The coupling of light and potassium supplements optimize strawberry fruit performance through plant nitrogen uptake, assimilation and translocation to fruit

The greenhouse strawberry usually had undesirable quality in deep-winter due to inefficiency nitrogen use, so this experiment investigated if light and potassium supplements could alleviate this drawback and the mechanism beneath. The plants were subjected to two light conditions coupling with two leaf sprays, yielding 4 different groups: 1) control (NDN), 2) 10 mM KCl spray (NDK), 3) 3h supplemental light (SDN) and 4) 10 mM KCl spray + 3h supplemental light (SDK). The supplemental light enhanced leaf nitrate uptake (36.5 %), fruit nitrogen concentration (8.9 %), per fruit nitrogen accumulation (41.3 %) and single fruit mass (50.1 %); the KCl spray decreased leaf nitrogen concentration (4.0 %) but improved the activity of leaf nitrate reductase (60.0 %), nitrite reductase (12.9 %) and glutamine synthase (19.3 %), and fruit sugars concentration (19.7 %). Comparing with NDN, SDK increased both fruit size (38.6 %) and sugars content (23.8 %). The leaf transcriptomic analysis suggested the differentially expressed genes in SDK vs NDN were significantly enriched in nitrogen metabolism, alanine, aspartate and glutamate metabolism, cyanoamino acid metabolism and tryptophan metabolism based on the KEGG pathways. The key genes were screened out from these pathways and examined by qRT-PCR test, and the results confirmed that the SDK had up-regulated the leaf expression of Nitrate reductase, Ferredoxin-nitrite reductase and (NADH) glutamate synthase. Hence, the combination of supplemental light and potassium spray enhanced plant source-sink nitrogen coordination through nitrogen uptake, assimilation, translocation and key genes expression, thereby improving strawberry fruit performance in deep-winter greenhouse.