A review of productivity in strawberry: do the plants need larger canopies, more flowers, or higher CO2 assimilation for higher yields?

Productivity in strawberry (Fragaria xananassa Duch.) is dependent on the production of new crowns, leaves and inflorescences, with fruit growth dependent on CO2 assimilation in the leaves. The yields of new cultivars peaked in the 1970s and 1980s in many locations, possibly because of the focus of breeding on fruit quality and other plant traits and the loss of diversity across commercial breeding populations. It is not known if higher yields are related to higher plant growth or higher CO2 assimilation in the leaves. This review assessed whether cultivars need larger canopies, more flowers, or higher CO2 assimilation for higher yields. There were moderate to strong relationships between plant growth or yield, and net CO2 assimilation per leaf area (P < 0.05; R2 = 0.42 to 0.84) or the stability of net CO2 assimilation (P = 0.052, R2 = 0.48) across cultivars and environments. Carbon dioxide enrichment increased mean (± SE) net CO2 assimilation by 88 ± 40% compared with plants at ambient and increased yield by 112 ± 73%. There was a linear decrease in yield as light levels decreased below full sun (P < 0.001, R2 = 0.66), and linear decreases in yield the longer two cultivars were grown under 20% full sun (P = 0.002, R2 = 0.84; P < 0.001, R2 = 0.86). Daily gross primary productivity (GPP, mol CO2 per m2) estimated by measuring the changes in the concentration of CO2 above a crop was still increasing when the PAR (photosynthetic active radiation) was above 2,000 µmol per m2 per s (rectangular hyperbola models, R2s = 0.44 or 0.72). There were linear decreases in yield as the number of leaves (P < 0.05, R2 = 0.70 to 0.90), crowns (P < 0.001, R2 = 0.82) or flowers (P < 0.05, R2 = 0.51) were removed from plants. There were moderate to strong phenotypic correlations (RP > 0.50, N = 47 studies) and weak correlations (RP < 0.50, N = 69 studies) between yield and vegetative growth and flowering. Estimate of broad- and narrow-sense heritability for canopy growth and yield are low to moderate (H2 and h2 = 0.15 to 0.40). Cultivars produce high yields in different ways, suggesting that breeders consider the relationship between yield and plant architecture when developing cultivars. Initial experiments should include data on the number of leaves, crowns and inflorescences per plant. Subsequent experiments should include data on leaf area index (LAI) and/or LAD (leaf area density) using image analysis technology. More research is required to determine the value of measuring net CO2 assimilation per leaf area, with current technology time-consuming and the results variable. The use of image analyses to characterise the canopy will assist the development of high-yielding cultivars. Genome-wide association (GWA) and genomic prediction (GP) will also accelerate the identification of high-yielding populations and individuals.