**4. Discussion**

214 Artificial Photosynthesis

**C**

**1S**

**2S**

**C**

**1S**

**2S**

*C. morifolium S. wallisii*

**300** 

**300 μmol ·**

 **m-2 · s-1**

**20 μmol · m-2 · s-1**

0.2 0.4 0.6 0.8 1.0

0.0

0.2 0.4 0.6 0.8

0.0

chlorophyll fluorescence measurements in six entire leaves.

**μmol ·**

 **m-2 · s-1**

**20** 

**μmol ·**

 **m-2 · s-1** **Y(II) F Nq Y(II) F Nq**

**0 100**

*C. morifolium S. wallisii*

Y(II) F Nq

C 1S 2S C 1S 2S

images) than in shade plants (0.28, green images). After one and two stress photoperiods, the effective PSII quantum yield and the fluorescence emission decreased, moreso in shade than in sun plants, while Nq increased, moreso in shade (blue images) than in sun plants

Fig. 5. Images at 20 and 300 mol·m-2·s-1 PPFD of the effective PS II quantum yield (Y(II)), the fluorescence yield (F) and non-photochemical quenching (Nq) from a typical leaf attached to *Chrysanthemum morifolium* and *Spathiphyllum wallisii* plants, in control conditions (C) and exposed to one (1S) and two (2S) stress photoperiods (18 h, 1060 mol·m-2·s-1 PPFD, 35 ºC and low watering). The stress photoperiods were separated by 6 h night-periods at 24 ºC. Images are colour coded according to the pattern (0 to 1 x 100 range) shown below the images. The histograms show the means SE of parameters calculated from variable

Fluorescence imaging represents a simple and non-invasive tool for the early detection of effects caused by adverse factors, which affect photosynthesis causing an imbalance in the processes of excitation energy dissipation (Long et al., 1994). This technique permits us to compare, by means of imagines, the variation in these processes and to study any damage caused in the same leaf as time progresses. Usually, changes in Fv/Fm of leaves adapted to dark, which represents the maximal quantum yield of PS II (Krause & Weis, 1991), are used as an indicator of the functional state of the photosynthetic apparatus (Barbagallo et al., 2003; Krause & Jahns, 2004; Oxborough, 2004b), since this parameter, which has a value of between 0.70 and 0.85 in unstressed leaves, falls under the influence of adverse factors (Ehlert & Hincha, 2008; Havaux & Lannoye, 1985; Joshi & Mohantly, 2004; Quiles & López, 2004; Teicher et al., 2000).

Sun plants (*C. morifolium*) and shade plants (*S. wallisii*) were exposed to photoperiods with low watering, high illumination and heat. Even after two stress photoperiods no visible damage was observed in either plant species (not shown). Neither did the concentration of photosynthetic pigments or the Fv/Fm values show any significant decrease after the stress photoperiods, suggesting that chloroplasts are protected by mechanisms that dissipate excess excitation energy to prevent damage to the photosynthetic apparatus under adverse conditions. In this respect, we have reported that chlororespiration and cyclic electron flow pathways are involved in the tolerance to adverse factors in both sun and shade species (Díaz et al., 2007; Gamboa et al., 2009; Ibañez et al., 2010; Quiles, 2006; Tallón & Quiles, 2007). However, when the light response curves for the relative electron transport rate were depicted, differences were observed between control plants and those exposed to stress photoperiods, the capacity of photosynthetic electron transport being lower in plants exposed to stress photoperiods in both species. In *C. morifolium* after one or two stress photoperiods, the values were similar and the capacity of photosynthetic electron transport was approximately 22% lower than in control plants. However, in *S. wallisii* differences between plants exposed to one and two stress photoperiods were observed and the capacity of photosynthetic electron transport after one and two stress photoperiods was approximately 27 and 44%, respectively, lower than that of control plants.

The images of the fluorescence yield, the effective PS II quantum yield or photochemical efficiency and the non-photochemic quenching of fluorescence, which represents the heat dissipation in the antenna system (Müller et al., 2001), also showed significant differences, indicating that plants exposed to stress photoperiods behaved differently as regards the processes of dissipation of excitation energy, in each species. At low illumination (20 mol·m-2·s-1), fluorescence emission predominates over heat dissipation in the sun species, while the contrary occurs in the shade species, heat dissipation predominates over

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