UC Davis

Vegetative development is crucial to orchard tree growth and productivity. Vegetative bud formation and subsequent preformed and neoformed growth patterns determine tree size and architecture (Arquero and Jarvis-Shean 2017). Because flower buds are derived from vegetative buds, the number and distribution of vegetative buds are also a major determinant in tree crop (Lamp et al. 2001). The failure or significant delay of vegetative bud development and growth weakens tree productivity in the current year through the decreased availability of new shoots and leaves for photosynthesis. Failures affect tree productivity in the following years through decreased flower-bearing wood and thus decreased potential yields. In temperate crops various types of dormancy, including paradormancy, endodormancy and ecodormancy, have evolved to suppress bud development and so make these crop species less vulnerable to adverse environments. Different cultivars have different chill and heat requirements for successful transition through dormancy. With ongoing climate change, these differences have made some cultivars more vulnerable to subsequent bud survival and growth. Such bud “pushing” failures have a range of possible causes, including virus and bacterial infections, nutrient deficiencies and insufficient winter vernalization to overcome endodormancy (Gradziel and Fresnedo-Ramírez 2019). Endodormancy is defined as growth suppression by physiological factors within the plant vegetative buds, even in favorable environments (Alonso et al. 2005). Genetic bud-failures include those associated with specific cultivars and in particular Noninfectious Bud Failure (NBF) in Nonpareil and Carmel, and Environmental Bud Failure (EBF) in Monterey and Bennett-Hickman (Fresnedo-Ramírez et al. 2019). NBF and EBF both result in bud-failures but differ in their induction, developmental timelines, and ultimate cause of collapse. Understanding these differences is important for both diagnosis and management.Genetic control of Noninfectious Bud Failure has been demonstrated in crossing studies with almond and peach (Gradziel and Fresnedo-Ramírez 2019; Gradziel and Shackel 2021). Noninfectious bud-failure does not result from a genetic change but rather a change in the state of a “dormancy” gene; in effect, this gene is turned off at the wrong time and this change is irreversible once a certain genetic “age” is achieved (Fresnedo-Ramírez 2017; Gradziel and Shackel 2021). Results from earlier studies (Kester et al. 2005) support the current working model that this gene is also involved in the proper functioning of a hypothesized paradormancy in almond during late summer, and that this is the initial induction or trigger of the disorder. In contrast to endodormancy, paradormancy refers to bud dormancy caused by a signal from a structure other than the buds and is often associated with apical dominance (Kester et al. 2005).A critical diagnostic for NBF is that vegetative buds are already dead (necrotic and brown at the core) going into winter dormancy in the fall, further indicating that the induction occurred during earlier growth. Mechanisms for controlling gene action without changing gene identity are known as epigenetic mechanisms and include changes in gene methylation (D’Amico- Willman et al. 2021a; D’Amico-Willman et al. 2021b), chromosome (telomere) structure (D’Amico-Willman et al. 2021c), micro-RNA composition as well as several still poorly understood processes (Gradziel and Fresnedo-Ramírez 2019).In contrast, the greater site, source and year-to-year variability in Environmental Bud Failure (EBF) suggests that while it is associated with certain highly susceptible cultivars, it is strongly influenced by environmental factors such as diseases and other stresses during the previous growing seasons as well as environmental conditions during dormancy. Environmental bud-failure is activated at some time between fall dormancy and bud-pushing the following spring, but, like NBF, the specific time (and so developmental stage) of failure has not been determined. To better characterize the time and position of such failures in bud development, useful developmental milestones for bud development prior to and during dormancy need to be identified. This study has shown that the number of leaf primordia in dormant Nonpareil as well as Monterey buds shows a uniform rate of increase throughout dormancy. This internal bud- growth pattern can be thus used to establish a developmental timeline for normal dormant-bud development as well as providing more precise estimates for the time of bud failure. While much of this work is preliminary, it opens the door to a better understanding of the ontogeny of meristem and vegetative bud development in perennial plants. Ontogenesis is defined as the development of an individual organ or anatomical feature from the earliest stage to maturity. Negron et al. (2014) have demonstrated that the relative axillary bud position on current season shoot growth was the critical determinant of subsequent shoot fate, whether flower, or vegetative shoot, or blind node. This current research demonstrates that axillary bud ontogeny, including the structure and development of meristematic tissues within individual buds, is a critical determinant of subsequent development years to decades after initial formation. Improved knowledge of such deferred fates including endodormancy, paradormancy and ecodormancy, as well as epicormic shoot induction, will lead to more effective growth management in agricultural and ecological systems, including improved diagnosis and remediation of developmental disorders.