Can phenological models predict tree phenology accurately in the future? The unrevealed hurdle of endodormancy break

The onset of the growing season of trees has been earlier by 2.3 days per decade during the last 40 years in temperate Europe because of global warming. The effect of temperature on plant phenology is, however, not linear because temperature has a dual effect on bud development. On one hand, low temperatures are necessary to break bud endodormancy, and, on the other hand, higher temperatures are necessary to promote bud cell growth afterward. Different process‐based models have been developed in the last decades to predict the date of budbreak of woody species. They predict that global warming should delay or compromise endodormancy break at the species equatorward range limits leading to a delay or even impossibility to flower or set new leaves. These models are classically parameterized with flowering or budbreak dates only, with no information on the endodormancy break date because this information is very scarce. Here, we evaluated the efficiency of a set of phenological models to accurately predict the endodormancy break dates of three fruit trees. Our results show that models calibrated solely with budbreak dates usually do not accurately predict the endodormancy break date. Providing endodormancy break date for the model parameterization results in much more accurate prediction of this latter, with, however, a higher error than that on budbreak dates. Most importantly, we show that models not calibrated with endodormancy break dates can generate large discrepancies in forecasted budbreak dates when using climate scenarios as compared to models calibrated with endodormancy break dates. This discrepancy increases with mean annual temperature and is therefore the strongest after 2050 in the southernmost regions. Our results claim for the urgent need of massive measurements of endodormancy break dates in forest and fruit trees to yield more robust projections of phenological changes in a near future.     

Postharvest changes in endogenous cytokinins and cytokinin efficacy in potato tubers in relation to bud endodormancy

Using an indirect enzyme‐linked immunosorbent assay (ELISA), the effects of postharvest storage duration and temperature on endogenous cytokinins in potato ( Solanum tuberosum L. cv. Russet Burbank) tuber apical bud tissues in relation to endodormancy status were determined. Following fractionation by HPLC, a total of eight cytokinins were detected and these were: zeatin riboside‐5'‐monophosphate (ZRMP), zeatin‐ O ‐glucoside (ZOG), zeatin (Z), zeatin riboside (ZR), isopentenyl adenosine‐5'‐monophosphate (IPMP), isopentenyl adenine‐9‐glucoside (IP‐9‐G), isopentenyl adenine (IP) and isopentenyl adenosine (IPA). Regardless of postharvest storage temperature or endodormancy status, IP‐9‐G was the most abundant cytokinin detected while ZRMP and ZOG were the least abundant ones. In tubers preincubated at a growth‐permissive temperature (20°C) prior to extraction, the loss of endodormancy was preceded by significant increases in the endogenous levels of Z, ZR, IPMP and IP‐9‐G. When stored continuously at a growth‐inhibiting temperature (3°C), significant increases in ZR, IP‐9‐G and IP + IPA were observed. The total content of cytokinins increased by over 7‐fold during postharvest storage and this increase was a result of de novo biosynthesis. Dose‐response studies using IPA and ZR demonstrated a time‐dependent increase in apparent cytokinin sensitivity during postharvest storage. With the exception of IP‐9‐G, injection of any of these endogenous cytokinins resulted in the rapid and complete termination of tuber endodormancy. The significance of these results with respect to endodormancy regulation and the possible mechanisms controlling cytokinin levels in potato tubers are discussed.     

Expression analysis of phytochromes A, B and floral integrator genes during the entry and exit of grapevine-buds from endodormancy

A common molecular regulatory pathway that involves PHYA, PHYB and floral integrator genes CONSTANS (CO), FLOWERING LOCUS T (FT) and SUPRESSOR OF OVEREXPRESSION OF CO1 (SOC1) has been suggested to participate in the regulation of photoperiod dependent processes such as flowering and dormancy. In grapevines (Vitis vinifera L.), decreasing photoperiod and low temperatures trigger the transition of buds into endodormancy (ED), a process that is accompanied by drastic changes in gene expression of VvPHYA and B in leaves. To analyse the relationship of VvPHYA, VvPHYB, and grape homologues of Arabidopsis floral integrator genes VvCO, VvFT, VvMADS8, with ED, a comparative expression analysis of these genes was performed in grapevine-leaves and buds before, during and after the transition of buds into ED. The expression of all the above genes in the bud-tissue, and the fact that photoperiod regulates differently the expression of VvPHYA and B in buds than in leaves, suggests that the bud might be an autonomous or semi-autonomous organ in perceiving and transducing the photoperiod signal. On the other hand, the coordinated down-regulation of VvFT in leaves and buds during the transition of buds into ED, and its subsequent up-regulation following the application of dormancy-breaking compounds, hydrogen cyanamide (HC) and sodium azide, suggests that VvFT could play a key role in stimulating bud-growth by repressing their entry into ED.     

短时间高温处理下桃树活性氧代谢与桃芽自然休眠解除的关系

以6年生曙光油桃为试验材料,研究40 ℃、45 ℃、50 ℃ 3个梯度高温短时间处理对桃树花芽和叶芽存活率、萌芽级数、活性氧含量及其相关酶活性的影响,探讨短时间高温处理对桃芽自然休眠解除的调控效应.结果表明：随着短时间高温处理时期的延后以及处理温度的升高和处理持续时间的延长,高温处理对桃芽自然休眠的解除作用增强.11月30日处理中,40 ℃处理对桃芽自然休眠的解除具有负调控效应,其萌芽级数、·OH和 O₂^-.产生速率、H₂O₂含量、过氧化物酶（POD）和过氧化氢酶（CAT）活性均低于对照,而超氧化物歧化酶（SOD）活性高于对照;45 ℃和50 ℃处理对桃芽自然休眠的解除呈正调控效应,其萌芽级数、·OH和 O₂^-·产生速率、H₂O₂含量及POD、CAT活性与对照相比明显升高,而SOD活性显著降低.12月10日处理中,40 ℃处理对桃芽自然休眠解除的调控效应不明显,45 ℃和50 ℃处理与11月30日处理相同,但前者对桃芽自然休眠的调控效果优于后者.相关分析表明,活性氧的迅速增加可能是高温解除桃芽自然休眠的原因.     

Beyond rest and quiescence (endodormancy and ecodormancy): A novel model for quantifying plant–environment interaction in bud dormancy release

Bud dormancy of plants has traditionally been explained either by physiological growth arresting conditions in the bud or by unfavourable environmental conditions, such as non-growth-promoting low air temperatures. This conceptual dichotomy has provided the framework also for developing process-based plant phenology models. Here, we propose a novel model that in addition to covering the classical dichotomy as a special case also allows the quantification of an interaction of physiological and environmental factors. According to this plant–environment interaction suggested conceptually decades ago, rather than being unambiguous, the concept of “non-growth-promoting low air temperature” depends on the dormancy status of the plant. We parameterized the model with experimental results of growth onset for seven boreal plant species and found that based on the strength of the interaction, the species can be classified into three dormancy types, only one of which represents the traditional dichotomy. We also tested the model with four species in an independent experiment. Our study suggests that interaction of environmental and physiological factors may be involved in many such phenomena that have until now been considered simply as plant traits without any considerations of effects of the environmental factors.     

The dormancy-breaking stimuli “chilling,hypoxia and cyanamide exposure” up-regulate the expression of α-amylase genes in grapevine buds

It has been suggested that respiratory stress is involved in the mechanism underlying the dormancy-breaking effect of hydrogen cyanamide (H₂CN₂) and sodium azide in grapevine buds; indeed, reductions in oxygen levels (hypoxia) and inhibitors of respiration promote bud-break in grapevines. In this study, we showed that, hypoxia increased starch hydrolysis soluble sugar consumption and up-regulated the expression of α-amylase genes (Vvα-AMYs) in grapevine buds, suggesting that these biochemical changes induced by hypoxia, may play a relevant role in the release of buds from endodormancy (ED). Three of the four Vvα-AMY genes that are expressed in grapevine buds were up-regulated by hypoxia and a correlation between changes in sugar content and level of Vvα-AMY gene expression during the hypoxia treatment was found, suggesting that soluble sugars mediate the effect of hypoxia on Vvα-AMY gene expression. Exogenous applications of soluble sugars and sugar analogs confirmed this finding and revealed that osmotic stress induces the expression of Vvα-AMY1 and Vvα-AMY3 and that soluble sugars induces Vvα-AMY2 and Vvα-AMY4 gene expression. Interestingly, the plant hormone gibberellic acid (GA₃) induced the expression of Vvα-AMY3 and Vvα-AMY4 genes, while dormancy breaking stimuli, chilling and cyanamide exposure, mainly induced the expression of Vvα-AMY1 and Vvα-AMY2 genes, suggesting that these two α-amylase genes might be involved in the release of grapevine buds from the ED.