The impact of non-structural carbohydrates (NSC) concentration on yield in Prunus dulcis, Pistacia vera, and Juglans regia

Successful yield in orchards is the culmination of a series of events that start with plants entering dormancy with adequate energy reserves (non-structural carbohydrates; NSC). These NSC

are responsible for the maintenance of activities during dormancy and extending onto the period of activeness. Using multi-year yield information and monthly NSC content in twigs, we show

that high levels of carbohydrate in Prunus dulcis, Pistachio vera, and Juglans regia during the winter months are indeed associated with high yield, while high levels of the NSC in late

summer often correlate with low yield. An evaluation of monthly NSC level importance on yield revealed that for P. dulcis high levels in February were a good predictor of yield and that low

levels throughout summer were associated with high yield. In P. vera, high levels of NSC in December were best predictors of yield. J. regia exhibited peculiar patterns; while high

pre-budbreak reserves were associated with high yields they only played a minor role in explaining crop, the most important months for predicting yields were June and July. Results suggest

that NSC levels can serve as good predictors of orchard yield potential and should be monitored to inform orchard management.

Perennials are characterized by their persistence across seasonal cycles, including recurring periods of dormancy and activity, that may be intermittently punctuated by biotic and abiotic

disturbances. As such, they must accommodate both short and long-term fluctuations in energy supply and demand1,2,3,4. The nonstructural carbohydrates (NSC) reserve pool in trees, mostly

consisting of soluble sugars and starch, constitutes the primary resource-supply for energetic disparities3,4. Short-term variability typically results from the day/night cycle wherein

metabolic processes continuously draw energy from NSC reserves which are all the while replenished by the daily photosynthetic activity5. Long-term variability can result from either periods

of drought, forcing plants to keep stomata closed (days–weeks), or from seasonal shifts like dormancy when photosynthetic activity is relatively absent (summer–winter). Thus, the survival

of perennial species depends on their ability to accumulate adequate NSC reserves to meet resource demands during any period in which photosynthetic output is absent or lacking.

Furthermore, not only are sufficient NSC reserves critical for supplementing energy deficiencies but a growing body of evidence suggests that whole-tree NSC reserve levels/status play an

important role in sustaining and synchronizing certain phenological progressions. NSC reserves, specifically those present when approaching the end of the season/dormancy, can affect all

aspects of tree physiology including effective bloom, spring growth, and ultimately yield. The level of NSC in twigs, in particular, seems to be the most variable yet the most indicative of

whole tree storage status3 and is of major importance in terms of energy supply for flower development and during the initial phases of vegetative bud growth in species like Prunus dulcis,

Pistacia vera, and Juglans regia1,6. Additionally, there is increasing evidence suggesting that conversions between NSC forms, soluble sugars and starches, may potentially serve as a

‘dormancy clock’6. Hence, the timing and synchrony of bloom is strongly impacted by disturbances especially to the NSC accumulation and dispersion that flank dormancy7,8.

Surprisingly, despite the importance of NSC activity in providing protection against adverse weather conditions through dormancy and in influencing the timing and synchrony of bloom, little

attention has been given to how trees physiologically prepare for this quintessential period of quiescence6,7,9. The amount of reserves needed to maintain dormancy and a healthy growth

resumption (bloom/leafing) can be variable and not easily predicted. The build-up of pre-dormancy reserves may be subject to changing abiotic and biotic conditions, growth, and reproductive

activity during the active season3 especially in alternate bearing species like P. vera. Additionally, not only is the NSC reservoir contingent upon the active season but the length and

conditions of the dormant period itself can also vary from year-to-year, further affecting the amount of reserves readily available to sustain phenological transitions. The unpredictable

nature of the local climate combined with the selection for yield maximization most likely enforce the need to store more NSC reserves in domesticated plants than what is required for the

average dormancy period in most undomesticated perennials10,11. While the accumulation of reserves is often seen as a byproduct of an excess of carbohydrates, mounting evidence suggests that

it may actually be a sink that actively competes with growth and reproduction rather than merely a passive process12,13. To their undomesticated counterparts, this ‘excess’ might provide a

competitive advantage in which long-term survival is promoted over current vegetative growth and reproductive capacity (yield). However, in domesticated fruit and nut species this may

instead shift to promoting short-term gains in reproductive capacity in lieu of long-term NSC reserve formation. This, in turn, potentially makes selected varieties potentially more

susceptible to the negative impacts of unexpected changes in dormancy conditions and reduces their resilience to additional stresses. Finally, as a healthy and synchronous bloom is a

prerequisite for pollination and fruit set, any changes to NSC content and its forms as affected by weather, biological stress or management can result in significant yield variation.

Furthermore, since NSC levels and their form can affect a range of physiological activities, it is important to ask if and when NSC content has the greatest impact on tree productivity and

if it is always better to assure high NSC content to generate high yields. Therefore, to answer these questions, we used multi-year observations of NSC content in twigs of P. dulcis, P.

vera, and J. regia and combined them with reported yields for over 300 orchards located across the Central Valley, CA, USA.

Using a Citizen Science approach, growers across the entire Central Valley of California sent samples of current-season twigs of Prunus dulcis (Mill. D.A Webb), Pistacia vera L. and Juglans

regia L. (Fig. 1). The study complies with local and national guidelines. The carbohydrate data set used in this study spans from September 2016 to August 2019 with yield data for the

2017–2019 period. Out of over 590 orchards participating in the NSC study, we selected the orchards from which growers shared yield information for at least one year during the 2017–2019

period. This resulted in 132 P. dulcis, 122 P. vera, and 84 J. regia orchards used in the presented analysis. We encouraged growers to collect samples once a month, however frequency and

participation level varied over time and therefore the data sets varied from month to month.

Geographical location of orchards used in the analysis. Figure was generated using R version 3.6.3 with packages ggplot2 and ggmap (Google Maps).

Specific details of sample collection and handling were described previously1. Briefly, a unified protocol for sample collection required that one current season twig from three trees per

orchard be cut at the base where the current season’s wood met last year’s wood. The bark from the lower 10 cm of the twig was removed using a razor blade. Both the bark and the wood of the

three twigs were put in a paper envelope and mailed to the laboratory for NSC analysis. Buds were excluded from the samples. The integrity of the NSC content over shipping time was tested to

assure the quality of the results1. Upon arrival, samples were put in the dryer for 48 h at 75 °C. The bark and wood were chopped into small