Low Nocturnal Temperature Alters Tomato Foliar and Root Phosphorus Fractions Allocation by Reducing Soil Phosphorus Availability

Low nocturnal temperature (LNT) is a major constraint for protected tomato production in China during winter and spring, which leads to tomato phosphorus (P) deficiency symptoms. The soil P fractions reflect soil P availability. The foliar and root P fractions reflect the adaptation strategies of tomatoes to LNT. However, the relationship between plant P fractions and soil P fractions under LNT is not well understood. Therefore, we conducted a 40-day indoor incubation experiment with four nocturnal temperatures (15, 12, 9 and 6 °C). Tomato growth status, plant P fractions and soil P fractions were determined. Then, structural equation model (SEM) was used to analyze the direct and/or indirect effects of LNT on soil P fractions, plant P fractions and tomato shoot dry weight (SDW). The results showed that LNT decreased soil P availability by decreasing soil labile P and increasing soil moderately labile P. The foliar inorganic P, metabolite P, nucleic acid P and residual P were decreased under 9 and 6 °C. The root nucleic acid P and lipid P were decreased, while metabolite P was increased under 9 and 6 °C. Tomato foliar and root P fraction allocation was directly influenced by the increase in soil moderately labile P, while the decline in SDW was directly influenced by the decrease in soil labile P. In conclusion, LNT affects tomato P fractions allocation by reducing soil P availability, which limits the shoot dry matter production in tomatoes.