Ethephon-Mediated Bloom Delay in Peach Is Associated With Alterations in Reactive Oxygen Species, Antioxidants, and Carbohydrate Metabolism During Dormancy

Ethephon (ET) is an ethylene-based plant growth regulator (PGR) that has demonstrated greater efficacy in delaying bloom in deciduous fruit species. However, the underlying mechanisms by which ET modulates dormancy and flowering time remain obscure. This study aimed to delineate the ET-mediated modulations of reactive oxygen species (ROS), antioxidants, and carbohydrate metabolism in relation to chilling and heat requirements of “Redhaven” peach trees during dormancy. Peach trees were treated with ethephon (500ppm) in the fall (at 50% leaf fall), and floral buds were collected at regular intervals of chilling hours (CH) and growing degree hours (GDH). In the control trees, hydrogen peroxide (H2O2) levels peaked at the endodormancy release and declined thereafter; a pattern that has been ascertained in other deciduous fruit trees. However, H2O2 levels were higher and sustained for a more extended period than control in the ET-treated trees. ET also increased the activity of ROS generating (e.g., NADPH-oxidase; superoxide dismutase) and scavenging (e.g., catalase, CAT; glutathione peroxidase) enzymes during endodormancy. However, CAT activity dropped significantly just before the bud burst in the ET-treated trees. In addition, ET affected the accumulation profiles of starch and soluble sugars (hexose and sucrose); significantly reducing the sucrose and glucose levels and increasing starch levels during endodormancy. However, our study concluded that variations in ROS levels and antioxidation pathways, rather than carbohydrate metabolism, could explain the differences in bloom time between ET-treated and -untreated trees. The present study also revealed several important bud dormancy controlling factors that are subject to modulation by ethephon. These factors can serve as potential targets for developing PGRs to manipulate bloom dates in stone fruits to avoid the ever-increasing threat of spring frosts.

Assessment of Climate Change Impacts on Chilling and Forcing for the Main Fresh Fruit Regions in Portugal

Air temperature plays a major role in the growth cycle of fruit trees. Chilling and forcing are two of the main mechanisms that drive temperate fruit development, namely dormancy and active plant development. Given the strong sensitivity of these crops to air temperature and the foreseeable warming under future climates, it becomes imperative to analyze climate change impacts for fruit trees. The fruit sector in Portugal has risen significantly over the last decades, gaining increasing importance both internally and through exports. The present research assesses the impacts of climate change on the chilling and forcing for economically relevant fruit trees in Portugal, namely apples, oranges, pears, and plums. To assess temperate fruit chilling and forcing conditions, the chilling portions (CP) and growing degree-hours (GDH) were computed over Portugal, for the recent-past (1989–2005) and future (2021–2080) periods, following two anthropogenic radiative forcing scenarios (RCP4.5 and RCP8.5). Future climate data were obtained from four regional-global climate model pairs to account for model uncertainties. Bias-correction methodologies were also applied. A spatial analysis over the main regions with PDO “Protected Denomination of Origin” or PDI “Protected Geographical Indication” of origin of each fruit tree was performed. Future projections show a clear decrease in chilling for all regions and fruit types in Portugal. Nonetheless, given the current chilling values in Portugal and the relative importance of chilling accumulation for each fruit type, these changes are more significant for certain varieties of apples than for other types of fruit. Regarding forcing, the future projections highlight an increase in its values throughout the different fruit tree regions in Portugal, which should lead to earlier phenological timings. These changes may bring limitations to some of the most important Portuguese temperate fruit regions. The planning of suitable adaptation measures against these threats is critical to control the risk of exposure to climate change, thus warranting the future sustainability of the Portuguese fruit sector, which is currently of foremost relevance to the national food security and economy.

Ethylene-Mediated Modulation of Bud Phenology, Cold Hardiness, and Hormone Biosynthesis in Peach (Prunus persica)

Spring frosts exacerbated by global climate change have become a constant threat to temperate fruit production. Delaying the bloom date by plant growth regulators (PGRs) has been proposed as a practical frost avoidance strategy. Ethephon is an ethylene-releasing PGR found to delay bloom in several fruit species, yet its use is often coupled with harmful effects, limiting its applicability in commercial tree fruit production. Little information is available regarding the mechanisms by which ethephon influences blooming and bud dormancy. This study investigated the effects of fall-applied ethephon on bud phenology, cold hardiness, and hormonal balance throughout the bud dormancy cycle in peach. Our findings concluded that ethephon could alter several significant aspects of peach bud physiology, including accelerated leaf fall, extended chilling accumulation period, increased heat requirements, improved cold hardiness, and delayed bloom date. Ethephon effects on these traits were primarily dependent on its concentration and application timing, with a high concentration (500 ppm) and an early application timing (10% leaf fall) being the most effective. Endogenous ethylene levels were induced significantly in the buds when ethephon was applied at 10% versus 90% leaf fall, indicating that leaves are essential for ethephon uptake. The hormonal analysis of buds at regular intervals of chilling hours (CH) and growing degree hours (GDH) also indicated that ethephon might exert its effects through an abscisic acid (ABA)-independent way in dormant buds. Instead, our data signifies the role of jasmonic acid (JA) in mediating budburst and bloom in peach, which also appears to be influenced by ethephon treatment. Overall, this research presents a new perspective in interpreting horticultural traits in the light of biochemical and molecular data and sheds light on the potential role of JA in bud dormancy, which deserves further attention in future studies that aim at mitigating spring frosts.

Season, sowing date, and row cover influences the production of cool season vegetables in movable high tunnels

Moveable high tunnels offer the possibility of increasing the number of crops harvested from a given piece of ground in northern latitudes where there is a short growing season. In an effort to expand crop scheduling options, three leafy greens and three root vegetables were grown in the spring in a movable high tunnel, and in the fall were sown outside and the tunnel was moved over the crops in late September. The effects of seeding date and addition of row cover were further explored on fresh weight and days to harvest. Using row cover within the high tunnel increased growing degree hours (GDH) by an average of 29% in the spring and 17% in the fall over a high tunnel without row cover. Soil degree hours (SDH) in the high tunnel with row cover increased an average of 9% in the spring and 12% in the fall over the high tunnel without row cover. The addition of row cover increased yield of leafy greens and turnip by an average of 35% in spring 2018 when the outside air temperature was considerably below average. Early-seeded fall leafy greens out-yielded late-seeded by 52% due to the ability to make a second harvest. Using row cover within the high tunnel increased GDH and SDH during both spring and fall seasons and increased the yield of cool season vegetables when outside air temperatures were considerably below average.

Male meiosis in sweet cherry is constrained by the chilling and forcing phases of dormancy

Male meiosis in temperate fruit trees occurs in the anthers once a year, synchronized with the seasons. The alternation of dormant and growth cycles determines the optimum moment for the male gametophyte formation, a process sensitive to both cold and warm temperatures. This ensures pollen viability and subsequent reproduction success that guarantee fruit production. In this work, we explore how male meiosis is framed by seasonality in sweet cherry. For this purpose, the dormant phases, male meiosis and blooming dates were established in four cultivars with different flowering dates and chilling requirements over 7 years. The chilling and heat requirements for each cultivar were empirically estimated, and chilling and heat temperatures were quantified according to the Dynamic and Growing Degree Hours (GDH) models, respectively. Endodormancy was overcome approximately a fortnight earlier during the colder winters than during the milder winters. Against our initial hypothesis, these differences were not clearly reflected in the time of male meiosis. The period between chilling fulfillment and meiosis lasted several weeks, during which a high amount of GDH accumulated. Results showed that male meiosis is conditioned by endodormancy but especially by warm temperatures, during the forcing period. This differs from what has been described in other related species and creates a framework for further studies to understand the strategies of synchronizing dormancy with seasons.

High crop load and low temperature delay the onset of bud initiation in apple

The reproductive cycle of apple (Malus × domestica Borkh.) starts with the induction of floral development, however, first morphological changes within the bud appear during the following period of bud initiation. This study identifies the onset and duration of bud initiation in the apple cultivars ‘Fuji’ and ‘Gala’, characterized by biennial and non-biennial bearing behaviour, respectively, and describes the effect of crop load and heat accumulation on the temporal pattern of floral development. The onset of flower bud initiation in heavy cropping ‘Gala’ trees was delayed for 20 days compared to trees with no crop load, but the rate of initiation was not affected by crop load. Bud initiation on heavy cropping ‘Fuji’ trees was minor, whereas trees with no crop load started initiating buds 19 days earlier than those of ‘Gala’ despite the same cropping status and growing degree hours in a given year. The onset of bud initiation in ‘Fuji’ ‘off’ trees was 5 and 20 days after summer solstice, respectively, in two consecutive growing seasons, suggesting that this process is driven by heat accumulation rather than by daylength. The results indicate, that the genetic make-up of the cultivar determines the onset of bud initiation. This can be delayed by increasing crop loads and low temperatures at the beginning of the flower formation process.

Blooming under Mediterranean Climate: Estimating Cultivar-Specific Chill and Heat Requirements of Almond and Apple Trees Using a Statistical Approach

Climate change, and specifically global temperature increase, is expected to alter plant phenology. Temperate deciduous fruit trees have cultivar-specific chill and heat requirements to break dormancy and bloom. In this study, we aimed to estimate chill and heat requirements (in chill portions, CP, and growing degree hours, GDH, respectively) of 25 almond (30–36 years) and 12 apple (14–26 years) cultivars grown under a Mediterranean climate. The set included early and late blooming genotypes. Long-term phenological and temperature records were analyzed by means of partial least squares (PLS) regression. The main difference between early and late genotypes was chill requirement, ranging from 8.40 CP of early genotypes to 55.41 CP of extra-late genotypes. However, as chill requirements are quite easily attained by all almond cultivars in this study, year-to-year variations in actual blooming dates for each genotype are governed by variability of mean forcing temperatures. In contrast, different chill and heat combinations resulted in similar mean blooming dates for the studied apple cultivars. Mean temperature in both chilling and forcing phases determined their blooming time in the location studied. Overlaps and gaps between both phases were obtained. Despite some limitations, the PLS analysis has proven to be a useful tool to define both chilling and forcing phases. Nevertheless, since the delineation of these phases determine the total amount of CP and GDH, further efforts are needed to investigate the transition of these phases.

Impact of Temperature on Autumn- and Spring-initiated Inflorescence Systems within a Biennial Pruning System of Protea ‘Pink Ice’ Cut Flowers

The potential impact of increasing temperatures driven by climate change on cultivated Protea cut flower production systems is not known. This study used a biennial pruning system in Protea ‘Pink Ice’ to track the physiological and reproductive responses in comparable phenological stages, but exposed to different seasonally determined temperature conditions. Protea ‘Pink Ice’ generally initiates inflorescences terminally on the spring flush. A limited number of shoots can initiate inflorescences on the preceding autumn flush, leading to an advanced harvesting time compared with that of the spring-initiated inflorescences. In a commercial Protea orchard in Hopefield, South Africa, gas exchange, carbohydrate availability, and vegetative and reproductive growth were compared between the two shoot types in the context of seasonal temperature differences. Leaves of shoots, which initiated inflorescences on the autumn flush, generally had higher light-saturated net carbon dioxide (CO2) assimilation capacities in autumn (April–May) and spring (October–November). There is evidence of a requirement of minimum shoot diameter of 7.6 mm (four- or five-flush shoot), as measured directly above the intercalation between the terminal (uppermost mature flush) and subterminal flush, when the subsequent flush was at budbreak stage during April (autumn) and at least five flushes to be required for floral initiation in Protea ‘Pink Ice’. Spring-initiated inflorescences had a shorter developmental period (4 months) than that of autumn-initiated inflorescences (7 months) and developed into significantly smaller (width) inflorescences with a lower width and dry weight at harvest. These inflorescences were harvested on average a month later compared with autumn-initiated inflorescences. The ambient temperature during inflorescence development played a significant role in the inflorescence growth rate, affecting the time required from visible inflorescence detection to harvest. At the calculated optimum base temperature of 9 °C, autumn-initiated inflorescences required 41,010 growing degree hours (GDH), whereas spring-initiated inflorescences required 35,872 GDH from initiation to anthesis. Under future warmer growing conditions, anticipated decreased size and dry weight of inflorescences may reduce marketability and income for Protea producers.

Climate change impacts on thermal growing conditions of Portuguese grapevine varieties

Heat accumulation conditions of a collection of 44 grapevine cultivars currently grown in Portugal are assessed at very high spatial resolution (~1 km) and for 1981–2015. A Growing Degree Hours – GDH (February–October) index is used for this purpose. Three clusters of grapevine cultivars are identified, assembling varieties with close heat accumulation requirements (early, intermediate and late ripening). These clusters provide more physiologically consistent information when compared to previous studies, as non-linear plant-temperature relationships are herein taken into account. For the future climates in Portugal, ensemble mean projections under two anthropogenic-driven scenarios (RCP4.5 and RCP8.5, 2041–2070), from four EURO-CORDEX simulations, reveal a widespread increase of GDH, but with spatial heterogeneities. The spatial variability throughout Portugal is projected to decrease in GDH, with strongest increases in the coolest regions of the northeast. The typical heat accumulation conditions of each cluster are projected to gradually shift north-eastwards and to higher-elevation areas. An unprecedented level of detail for a large collection of grapevine varieties in Portugal is provided, which may promote a better planning of climate change adaptation measures in Portuguese viticulture.

Soil Heating and Secondary Plant Covers Influence Growth and Yield of Winter High Tunnel Spinach

High tunnel (HT) winter production may be limited by extreme low air temperatures, suboptimal soil temperatures, large diurnal temperature changes, and short daylengths and associated low light conditions. To determine the productivity of spinach in extreme climates, HT production trials were conducted in the fall (October to December) and winter (January to March) of 2010–12 at the Greenville Research Farm in Logan, UT (lat. 41 N. elevation 1455 m). Soil heating (±) using electric cables and secondary covers (fabric rowcovers and plastic low tunnels) were evaluated to determine combined effects on fall and winter spinach production. Soil heating significantly increased yield in all cover treatments in the Fall 2010 (F2010) trial when spinach was planted in November, but had little to no effect on plant productivity in the other three trials (more appropriate planting dates) even though it did increase soil temperature marginally. The addition of secondary covers significantly increased plant biomass and leaf area when compared with the uncovered control. Excluding the F2011 trial when spinach was planted earlier under more favorable temperature and light conditions, the use of low tunnels resulted in significantly higher spinach yields (biomass and leaf area) than when grown under fabric rowcover. In the fall, relative growth rates (RGRs) decreased exponentially regardless of whether the soil was heated or not heated or if a secondary cover was used. This response was because of the seasonal decline in light levels and temperatures. In the winter production cycle, spinach relative growth without covers was similar or increased as climatic conditions improved. For plants grown under fabric or plastic rowcovers, RGR remained more constant or decreased during the production cycle. Increased yields were possible with secondary covers as air temperatures increase more quickly in the morning, maintained optimal temperatures longer each day (higher growing degree hours), and retained trapped heat later into the evening. Statistical interaction between heating cables and secondary covers were rarely observed. Fall and winter HT spinach production increases when further protection with secondary plant covers is provided; however, supplemental soil heating is not necessary.