Reductions in photosynthetic activity and/or in assimilate partitioning to the fruits, negatively affecting crop yield, are frequently observed for open-field crops grown in Mediterranean regions where plants are subjected to high radiation load and low air relative humidity during a great part of the crop cycle (Aloni et al., 1990; Dinar and Rudich, 1985; Erickson and Markhart, 2001). An alternative to alleviate the effects of radiation load is the use of reflection or shading (whitening, internal or external screens). Shading conditions, creating a light regime compatible with the requirements of leaf physiological functions (Barber and Anderson, 1992), have been proven to induce a positive impact on leaf gas exchange (Gonzalez-Real and Baille, 2006; Jaimez and Rada, 2011) and on plant growth and crop yield in several crops (Baille et al., 2001; Gent, 2007; Lorenzo et al., 2003) including sweet pepper (Jaimez and Rada, 2011; Rylski and Spigelman, 1986a, 1986b). The latter, like other species, is sensitive to high temperature (Erickson and Markhart, 2001, 2002; Rylski and Spigelman, 1982) but appear to maintain their leaf net CO2 assimilation rates at temperatures as high as 33 °C in detriment of developing fruits (Erickson and Markhart, 2001).
Screenhouses seem to be a valuable alternative to reduce radiation load, because they can simultaneously act as cover and shading devices. These low-cost structures have been progressively adopted by growers in the last decade (Castellano et al., 2008) as shown by the increasing area of field-grown crops that shifted to screenhouse (Cohen et al., 1997, 2005; Kittas et al., 2012; Tanny and Cohen, 2003). In particular, the use of insect-proof nets raised much interest among growers, because they limit the use of pesticides and associated costs through implementation of Integrated Pest and Disease Management strategies favoring a production of quality (Raviv and Antignus, 2004; Reuveni et al., 1989).
Screenhouses substantially modify the radiation regime with respect to the outside conditions, both quantitatively, through reduction of photosynthetic photon flux density (PPFD) reaching the crop, and qualitatively, through changes in the ratio of diffuse-to-direct radiation (Healey et al., 1998) and the spectral distribution of solar radiation (Ehret et al., 1989; Kittas and Baille, 1999). With regard to open-field crops, the radiative microclimate under a screenhouse can lead to modifications in leaf physiological attributes and carbon allocation patterns, which in turn affects crop yield and quality (Rylski and Spigelman, 1986a, 1986b). The wide range of plant response to artificial or natural shading reported in the literature (Cockhull et al., 1992; Jaimez and Rada, 2011; Li et al., 2000; Raveh et al., 2003; Stanhill and Cohen, 2001) can be related to 1) differences in screen physical properties (e.g., porosity) and shading duration and intensity (Gent, 2007); 2) stage of plant development (Cohen et al., 2005); 3) plant density (Papadopoulos and Pararajasingham, 1997); and 4) response of leaf structural and physiological attributes to changes in local light regime (e.g., Egea et al., 2012).
A large body of studies, focusing on light acclimation of leaf physiological function, stressed the close relationship between the distribution of photosynthetic traits and the local light regime in species grown in the open field (Field and Mooney, 1986; Niinemets et al., 2004) and under greenhouses (Acock et al., 1978; González-Real et al., 2007; Gonzalez-Real and Baille, 2000) including pepper crops (Dueck et al., 2006; González-Real et al., 2009). Changes in the light environment may induce a wide range of structural and physiological changes such that the photosynthetic capacity of uppermost leaves can reach values more than 2-fold higher than that of the bottom leaves (Niinemets, 2007).
In sweet pepper, the impact of light may interfere with the internal control induced by either the presence or removal of fruits (Gucci and Flore, 1989; Hall, 1977), which are known to exhibit strong temporal changes in their demand for assimilates at different stages of growth (González-Real et al., 2009). Therefore, knowledge about plasticity in leaf physiological function and its interaction with leaf ontogeny in response to light distribution within the crop is determinant not only for assessing crop carbon uptake, but also for characterizing the impact of greenhouse agronomic practices (e.g., pruning and defoliation strategies) on crop yield (Adams et al., 2002; Heuvelink et al., 2005).
To our knowledge, an important issue not yet fully investigated in screenhouses concerns plant acclimation to the light regime imposed by the screen. The present work deals with the effects of three screen materials having different color, porosity, and SI on leaf physiological attributes of the sweet pepper, which is a horticultural species of great economical importance in Mediterranean countries. The specific objectives of our study were 1) to characterize gas exchange of sweet pepper plants under open-field and screenhouse growing conditions; and 2) to evaluate the degree of acclimation of photosynthetic leaf attributes to different light regimes during a summer–fall growth cycle in a Mediterranean region (central Greece). Additionally, leaf water-use efficiency and light-use efficiency were analyzed.
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