Diel changes in nitrogen and carbon resource status and use for growth in young plants of tomato (Solanum lycopersicum)

Abstract

Background and aims: Modellers often define growth as the development of plant structures from endogenous resources, thus making a distinction between structural (W(S)) and total (W) dry biomass, the latter being the sum of W(S) and the weight of storage compounds. In this study, short-term C and N reserves were characterized experimentally (forms, organ distribution, time changes) in relation to light and nutrition signals, and organ structural growth in response to reserve levels was evaluated.

Methods: Tomato plants (Solanum lycopersicum) were grown hydroponically in a growth room with a 12-h photoperiod and an adequate supply of NO(3)(-) (3 mol m(-3)). Three experiments were carried out 18 d after sowing: [NO(3)(-)] was either maintained at 3 mol m(-3), changed to 0.02 mol m(-3) or to 0 mol m(-3). Plants were sampled periodically throughout the light/dark cycles over 24-48 h. Organ W(S) was calculated from W together with the amount of different compounds that act as C and N resources, i.e. non-structural carbohydrates and carboxylates, nitrate and free amino acids.

Key results: With adequate nutrition, carbohydrates accumulated in leaves during light periods, when photosynthesis exceeded growth needs, but decreased at night when these sugars are the main source of C for growth. At the end of the night, carbohydrates were still high enough to fuel full-rate growth, as W(S) increased at a near constant rate throughout the light/dark cycle. When nitrate levels were restricted, C reserves increased, but [NO(3)(-)] decreased progressively in stems, which contain most of the plant N reserves, and rapidly in leaves and roots. This resulted in a rapid restriction of structural growth.

Conclusions: Periodic darkness did not restrict growth because sufficient carbohydrate reserves accumulated during the light period. Structural growth, however, was very responsive to NO(3)(-) nutrition, because N reserves were mostly located in stems, which have limited nitrate reduction capacity.

Fig. 1.

Dry biomass (W, g plant⁻¹) accumulation over time in tomato plants. Growth of two independent groups of seedlings (open and closed symbols) was monitored in a growth room with the same environmental conditions: 12/12 h photoperiod, 20 °C air and solution temperature, 320 µmol m⁻² s⁻¹ PPFD, 3 mol m⁻³ NO₃⁻. Symbols and vertical bars indicate the mean and s.e. for eight (open symbols) and 12 (closed symbols) randomly sampled plants. The curve is the common exponential best fit: W = 0·001e^0·299DAS.

Fig. 2.

N nutrition effects on diel dry biomass accumulation in organs and whole tomato plants. Plants were grown with 3 mol NO₃⁻ m⁻³ (12 h photoperiod). At 18 DAS (0 h on time axis), plants were sampled at 3–4 h intervals over a 24–48 h period to determine total (W), non-structural (W_NS) and structural (W_S = W – W_NS) dry weights. At time 0, nutrition was either maintained at 3 mol NO₃⁻ m⁻³ (A-1–4; two independent experiments), or changed to 0·02 mol NO₃⁻ m⁻³ (B-1–4) or 0 mol NO₃⁻ m⁻³ (C-1–4). Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants and lines are piecewise linear regressions, as described in the Materials and Methods.

Fig. 3.

Whole-plant dry biomass (W) and organ components expressed on a percentage basis. Non-structural biomass (W_NS) is divided into free amino acids, carboxylates (malate + citrate), carbohydrates (starch + soluble sugars), NO₃⁻ and other minerals (non-structural K⁺ and SO₄²⁻). Structural biomass (W_S) was calculated as the difference between total and non-structural dry weights (W_S = W − W_NS). Tomato plants in the vegetative growth stage (18–19 DAS) with adequate nutrition (3 mol NO₃⁻ m⁻³) are compared to plants grown in a N-free solution for 48 h. Data are the mean of 6–8 plants, sampled at the end of the dark period.

Fig. 4.

Diel changes in the NO₃⁻ content (μmol N g⁻¹ d. wt) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2, with nutritional treatments as indicated. Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants.

Fig. 5.

Diel changes in the free amino acid N content (μmol N g⁻¹ d. wt) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2. N content was calculated from the free amino acid composition determined by HPLC; nutritional treatments as indicated. Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants. Free ammonium was grouped with the amino acid pool.

Fig. 6.

Diel changes in starch C content (μmol C g⁻¹ d. wt) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2, with nutritional treatments as indicated. Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants.

Fig. 7.

Diel changes in soluble-sugar C content (μmol C g⁻¹ d. wt) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2, with nutritional treatments as indicated. C content was measured by analysing glucose, fructose and sucrose. Dark periods are indicated by shading. Symbols are the means of 6–8 plants.

Fig. 8.

Diel changes in the non-structural carboxylate C content (μmol C g⁻¹ d. wt) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2, with nutritional treatments as indicated. The C content was determined by analysing malate, citrate and oxalate. Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants.

Fig. 9.

Diel changes in the free amino acid C content (μmol C g⁻¹ dw) of (A) whole tomato plants, (B) leaves, (C) stems and petioles, and (D) roots. Experimental details are the same as in Fig. 2, with nutritional treatments as indicated. C content was calculated from the free amino acid composition determined by HPLC. Dark periods are indicated by shading. Each symbol is the mean of 6–8 plants.