Dormancy Requirements Research Articles

Cold hardiness-informed budbreak reveals role of freezing temperatures and daily fluctuation in a chill accumulation model.

Fundamental questions in bud dormancy remain, including what temperatures fulfill dormancy requirements (i.e. chill accumulation). Recent studies demonstrate freezing temperatures promote chill accumulation and cold hardiness influences time to budbreak-the phenotype used for dormancy evaluations. Here we evaluated bud cold hardiness and budbreak responses of grapevines (Vitis hybrids) throughout chill accumulation under three treatments: constant (5 °C), fluctuating (-3.5 to 6.5 °C daily), and field conditions (Madison, WI, USA). Chill treatments experiencing lower temperatures promoted greater gains in cold hardiness (field>fluctuating>constant). All treatments decreased observed time to budbreak with increased chill accumulation. However, perceived treatment effectiveness changed when time to budbreak was adjusted to remove cold acclimation effects. Among three classic chill models (North Carolina, Utah, and Dynamic), none was able to correctly describe adjusted time to budbreak responses to chill accumulation. Thus, a new model is proposed that expands the range of chill accumulation temperatures to include freezing temperatures and enhances chill accumulation under fluctuating temperature conditions. Most importantly, our analysis demonstrates that adjustments for uneven acclimation change the perceived effectiveness of chill treatments. Therefore, future work in bud dormancy would benefit from simultaneously evaluating cold hardiness.

Cultivar and Maternal Plant Environment Influence Cold Stratification Requirements and Germination Rates of Vitis Species

Open-pollinated seeds from grapevines in Parlier and Davis (in California) and Geneva (in New York) were collected in 2016, 2017, and 2018. Seeds were subjected to a series of cold stratification treatments of varying lengths and germinated in incubators to compare germination rates. Two V. vinifera cultivars (Chardonnay and Cabernet Sauvignon) and three other cultivars (V. labrusca hybrids) with a similar genetic background were compared across three locations to test for maternal environmental effects on germination rates under different cold stratification durations. Two interspecific hybrids (‘Salamander’ and ‘Sovereign Rose’) and three genotypes each from two species, V. riparia and V. cinerea, were evaluated to compare germination rate variability at different cold stratification durations among and within species and hybrids. Large variability in germination rates was evident among and within grape species, with some accessions requiring little to no cold stratification, and others requiring 10 to 12 weeks. These differences could be useful for breeding grapevines with high or low dormancy requirements. The maternal plant environment impacted the seed weight and total seed germination across years and locations.

Hairy fleabane germination in response to temperature, osmotic potential, pH, salinity levels, and seed dormancy periods

AbstractHairy fleabane (Erigeron bonariensis L. Cronq.) is an invasive weed in low‐disturbance annual, perennial, and noncrop systems in the San Joaquin Valley (SJV), California. It is often observed emerging year‐round in diverse environmental conditions in the SJV. However, very little information is available on the reasons for this. The study aimed to assess the effect of temperature, water potential, salinity, pH level, and dormancy periods on hairy fleabane's germination. Tests were conducted at different day/night temperatures (5:0, 10:5, 15:10, 20:15, 25:20, and 30:25 °C), water potential (0 to –5.56 MPa), salinity (0–25 dS m–1), and pH (5–9). Germination tests were also performed on seeds the day they were collected from mature plants, and 7, 14, and 21 d later. Tests were conducted in growth chambers set to 20 °C or under ambient laboratory conditions at 22 °C. About 1% of the seeds germinated at 5 °C but maximum germination occurred at 25:20 °C. About 9% of the seeds germinated at –0.51 MPa and the estimated value for 50% germination reduction was –0.2 MPa. Seed germination had high tolerance to salinity, as 42% of the seeds germinated at 15 dS m–1. Germination did not differ across pH levels. About 50% of the seeds germinated the day they were collected, indicating that this species had a very short seed dormancy requirement. These findings could explain why hairy fleabane is observed emerging year‐round in the SJV, particularly in irrigated cropping systems.

SEED DORMANCY BREAKING OF AN ENDANGERED MEDICINAL TREE SPECIES (TAXUS BACCATA L.) USING EMBRYO CULTURE

Natural regeneration of Taxus baccata L. is constrained due to the depth of seed dormancy requirements (often taking two or more years) and low seed germination. Further, the conventional method of vegetative propagation by cuttings is associated with difficulties in rooting. Hence, for the first time, this study describes an efficient and reproducible in vitro protocol for breaking the dormancy of seeds from the endangered forest tree T. baccata L. via zygotic embryo culture. Embryos isolated from 100% sterile seeds were cultured on DCR medium that contains sucrose (30 g/l), agar (8 g/l), and activated charcoal (5 g/l), fortified with different concentrations of Plant Growth Regulators (PGRs), and held at a temperature of 25 ± 2 ºC in a growth room. The results revealed that the in vitro embryo germination percentage was mostly affected by gibberellic acid (GA3) and thidiazuron (TDZ). Among the nine treatments, the treatments with 0.5 mg/l TDZ and 1 mg/l GA3 showed the highest germination (100%), while the other treatments all increased the germination percentages significantly compared to the control (37.5%). The 1/2 DCR medium with the addition of 0.1 mg/l indole-3-butyric acid (IBA) resulted in the highest rooting ratio (94%). However, the greatest root and hypocotyl elongation (59.37 ± 3.77 and 62.75 ± 4.43 mm, respectively) occurred when seedlings were cultured on 1/2 DCR medium containing 0.5 mg/l BA. Plantlets were transplanted into plastic pots containing an autoclaved garden soil, sand, and vermiculite mixture (1:1:1) and held at a temperature of 25 ± 2 ºC in a growth room for 4 weeks before being transplanted into the greenhouse. These results indicated that the protocol developed during the current study will be useful to overcome seed dormancy and for multiplication and conservation of the species T. baccata L.

Seed dormancy in Campanulaceae: morphological and morphophysiological dormancy in six species of Hawaiian lobelioids

We determined the requirements for dormancy break/germination and kind of dormancy in seeds of the Hawaiian lobelioids Cyanea kunthiana, Delissea rhytidoperma, Lobelia grayana, L. hypoleuca, Trematolobelia grandifolia, and T. singularis. Fresh seeds were incubated in light/dark at 15/6, 20/10, and 25/15 °C, and germination monitored at two-week intervals for 14 weeks. For each species, the mean embryo length (E): seed (S) length ratio was determined for freshly matured seeds and for seeds at the time the seed coat split but before radicle emergence (germination). The embryo in seeds of all six species incubated at 25/15 °C grew inside the seed prior to germination (42%–148% increase in E:S ratio, depending on species). Seeds of L. grayana and L. hypoleuca have morphological dormancy (MD); they germinated to 82%–98% at the three temperature regimes in 4 weeks. Seeds of the other species have nondeep simple morphophysiological dormancy (MPD) and require >4 weeks for maximum germination to occur. Our results add to the growing body of knowledge about the kind (class) of seed dormancy in Campanulaceae, which suggests that seeds of members of this family have either MD or MPD and embryos grow at warm (≥15 °C) temperatures.

Germination behaviour of forbs native to the southwestern United States following exposure to short-term seed bank conditions (−20°C)

AbstractSeeds are increasingly being stored in seed banks for ex situ conservation, food security, and restoration reserves. Although it is hypothesized that seeds of many species can survive the dry, freezer conditions (−20°C) of seed banks for many years, shelf life is unknown for many species. In particular, changes to dormancy and germination requirements following freezer storage have not yet been studied extensively. To improve our understanding of how seed dormancy and germination respond to seed banking conditions, we evaluated seed of six restoration-priority species in the southwestern United States, which is a region of increasing restoration need. Germination tendencies of seeds that were and were not exposed to a −20°C treatment for 4–6 months were evaluated under 22 cold-moist stratification and incubation treatments to broadly assess changes to dormancy and germination requirements and speed. Direct or indirect (via interactions with stratification and/or incubation treatments) differences in dormancy breaking and germination were observed in seeds for four of the six species studied. Specifically, storage temperature accounted for differences in the final germination proportion, germination during stratification, and/or rate of germination in five of the six species. Notably, seeds of the one species that exhibit a combination of physiological and physical dormancy showed significant differences in all germination measures. However, while significant differences were found between seeds exposed to short-term seed bank storage and those that were not, these differences were small and may not be biologically meaningful, indicating that seed banking these species should not change how they are used for restoration or reintroduction purposes.

Comparing Apple and Pear Phenology and Model Performance: What Seven Decades of Observations Reveal

Based on observations for the beginning of the flowering stage of Malus domestica (apple) and Pyrus communis (pear) for the 1950–2018 period, phenological trends in north-eastern Belgium were investigated in function of temperatures during dormancy. Moreover, two different phenological models were adapted and evaluated. Median flowering dates of apple were on average 9.5 days earlier following warm dormancy periods, and 11.5 days for pear, but the relationship between bloom date and temperature was found not to be linear, suggesting delayed fulfilment of dormancy requirements due to increased temperatures during the chilling period. After warm chilling periods, an average delay of 5.0 and 10.6 days in the occurrence date of dormancy break was predicted by the phenological models while the PLSR reveals mixed signals regarding the beginning of flowering. Our results suggest overlapping chilling and forcing processes in a transition phase. Regarding the beginning of flowering, a dynamic chill model coupled to a growing degree days estimation yielded significantly lower prediction errors (on average 5.0 days) than a continuous chill-forcing model (6.0 days), at 99% confidence level. Model performance was sensitive to the applied parametrization method and limitations for the application of both models outside the past temperature ranges became apparent.

Germination response of Arabian desert species to gibberellic acid and potassium nitrate seed treatment

Desert plant species commonly use seed dormancy to prevent germination during unfavorable environmental conditions and thus increase the probability of seedling survival. Seed dormancy presents a challenge for restoration ecology, particularly in desert species for which our knowledge of dormancy regulation is limited. In the present study the effect of gibberellic acid (GA3) and potassium nitrate (KNO3) on seed dormancy release was investigated on eight Arabian desert species. Both treatments significantly enhanced the germination of most species tested. GA3 was more effective than KNO3 in enhancing germination percentage, reducing mean germination time and synchronizing the germination in most of the studied species. Light requirement during germination was species-specific, but in general the presence of light promoted germination more effectively when combined with KNO3 and GA3. The wide variation in dormancy and germination requirements among the tested species is indicative of distinct germination niches, which might assist their co-existence in similar habitat/environmental conditions. Seed pre-treatments that optimize germination in this habitat must therefore be assessed for individual species to improve the outcomes of ecological restoration.

Identification of Alternaria spp. as causal agent of dead flower buds disease of pear (Pyrus communis) in the Netherlands and methods for disease control

The occurrence of dead dormant flower buds is a common phenomenon of economic importance in the major pear production areas of Europe. Thus far, the cause of dead flower buds disease remained unknown. Several causes have been proposed, including insufficient tree chilling, unmet dormancy requirements, incompatibility between scion and cultivar, but also various biotic stress agents such as pathogens and pests. In this study, we tested the relationship between reduction of tree growth and dead flower bud incidences, but found no indication that growth regulation can prevent the occurrence of dead flower buds. It has been proposed that the bacterium Pseudomonas syringae pv. syringae may be the causal agent of dead flower buds of pear. However, although we found the bacterium as epiphyte and even as endophyte on and in flower buds, our findings argue that P. syringae pv. syringae is not the causal agent of dead flower buds disease in the Netherlands. In our research, Alternaria spp. were consistently found in diseased flower buds, and strong correlations between dead dormant flower buds and infection rates of flower buds with Alternaria spp. were recorded. The isolated Alternaria species were identified as A. arborescens SC and A. alternata SC. Field experiments for disease control showed that the disease may be controlled by specific fungicide applications. Thus, we propose that dead flower buds of pear in the Netherlands should be regarded as a fungal disease caused by A. alternata SC and potentially also A. arborescens SC.

Annualization of the Long Day Onion Breeding Cycle through Threshold Vernalization and Dormancy Disruption

Background: Genetic gain is a function of several parameters including generation time. Onion is a biennial crop that typically requires two full calendar years per generation or breeding cycle. Annualization of onion breeding could double the rate of genetic gain but is technically difficult because of bulb vernalization and dormancy requirements. Methods: Based on recent results that demonstrate vernalization thresholds for long-day onion, we conducted an experiment to assess whether shorter vernalization times could result in faster, uniform flowering with dormancy-broken onion bulbs. Bulbs from eight varieties were subjected to a 15% hydrogen peroxide treatment for 4 h in order to break dormancy. This treatment was followed by either 6 or 12 weeks of vernalization after the first physical signs of dormancy release. The vernalized plants were transferred to 16-h daylengths in a 20 °C greenhouse for flowering. Results: The treatments imposed in this study resulted in the completion of a full generation in less than 12 months for all eight varieties examined. As the length of vernalization at 10 °C increased, we observed greater uniformity in floral initiation and a higher proportion of scape emergence. Plants that were vernalized for 6 weeks flowered 24% earlier than those in the 12-week treatment group. However, we observed greater uniformity in scape emergence both within and among varieties when vernalized for 12 weeks. conclusions: We have developed a framework for an annual cycle breeding system that offers the ability to phenotype bulb traits. We have also identified aspects of the system that will benefit from further refinements. Overall, this annualization system may be useful for advancing generations in onion breeding and reducing the time required to produce new varieties.

Extreme winter warm event causes exceptionally early bud break for many woody species

AbstractIn many ecosystems, climate is changing faster during winter compared to other seasons. However, we lack basic information about the responses of many species to winter climate change, including extreme warm events. Extreme warm events may have particularly strong influences at the end of winter, when some species begin to break dormancy while the risk of freezing remains high. Here, we monitored bud burst of 101 temperate woody species following an extreme warm event during winter to investigate species responses to this anomalous event and determine whether functional traits predicted species responses. Following six consecutive days of extreme warm temperatures in winter, nearly half the surveyed tree and shrub species had an advanced stage of bud phenology. Responding species were most likely to be shade‐intolerant, phylogenetically related, and have weaker dormancy requirements. Based on established species response thresholds to spring temperatures in the region, species were expected to be unresponsive to warm temperatures this early in the year, yet many species broke dormancy. Species responses to this extreme winter warm event highlighted how climate change can alter well‐established species–climate associations. In an era of increasing climate change creating novel winter conditions, continued monitoring both long‐term and following extreme events is needed to understand new species–climate dynamics.

Chilling and heat requirements for flowering in apricot cultivars

Chilling requirements for breaking dormancy and heat requirements for flowering were studied in 10 apricot cultivars in the region of Belgrade during a 2‑year period. Chilling requirements were determined using the Utah chill unit (CU) model. To determine the date of dormancy breaking, 1-year-old shoots were collected weekly from December to February and kept under controlled conditions. Heat requirements were calculated as growing degree hours (GDH) accumulated from the breaking of dormancy to the beginning of flowering using a base temperature of 4°C. The cultivars studied showed a range of chilling requirements, from 834 CU in ‘Ninfa’ and ‘Goldrich’ to 1442 CU in ‘Re Umberto’. The heat requirements for flowering ranged between 2624 GDH in ‘Magyar kajszi’ and 3469 GDH in ‘Tomcot’. The latest flowering cultivar (‘Re Umberto’) finished dormancy 42 days after the earliest flowering cultivar (‘Ninfa’). The results indicate that flowering time of apricot in the studied region is influenced more by chilling requirements than by heat requirements. Identification and selection of cultivars with higher chilling requirements and later flowering is important for apricot production, especially in regions with the frequent occurrence of late spring frosts.

Micropropagation of Epilobium canum garretti (Firechalice) by Axillary Shoot Culture

Epilobium canum subsp. garrettii (firechalice) is an herbaceous wildflower with landscape potential, but its seeds are difficult to germinate because of dormancy requirements. The objective of this study was to develop a complete micropropagation procedure for a selected accession of firechalice. Single-node stem explants from the plant were examined for their ability to establish on Murashige and Skoog (MS) medium or Woody Plant Medium (WPM). Shoot explants on MS medium supplemented with 4.4 μm benzyladenine (BA) produced more than double the number of axillary shoots compared to explants on WPM (12.6 vs. 4.9 shoots, P = 0.0001). Benzyladenine, kinetin (kin), 6-(γ,γ-dimethlyallylamino)purine (2iP), thidiazuron (TDZ), and meta-topolin (mT) at concentrations of 0, 1.1, 2.2, 4.4, or 8.8 μm were evaluated for shoot proliferation efficacy. Stem explants treated with 8.8 μm of BA or mT produced the most shoots, 11 or 15, respectively. Benzyladenine, 2iP, and kin failed to affect shoot height even at the highest concentrations used, but 4.4 or 8.8 μm TDZ reduced shoot height to less than half of the heights of control shoots (3.1 vs. 1.2 cm, P = 0.0001). Firechalice shoots formed three to four roots easily without auxin added to the medium, but four to six roots formed per shoot when using up to 9 μm of indole-3-butyric acid (IBA). In contrast, 9 μm naphthaleneacetic acid (NAA) prevented root formation. When using 0–9 μm IBA for rooting, 82.5% of the rooted shoots survived transplanting. Based on these results, optimum micropropagation of firechalice may be achieved with shoots established on MS medium plus 4.4 µm BA, a concentration of 4.4 or 8.8 μm BA or mT used for shoot proliferation, and use of up to 6 µm IBA during root induction should result in >80% shoot survival after transplanting.

Effect of maturation time on dormancy and germination of Citrullus colocynthis (Cucurbitaceae) seeds from the Arabian hyper-arid deserts

BackgroundLight and temperatures of germination greatly affect germination of several Cucurbitaceae species. Environmental conditions prevailing at seed maturation time can affect dormancy and germination requirements. Citrullus colocynthis seeds have a deep dormancy. This perennial prostrate shrub grows all over the year in the arid Arabian deserts. We explored if seed dormancy and germination requirements of C. colocynthis depend on time of fruit collection. Matured seeds were collected at five different times during 2014/2015 year from a population around Dubai city. Fresh seeds were germinated at three temperature regimes in both continuous darkness and alternating 12 h light/12 h darkness. Impacts of seed storage and other treatments on germination were applied on seeds collected in March and exhibited deep dormancy.ResultsMarch collected seeds almost did not germinate in both light and dark at the three temperatures, but those of the other collections responded differently to both light and temperatures. At the lowest temperatures, seeds of all collections did not germinate in light, but those of June, October and December collections germinated in dark. There were negative correlations between final germination and seed length, width, mass and coat thickness. Physical scarification, water soaking and seed storage did not break dormancy of March collection.ConclusionsGermination of C. colocynthis is very sensitive to light and incubation temperature as well as to the environmental conditions associated with the time of seed maturation. It is important to investigate the effects of environmental factors prevailing during seed maturation under controlled conditions to understand exact reasons for unusual seed dormancy and germination requirements of C. colocynthis, which seems to be very sensitive to maternal environment.

Dormancy and Germination Requirements of Five Species From Brassicaceae

Bu calismada Uludag alpin bolgede yetisen Brassicaceae familyasina ait Thlaspi papillosum Boiss. & Huet , Thlaspi lilacinum Boiss., Draba brunifolia ssp. olympica Sibth. ex DC. , Aubrieta olympica Boiss. , Iberis spruneri Jord. turlerinin cimlenme gereksinimleri arastirildi. Karanlik (20°C) ve fotoperiyot (20/10 °C; 12/12 h, isik/ karanlik) kosullarinda, nemli soguklama (15 ve 30 gun, +4 °C) ve gibberellik asit (GA 3 ; 250, 500 and 1000 ppm) ile nemli soguklama ve GA 3 kombinasyonlarinin etkileri arastirildi. T.papillosum tohumlarinin dormant olmadigi fakat diger dort Brassicaceae turunun tohumlarinin nemli soguklama ve GA 3 kombinasyonlari ile kirilabilen fzyolojik dormansiye sahip olduklari tespit edildi. T. lilacinum ve D. bruniifolia turlerinde en yuksek cimlenme basarisi karanlikta ve 30 gun nemli soguklama ile 1000 ppm GA 3 kombinasyonlarinda bulundu. A. olympica tohumlari en yuksek karanlikta ve 15 gun gun nemli soguklama ile GA 3 kombinasyonlarinda cimlendi. I. spruneri cimlenmesi karanlik ve fotoperitotta 15 gun gun nemli soguklama ile GA 3 kombinasyonlarinda artti. I. spruneri ve A. olympica turlerinde nemli soguklama ve GA 3 interaksiyonu anlamli, T. lilacinum turunde sadece karanlikta anlamli ve D. bruniifolia turunde ise anlamsiz bulundu. Sonuc olarak ucu endemik olan bes Brassicaceae turunde dormansinin kirilmasi icin daha uzun sure nemli soguklamaya veya GA 3 kombinasyonlarina gerek duyuldugu, bu farkli cimlenme davranislarinin turlerin uygun olmayan cevre kosullarindan kacinabilmek icin gerekli oldugu belirlenmistir.

The Biological Flora of Coastal Dunes and Wetlands:Avicennia germinans(L.) L.

ABSTRACT Lonard, R.I.; Judd, F.W.; Summy, K.R.; DeYoe, H., and Stalter, R., 2017. The biological flora of coastal dunes and wetlands: Avicennia germinans (L.) L. Avicennia germinans (L.) L. is a pantropical, subtropical, and occasionally warm-temperate mangrove species that occurs on shorelines that have a broad horizontal tidal range. Also known as black mangrove, stands typically develop under anoxic, water-logged conditions in substrates of silt or clay. Black mangrove can tolerate salinity values ranging from 0 to 90 parts per thousand. Salt is excreted from salt glands on both leaf epidermal surfaces, and aerosol salt spray and salt crystals are frequently observed on the upper leaf epidermis. Avicennia germinans is viviparous. The embryos have no dormancy requirements, and there is no seed bank. This mangrove species has the northernmost distributional range of any mangrove species in North America. It occurs in Louisiana and northern Florida. Color-infrared photography and airborne video imagery te...

Evaluating seed viability in prairie forbs: a test of three methods

A fundamental aspect of establishing native plant species in restorations is ensuring that the seed used is viable. We test whether seed viability estimates for wild-collected seed of 4 forb species native to prairie habitat differ when using 3 methods: 1) germination, 2) tetrazolium, and 3) X-ray. Study species include Eryngium yuccifolium Michx. (button eryngo [Apiaceae]), Lespedeza capitata Michx. (roundhead lespedeza [Fabaceae]), Liatris aspera Michx. (rough prairie blazing star [Asteraceae]), and Ratibida columnifera (Nutt.) Woot. & Standl. (upright prairie coneflower [Asteracae]); multiple populations of each species are used to compare seed collected in the same year from different populations. Additionally, we test whether seed pretreatment (gibberellic acid or scarification) could improve germination estimates of viability by overcoming seed dormancy. Results show that viability estimates do not significantly differ by testing method for 3 of our 4 study species, with the exception being E. yuccifolium . We suspect that E. yuccifolium may have yielded different results for each viability testing method because its pretreatment was not enough to break dormancy, among other factors. Pretreatment yielded significantly higher viability estimates in L. capitata and in one population of L. aspera and E. yuccifolium in the germination study. These results confirm that restoration practitioners should calculate seeding rates based on viability measures on a per-species and per-accession basis. For many species, the method used to calculate viability can be determined by available equipment and expertise, but for species with unknown dormancy requirements, or those that may lose viability when stored, multiple methods may be needed.

GEOGRAPHIC TREND OF BUD HARDINESS RESPONSE IN VITIS RIPARIA

A major goal of grapevine breeding efforts for production outside of Mediterranean climates is the production of cultivars that have cold tolerance phenotypes. Typically, grapevine breeders use midwinter bud hardiness measures as the descriptive phenotype for cold tolerance. Historical practices of bud cutting and more modern day differential thermal analysis of low temperature exotherms allows for an accurate, quantitative measure of bud hardiness. In our recent studies we have examined the relationship between cold hardiness and dormancy requirements, and our results preliminarily suggest a geographic pattern associated with dormant tissue temperature response and midwinter hardiness in Vitis riparia. The data presented here represent two years of hardiness measures for different Vitis riparia and Vitis vinifera genotypes.

Tradeoffs between chilling and forcing in satisfying dormancy requirements for Pacific Northwest tree species.

Many temperate and boreal tree species have a chilling requirement, that is, they need to experience cold temperatures during fall and winter to burst bud normally in the spring. Results from trials with 11 Pacific Northwest tree species are consistent with the concept that plants can accumulate both chilling and forcing units simultaneously during the dormant season and they exhibit a tradeoff between amount of forcing and chilling. That is, the parallel model of chilling and forcing was effective in predicting budburst and well chilled plants require less forcing for bud burst than plants which have received less chilling. Genotypes differed in the shape of the possibility line which describes the quantitative tradeoff between chilling and forcing units. Plants which have an obligate chilling requirement (Douglas-fir, western hemlock, western larch, pines, and true firs) and received no or very low levels of chilling did not burst bud normally even with long photoperiods. Pacific madrone and western redcedar benefited from chilling in terms of requiring less forcing to promote bud burst but many plants burst bud normally without chilling. Equations predicting budburst were developed for each species in our trials for a portion of western North America under current climatic conditions and for 2080. Mean winter temperature was predicted to increase 3.2–5.5°C and this change resulted in earlier predicted budburst for Douglas-fir throughout much of our study area (up to 74 days earlier) but later budburst in some southern portions of its current range (up to 48 days later) as insufficient chilling is predicted to occur. Other species all had earlier predicted dates of budburst by 2080 than currently. Recent warming trends have resulted in earlier budburst for some woody plant species; however, the substantial winter warming predicted by some climate models will reduce future chilling in some locations such that budburst will not consistently occur earlier.

USING NON-PARAMETRIC REGRESSION TO MODEL DORMANCY REQUIREMENTS IN ALMONDS

Bud dormancy, by which buds must experience a certain amount of chill and heat before blooming or leafing out, affects orchard management and cultivar planting choices, and will be an important component to the effects of climate change on tree crops. Previous work has found that there is a partially compensatory relationship between chill and heat requirements in bud dormancy, as opposed to the common conception of one numeric combination of chill and heat alone resulting in bud-break. To date, this relationship has not been satisfactorily modeled for horticultural crops in a Mediterranean climate. This work aimed to address one of the first questions in defining this relationship – when do chill and heat accumulations start? Previous models have assumed starting dates based on historic conditions, or fitted these dates as model parameters. We have instead worked with a number of non-parametric approaches not previously utilized. This study used bloom timing data for Prunus dulcis ‘Nonpareil’, from three locations spanning the Central Valley of California, over ten years (1996-2005). We first used a fitted spline to help identify the most promising starting dates of chill and heat accumulations. Then, we used LOESS to fit polynomial regression functions based on local neighborhoods of data points. Using the Dynamic Model for chill accumulation starting in October, and the ASYMCUR GDH heat accumulation model starting in January, bloom timing was well modeled in 90% of the site-years. These dates differ from many found in the literature, suggesting pomologists and growers may be miscalculating when dormancy requirements have been met. Our results further support the theoretical model of a partially compensatory relationship between chill and heat towards stimulating bud-break.