ASHS 2025 Conference

Photosynthetic light response (Pn–PAR) curves provide insights into optimizing light use efficiency and plant productivity while supporting decision-making on canopy management, such as plant spacing and pruning. In strawberry (Fragaria ×ananassa), previous Pn–PAR studies have primarily focused on mature leaves but ignored how photosynthetic capacity varies with leaf and plant age. To fill this knowledge gap, we examined leaf and plant age-related changes in Pn–PAR of strawberry plants. A field experiment was conducted with ‘Florida Brilliance’ short-day strawberry during the 2023–2024 growing season in West Central Florida. We determined Pn–PAR at three development stages: early (21 Nov.), mid (12 Dec.), and late (1 Feb.) growth stages. Leaves were classified into three categories based on leaf age: young (first fully expanded leaf), mature (fully developed leaf), and old (senescing leaf). Measurements were made using a portable infrared gas analyzer at ten photosynthetically active radiation (PAR) levels ranging from 0 to 2,000 μmol m⁻² s⁻¹ at a constant CO₂ concentration of 400 μmol m⁻² s⁻¹. During the early growth stage, the light-saturated photosynthetic rate (Pmax) was highest in mature leaves, followed by old and young leaves (18.17, 16.33, and 15.48 µmol CO₂·m⁻²·s⁻¹, respectively). Despite the relatively low Pmax, young leaves showed efficient low-light photosynthesis with a notable quantum yield (QY) of 0.0685 mol·mol⁻¹, trailing just behind mature leaves (0.0768 mol·mol⁻¹). During the mid-growth stage, mature leaves had the highest Pmax, followed by young and old leaves (16.38, 15.06, and 9.86 µmol CO₂·m⁻²·s⁻¹, respectively), with corresponding QY values of 0.0574, 0.0588, and 0.0365 mol·mol⁻¹, respectively. During the late growth stage, Pmax remained highest in the mature leaves, followed by young and old leaves (14.83, 13.43, and 6.80 µmol CO₂·m⁻²·s⁻¹, respectively), with corresponding QY values of 0.0574, 0.0588, and 0.0365, respectively. The results show that young leaves achieve efficient photosynthesis under low light, as indicated by their consistently high QY values across all stages, while old leaves exhibit reduced efficiency with both lower Pmax and QY. These results reveal that light use efficiency is highly dependent on both leaf and plant age, with the greatest senescence-associated decline occurring in old leaves at the late growth stage. Optimizing light use efficiency in strawberry plants, whether through light intensity control in indoor production or canopy management in open fields, must account for the leaf- and plant-age-dependent Pn–PAR relationship.

The water balance of fleshy fruit is heavily influenced by fruit transpiration. Transpiration is driven by the vapor pressure gradient between the fruit and the atmosphere. Stomatal closure is the main form of resistance to water loss. Thus, stomatal density (the number of stomata per unit area) and function are critical for regulating transpiration. In Northern Highbush blueberry (NHB, Vaccinium corymbosum L.), transpiration rates decline as the fruit develops. However, these dynamics remain unknown in Southern Highbush blueberry (SHB, Vaccinium corymbosum L. interspecific hybrids). This study examines the relationship between stomatal density, stomata function, and fruit transpiration rates in SHB. Three SHB cultivars were analyzed: ‘Jewel’, ‘Sweetcrisp’, and ‘Keecrisp’. Fruits were sampled weekly between petal fall and the ripe stage. Stomatal imprints were collected from six fruit regions: calyx basin, calyx, distal (calyx) end, distal middle, proximal middle, and proximal (pedicel) end. Stomatal density and distribution were quantified using StoManager, an artificial intelligence tool that uses convolutional neural networks to count and measure stomata in micrographs. Stomata density varied by genotype and fruit region. Distal regions exhibited the highest stomata densities in all varieties. No stomata were observed in the proximal middle or proximal (pedicel) end for either genotype. Fruit transpiration rates were measured using an infrared gas analyzer equipped with a custom-built chamber. Results indicated a progressive decline in transpiration rates as the fruit matured. The results suggest that SHB and NHB exhibit similar stomata morphology and transpiration patterns during fruit development.

Recent studies show that far-red photons (FR; 700–750 nm), when combined with photosynthetically active radiation (PAR; 400–700 nm), can drive canopy-level photosynthesis as effectively as PAR alone. This has prompted suggestions to redefine PAR as extended PAR (ePAR; 400–750 nm). However, few studies have evaluated whether FR and PAR photons produce equivalent photosynthetic rates at the leaf level. We investigated whether photosynthesis is maintained under increasing FR substitution at equal total photon flux (1000 µmol·m⁻²·s⁻¹). Five crop species (apple, blueberry, corn, strawberry, and tomato) were grown under natural field conditions (tomato in a hoop house) and sampled for leaf gas exchange using A/Ci curves under three light spectra: 0%, 15%, and 30% FR substitution. Leaf transmittance of FR photons was 3–11 times greater than PAR across species, indicating reduced FR absorption compared to PAR photons. Nonetheless, maximum photosynthetic rates were similar across treatments for all species. For most species, FR substitution did not affect Vc,max or Jmax, indicating that rubisco activity and electron transport capacity remained stable. However, blueberry showed declines in both parameters with increasing FR, while corn exhibited increased Jmax under FR substitution. Despite reduced FR absorption, photosynthetic performance was largely unchanged with up to 30% FR substitution. These results support the inclusion of FR photons in the PAR definition and reinforce the relevance of ePAR in both natural and controlled environments.

Light attenuation effects on productivity, yield and fruit quality of Cranberries under Massachusetts conditions. Brian Makeredza, Giverson Mupambi and Peter Jeranyama University of Massachusetts Cranberry Station, 1 State Bog Rd, East Wareham, MA 02538 Cranberry (Vaccinium macrocarpon Ait.) is a fruit of significant commercial importance in North America. The fruit is consumed for its high vitamin C and antioxidants such as phenols, including anthocyanins and quercetin. Radiation stress poses significant challenges to production of high-quality marketable cranberries. Elevated exposure to high visible and ultraviolet (UV) light negatively impacts physiological and stress defensive mechanisms of the fruit, made up of biochemicals such as antioxidants, pigments and organic acids. We investigated the effects of light levels and quality on the productivity of two cranberry cultivars, Stevens’ and ‘Mullica Queen’ at two different sites. A sun exposed control was compared to three light reduction treatments. The treatments were two shade net treatments that filtered 17% and 34% visible light and a particle film spray (Raynox®), that filtered UV light. Sensors were installed to log micro-climatic weather conditions. Net carbon assimilation, stomatal conductance and transpiration were measured at the green, blush and full red stage of fruit development. The ratio of carbon isotopes as described by the δ13C value were assessed to determine carbon discrimination as a stress indicator. Fruit quality parameters measured at harvest were flesh firmness, titratable acidity (TA), total soluble solids (TSS), total anthocyanins (TAcy) and fruit rot. Raynox®, did not have an effect on carbon assimilation, yield and fruit quality. Reducing visible light did not affect stomatal conductance and transpiration but decreased carbon assimilation and yield but the effects were not statistically significant in some cases. Micro-climatic conditions under shade nets were conducive to the development of cranberry fruit rot which consequently contributed to yield reduction of marketable fruit. There were no differences in carbon isotope composition indicating no differences in abiotic stress levels. Fruit firmness decreased with an increase in shading. Trends for TA and TSS were inconsistent and unclear between the cultivars, but TAcy was only impeded by reducing light up to 34% level. Keywords: Cranberry, shade nets, light levels, particle film spray, carbon isotope

The American cranberry (Vaccinium macrocarpon Aiton), commonly known as the large-fruited cranberry, is native to North America. Fruit quality remains a major challenge for cranberry growers, with the cranberry fruit rot (CFR) complex posing a significant threat. Cranberry fruit rot is associated with over a dozen taxonomically diverse fungi. In the northeastern United States, growers typically apply three to five fungicide treatments per growing season to manage CFR. Even with well-timed applications, growers often observe rot levels between 1% and 15%, or sometimes higher. When rot exceeds 12%, growers face financial penalties, and crops with more than 20% rot are often rejected by processors. Managing CFR has become increasingly difficult, particularly in high-yielding and newer cultivars. This challenge is further compounded by regulatory restrictions over the past decade on key fungicides such as chlorothalonil and mancozeb. As conventional control options decline, interest in alternative strategies continues to grow. However, the influence of microclimate, cultural practices, and plant physiological factors on CFR incidence and overall fruit quality remains poorly understood. In this study, we investigated 22 cranberry bogs in Massachusetts over a three-year period (2021–2023) to elucidate the relationships among weather variables (temperature, humidity, growing degree days [GDD], soil moisture), plant traits (fruiting upright-to-total upright ratio, leaf area index [LAI], canopy height), cultural practices (fungicide choice and frequency), and fruit quality metrics (rot incidence, yield, anthocyanin content, and firmness). Statistical analyses included year-to-year comparisons, predictor-response modeling, and time-series evaluations to identify critical periods influencing fruit quality outcomes. Key findings indicate that interannual microclimatic variation significantly affects fruit quality. Temperature influenced anthocyanin accumulation throughout the growing season, while GDD accumulation influenced fruit yield. A higher fruiting upright ratio was associated with increased yield and firmness, while greater LAI correlated with higher anthocyanin content. Although fungicide choice and application frequency varied widely among participating bogs, a marked reduction in fruit rot and an increase in yield were observed with up to four fungicide applications; however, additional applications beyond this threshold did not result in further significant improvements. Notably, bog age did not have a significant effect on fruit quality. These findings highlight the need for integrated, site-specific strategies that combine environmental monitoring with targeted interventions to improve cranberry production and support long-term sustainability.

The balance between photosynthetic carbon fixation and leaf respiration drives our expectations of crop performance. The Laisk method is a technique used to estimate the CO2 concentration in the intercellular air space when Rubisco’s oxygenation velocity is inferred to be twice its carboxylation velocity (Ci*) and leaf respiration in the light (RL). These parameters serve as the basis for understanding leaf carbon dynamics at the physiological level and can also be incorporated into global carbon models. While Ci* and RL estimates have been well characterized in model plant species, there is a paucity of information available for horticultural crops. Further, intraspecies variation in these parameters has not been explored. We used the Laisk method to estimate Ci* and RL in three apple rootstock genotypes—G65, G11 and B10. The Laisk method was conducted in the steady-state along with chlorophyll fluorescence measurements to fit the solar induced fluorescence (SIF) model for estimating rates of net assimilation (An). In addition, the Laisk method was conducted in the nonsteady-state using the Dynamic Assimilation Technique (DAT). We found there were no statistically significant differences between genotypes nor technique used for the Laisk method when estimating Ci* and RL across the three genotypes. The findings of this study suggest that Ci* and RL values are conserved within species, the SIF model accurately predicts An for Laisk method data, and the DAT can be used to reliably estimate Ci* and RL.

Southern highbush blueberry (Vaccinium corymbosum interspecific hybrids) cultivars exhibit diverse canopy architectures. Plant architecture phenes influence light interception in other plant species. However, the relationship between canopy architecture and light interception is still poorly understood in southern highbush blueberry. We evaluated 29 genotypes, including cultivars and breeding selections from the University of Florida Blueberry Breeding and Genomics program. Plants were grown under commercial conditions in Citra, FL. We employed photogrammetry, field measurements, and a plant canopy analyzer to measure canopy density, canopy volume, base angle, and plant height in four plants per genotype. We found that genotypes differed in all measured phenes. Intercepted PAR in the bottom of the canopy varied among genotypes according to their plant architecture. Taller, wider, and denser genotypes received less light in the bottom of the canopy than shorter, narrower, and more sparse ones. We used principal component analysis to assess the relative contributions of each plant architecture phene to intercepted PAR. Canopy density and volume strongly contributed to intercepted PAR. These results suggest that plant architecture could be optimized, through breeding and agronomic practices, to maximize photosynthetic light interception in southern highbush blueberry.