Katherine Jarvis-Shean, UCCE Orchard Advisor, Sacramento, Solano and Yolo Counties
Maciej A. Zwieniecki, Dept Plant Sciences, UC Davis

 

Spring is often a time of unknowns and speculation. Did we get enough chill this winter? Will we have strong budbreak this spring? What are yields going to look like this fall? One relatively new area of research, carbohydrate dynamics, is shedding light that may help answer these questions. Recent years of research by the Zwieniecki lab (the Z Lab) at UC Davis, including the Carbohydrate Observatory, have been providing exciting new insights to better explain how pistachios may be counting winter chill, when budbreak occurs, and how much pistachios will yield in a given year. While many areas remain to be investigated, this research is starting to provide insight into how management can influence carbohydrates, which in turn influence dormancy, budbreak and yield.

 

What Are Carbohydrates?

A few definitions are helpful before we dive into discussion. Non-structural carbohydrates (NSC) are carbohydrates that are not part of structures like cell walls. NSC are utilized by the tree for energy, as building blocks for cell growth, as an osmolyte to influence water dynamics, and as signals for multiple physiological activities. NSC are either in the form of sugars or starch. Sugars are the product of photosynthesis, and the building blocks of starch. Sugars are also an active part of biological cell activity and their level in cells are under strict control. Starch is the storage form of carbohydrates and can later be broken down to provide sugars.

 

How Do Carbohydrates Vary Over the Year?

An intensive sampling was conducted of carbohydrates of almonds, pistachios and walnuts in the twig, branches and trunk over the course of a year. As has been seen in other temperate trees, it was found that NSC varies with changing stages of growth or phenology, and concurrent climatic conditions. NSC decreases following bud break, reaches the lowest levels during the growing season, and then increases starting mid-to-late summer to reach maximum levels in fall or early winter (Figure 1). By following the amounts of NSC in a plant over time, we can build a better understanding of how trees are using carbohydrates for current opportunities (vegetative and fruit growth) or future challenges (dormancy, defense against pathogens and other stressors).

 

Carbohydrate Dynamics Predict Bloom

Exactly how trees track the accumulated experience of winter cold and spring heat to “know” it’s time to break dormancy in the spring has remained somewhat mysterious. The Carbohydrate Observatory has found that in almonds, pistachios, and walnuts, shortly before bud break, there is a surge in starch and a dip in sugar concentration. The Z Lab has used this knowledge to create a model for bloom timing, based on fall and winter carbohydrate and temperature dynamics. This bloom prediction model integrates some important aspects about how plants balance sugar and starch concentrations. When it is warm, trees turn sugars into starch, and when it’s cold, trees turn starch into sugar. For trees to keep sugar levels in an optimum range, they adjust the concentration of the enzymes responsible for this starch synthesis and degradation. Because starch synthesis is very temperature sensitive, but starch degradation is not, trees can quickly respond to too much sugar at warm temperatures but can’t respond as quickly to too little sugar. When conditions warm up in the spring, starch synthesis quickly takes off, pulling sugars out of circulation, resulting in a dip in sugar. This dip in sugar and upsurge in starch is predictive of (and may even trigger) bud break.

Because of the different temperatures sensitivities of the different enzymes, cold winters, somewhat counter-intuitively, would amplify accumulation of starch synthesis enzymes, resulting in less warm time necessary in the spring to trigger a sharp sugar drop and bloom. Warmer winters would downregulate starch synthesis, requiring more warmth than normal in the spring to achieve low sugar levels. By integrating this knowledge of the principles of carbohydrate dynamics and specific thresholds and ranges learned from the Carbohydrate Observatory, pistachio budbreak was predicted within 7 days on average (Sperling et al 2021). While this may not be accurate enough for management decisions, it’s close enough to support integrating these carbohydrate dynamics into our understanding of how trees count the passing of winter and spring.

Using this model, they could then extrapolate the impacts of sugar concentrations going into winter. Going into winter with higher sugars, a metric of having built up higher NSC reserves over the growing season, results in earlier bloom, functioning almost like having experienced more chill. Lower sugars results in later bloom. This is supported by other recent research (Amico Roxas et al, 2021), that found that early defoliation in the fall decreased NSC going into winter and delayed budbreak in the spring, whereas girdling branches in October (which keeps more carbohydrates in the shoots) moved budbreak earlier.

 

Carbohydrates and Yield

Because carbohydrates are both the energy currency of plants and are used to make structures like cell walls, they are critical to growing pistachio fruit during the summer. NSC concentrations stay low during the growing season, as the carbohydrates made by photosynthesis get channeled into growing the crop, and sometimes into growing vegetation. New research is showing this interplay of carbohydrate sinks, photosynthesis and nitrogen demands may help explain alternate bearing habits of pistachio. UCCE Orchard Specialist Giulia Marino has recently looked into these dynamics by either partially defoliating bearing branches to reduce carbohydrate sources, or partially stripped off growing fruit to reduce carbohydrate sinks. She found that branches with a lot of leaves relative to the number of fruits actually increased photosynthesis during kernel fill, presumably in response to the strong carbohydrate demand of nearby fruit. However, branches with a lot of fruit set decreased photosynthesis after kernel fill. This may seem counter-intuitive (shouldn’t they ramp up photosynthesis even more?), until they found that nitrogen in the leaves of those branches was decreasing at the same time that photosynthesis was dropping. This is likely because nitrogen was remobilized from the leaves to the kernels, hindering the nitrogen-related components of the photosynthetic process. Why does this concern us, from a production standpoint? Because this change would then result in lower carbohydrate availability for buds being created for the following year’s crop, leading to alternate bearing.

Relatedly, Dr Zwieniecki, looking across almonds, pistachios and walnuts data from the Carbohydrate Observatory and yields provided by growers, sought to see if there was a relationship between NSC in different months of the year and yield (Zwieniecki et al 2023). He found that in pistachios there’s a strong, consistent correlation between NSC in wood and bark in the fall and winter (October through March) with yield the following year, particularly the starch component of total NSC,. This relationship was most pronounced in December, shortly after leaf drop. The higher the starch in the wood of pistachio twigs, the higher the yield turned out to be the following year. Given Marino’s findings of how carbohydrates relate to alternate bearing, this high starch-high yield relationship is likely in part a matter of correlation – following a low set “off” year, Marino’s research indicates you’d go into winter with more female flower buds and more carbohydrates. However, higher winter NSC was also found to be related to higher yields in almond and walnut, which don’t have this alternate bearing complication. This indicates that yields are likely also higher following a high carbohydrate winter because there’s more gas in the carbohydrate tank to fuel the growth needs of the crops the following season.

 

What’s this all mean for production?

The relationship between chill, heat and carbohydrates helps us understand why warm winter temperatures, and warmed wood and buds in low fog winters lead to delayed and protracted budbreak. This also explains why chemical sprays that interfere with respiration (cells turning sugar into energy) and reflectants (e.g. kaolin clay) that keep wood temperature lower could help compensate for lower chill. More work is needed to help fine tune the use of this knowledge, say to know how warm temperatures need to be to make reflectants worth the expense, or when the optimal time is for spraying dormancy breaking treatments.

The strong relationship between high NSC going into winter, strong yields and normal bloom timing suggest that late season management that helps send orchards into dormancy with higher amounts of NSC leads to positive outcomes of budbreak timing and yield. Irrigation and foliar disease management that can keep leaves healthy and producing new sugars well into October should benefit this NSC accumulation. Stresses that lead to early defoliation would have the opposite effect. The next step in research is to look into other factors can influence this relationship (Variety or rootstock selection? Nutrient management to keep leaves healthy and photosynthesizing?), and whether they have enough influence to merit the expense.