ABA and Not Chilling Reduces Heat Requirement to Force Cherry Blossom after Endodormancy Release

Abstract

Models used to predict the onset of fruit tree blossom under changed climate conditions should be physiologically based as much as possible. Pure optimized phenology models carry the risk of unrealistic predictions due to a misinterpretation of metabolic processes. This was the motivation determining the relevant phases for chill and heat accumulation, which induces cherry blossom (cv. Summit). Investigations are based on 8 years of observational and analytical data, as well as on controlled experiments. For ‘Summit’ buds, to be released from endodormancy, 43 chill portions from 1 September are necessary. After endodormancy release (t1), on average on 30 November, no further chilling is required, because no correlation between chill accumulation during ecodormancy and the subsequent heat accumulation until ‘Summit’ blossom exist. The declining amount of heat, which induces cherry blossom after t1—shown in several forcing experiments—seems to be the result of the declining bud’s abscisic acid (ABA) content, up to ~50% until the beginning of ontogenetic development. Shortly after t1, when the bud’s ABA content is high, a huge amount of heat is necessary to induce cherry blossom under controlled conditions. Heat requirement reduces during ecodormancy along with the reduction in the ABA content. According to these findings, plant development during ecodormancy is suppressed by low temperatures in the orchard and a slowly declining bud’s ABA content. These results should lead to a better consideration of the ecodormancy phase in phenology models.

Metabolites That Confirm Induction and Release of Dormancy Phases in Sweet Cherry Buds

“…Although abscisic acid only shows a clear change in the temporal course after the date of endodormancy release ( Figure 5 ), it appears to be the key metabolite explaining t 1 and t 1 * [ 2 ]. The physiological importance of ABA in the regulation of seed dormancy and seed development has been documented for decades [ 30 , 31 , 32 , 33 , 34 ], in the meantime, it is accepted that ABA during winter has a growth suppressing function in fruit trees.…”

“…The mean ABA content reached a significant ( p ≤ 0.05, Table 1 ) maximum of 6.98 μg/(g DW) during the endodormancy phase and continuously decreased during ecodormancy, reaching on average 50% of the content of t 1 at the beginning of ontogenetic development (t 1 *). The physiological importance of ABA to model cherry blossom has been described in detail in [ 2 ]. Consequently, the ABA content allows to define the date of endodormancy release, because after t 1 the content continuously decreases until t 1 * and finally up to OC.…”

“…The description of the experimental site, phenological observations in the orchard, determination of ecodormancy phase (t 1 –t 1 *), sampling of sweet cherry buds and preparing for analysis has already been published in detail [ 1 , 2 , 3 , 18 ].…”

“…An important aspect in the selection of suitable metabolites is the identification of candidates which are under environmental control and/or which have a proven physiological impact in the respective dormancy phase. Such metabolites would be of particular interest for phenological modelling because they would allow to model the non-observable stages of endodormancy release (t 1 ), when the cultivar specific chilling requirement is fulfilled, and the beginning of ontogenetic development (t 1 *), the latter of which cannot be recorded by phenological observations and must start a few weeks before any visible changes in the buds are detectable [ 2 ].…”

“…For this reason, it was at first necessary to detect the timing of t 1 and t 1 * before it was possible to identify any metabolites that might be related to the induction or release of winter dormancy. For nine years, we observed under controlled conditions the timing of t 1 by means of twig samplings, and analyzed t 1 * according to changes in the flower bud’s water content in the orchard [ 2 , 3 ]. Identifying biomarkers that can reliably indicate the transition between phenological phases would substantially improve phenological modelling and could allow for developing physiologically justified model approaches, instead of statistical ones.…”