Species-specific variation in the importance of the spectral quality gradient in canopies as a signal for photosynthetic resource partitioning

Abstract

Background and aims: Plants adjust the distribution of photosynthetic capacity and chlorophyll to canopy density. The importance of the gradient in the red : far-red ratio (R : FR) relative to the irradiance gradient was studied for its perception with respect to this partitioning of photosynthetic resources. Whether the relative importance of these two signals varied between six species of different growth habit (Phaseolus vulgaris, Lysimachia vulgaris, Hedera helix, Ficus benjamina, Carex acutiformis and Brachypodium pinnatum) was investigated further.

Methods: Single leaves of plants were shaded in daylight by a spectrally neutral filter or a leaf. In another experiment, leaves were treated with supplemental FR. In most cases, treatment effects were evaluated after 2 weeks.

Key results: Nitrogen and photosynthetic capacity (Amax) per leaf area, parameters pertaining to between-leaf resource partitioning, were strongly reduced in neutral shade but not additionally by spectral leaf shade. Supplemental FR reduced these parameters also, except in Carex. Acceleration of induction of senescence was observed in spectral leaf shade in primary bean leaves. Amax per unit chlorophyll, a parameter pertaining to within-leaf resource partitioning, was reduced in neutral shade, but not in spectral leaf shade or supplemental FR.

Conclusions: Signalling mechanisms associated with perception of the R : FR gradient in canopies were less important than those associated with the irradiance gradient for between-leaf and within-leaf partitioning of photosynthetic resources. The relative importance of the signals differed between species because Carex was the only species for which no indications were found for an involvement of the spectral gradient in perception of canopy density.

Fig. 1.

The effect of shading one leaf by a spectrally neutral filter or a tobacco leaf on parameters pertaining to between-leaf resource partitioning and within-leaf chloroplast organization. Both shading devices had the same PAR transmittance. Six species were compared: Phaseolus vulgaris (Pv), Lysimachia vulgaris (Lv), Hedera helix (Hh), Ficus benjamina (Fb), Brachypodium pinnatum (Bp) and Carex acutiformis (Ca). Parameters (means ± s.e.) are: leaf mass per area (LMA), nitrogen per leaf mass (N_LM), nitrogen per leaf area (N_LA), photosynthetic capacity per area (A_max) and per chlorophyll (A_max/chl), total chlorophyll per area and chlorophyll a/b ratio. The upper row of statistical notations refers to the results of a one-way ANOVA on the three treatment means per species. The lower row refers to the significance of the differences between the two shading treatments based on a Student's t-test performed on data expressed relative to unshaded control leaves on the same plant. ^*P < 0·05; ^**P < 0·01; ^***P < 0·001; n.s., not significant.

Fig. 2.

Effect of shading one leaf of the pair of primary Phaseolus vulgaris leaves by a tobacco leaf and a spectrally neutral screen with the same PAR transmittance on the time course of chlorophyll per area (means ± s.e.). Chlorophyll was measured non-destructively on the same leaves until abscission.

Fig. 3.

The effect of supplemental FR applied on a 3-cm-diameter spot of a leaf on parameters pertaining to between-leaf resource partitioning and within-leaf chloroplast organization. Six species were compared: Phaseolus vulgaris (Pv), Lysimachia vulgaris (Lv), Hedera helix (Hh), Ficus benjamina (Fb), Brachypodium pinnatum (Bp) and Carex acutiformis (Ca). Parameters (means ± s.e.) are: leaf mass per area (LMA), nitrogen per leaf mass (N_LM), nitrogen per leaf area (N_LA), photosynthetic capacity per area (A_max) and per chlorophyll (A_max/chl), total chlorophyll per area and chlorophyll a/b ratio. Differences between means were tested for significance with a t-test, for notation see legend to Fig. 1.