a 2×2 factorial experiment of irrigation volume and placement were imposed over a 30-day period: control (C) [>100% of crop evapotranspiration (ET(c))] applied to both containers; PRD100 (>100% ET(c)) applied to one container only; and two treatments receiving 50% ET(c) applied to either one (PRD50) or both containers (DI50). Irrigation between PRD (partial rootzone drying) root compartments was alternated when θ(v) reached ~35% of field capacity. Maximum daily sap flow of the irrigated roots of PRD100 exceeded that of C roots throughout the experimental period. Pre-dawn water potential (Ψ(pd)) was similar between C and PRD100; however, daily water use and mid-day gas exchange of PRD100 was 30% lower. Slightly higher [ABA(leaf)] was observed in PRD100, but the effect was not significant and could not explain the observed reductions in leaf gas exchange. Both 50% ET(c) treatments had similar, but lower θ(v), Ψ(pd), and gas exchange, and higher [ABA(leaf)] than C and PRD100. Regardless of treatment, the container having the lower θ(v) of a split-rooted system correlated poorly with [ABA(leaf)], but when θ(v) of both containers or θ(v) of the container possessing the higher soil moisture was used, the relationship markedly improved. These results imply that apple canopy gas exchange and [ABA(leaf)] are responsive to the total soil water environment.