## 4 Discussion  
The highest outside and inside radiation were registered in May, while the lowest radiation happened from November to January. This behavior pattern is usual in South-eastern Spain, as described by other authors whose work was also developed in the area [24,25]. The use of photovoltaic panels reduces the solar radiation inside the greenhouse, increasing while the number and opacity of these panels increase. In the present study, photosynthetic active radiation (PAR) in the control treatment was higher, and PAR radiation reduction was proportionally higher than the shading surface increase in the different treatments studied (15%, 30% and 50%). This effect was also described by [PERSON] et al. [44]. According to this, [PERSON] et al. [26] claim that the annual global radiation decreases by 0.8% for each additional 1.0% of photovoltaic panels located in a roof-top greenhouse. In this study, 1.0% of an additional shadowed area in the roof-top (as an average value for 15%, 30% and 50% treatments) caused a 1.6% decrease of PAR radiation. The reduction of light striking on tomato plants decrease photosynthesis and yields [45]. Excessive shading that would produce photovoltaic panels located in the roof-top in the greenhouse in the present study reduced the total and commercial yields in the crop. The main reason for yield loss is the gap in the energy balance that occurs inside the greenhouse. This affirmation is according to other authors, which assesses that a high shading level reduces tomato yields in greenhouse crops, because of a radiation reduction [27,28,31,44,45]. Besides, the increasing % of shadow caused proportional reduction in crop yields, as described by [PERSON] et al. [30], the high shadow rate studied caused, in addition, a reduction on the early yields, delaying production at the end of the crop, as described by [PERSON] et al. [27]. Due to this, shading % higher than 30% produced a higher number of green non-commercial fruits at the last harvesting season. Results of this study shows that covering a roof-top greenhouse with a high number of photovoltaic modules excessively reduces the inside radiation to levels that affect crop yields, even in low-latitude Mediterranean countries, where radiation is not usually a limiting factor for crop development. Previous studies show that shading the whole roof-top area of the greenhouse reduces the number of produced fruits and these are the tiniest [27,46,47]. Other studies show that shading is moderated, fruit size is lowest and no yield effects are shown [24,25]. Both affirmations are a coincidence with the present study, where excessive shading decreased the number of harvested fruits, and they were also tinier. In addition, the reduction on fruit weight was highest as shading percentage increased. In Mediterranean countries, where solar radiation satisfied the plant's needs enough, roof-top greenhouse moderate shading with photovoltaic panels does not affect the production and quality parameters in a tomato crop like TSS, color, firmness and pH [24,25]. However, excessive shading  
Figure 15: Shading effects of fruit color. can affect fruit quality [1; 26]. Previous studies in the south-eastern Spanish greenhouse production area [30] showed that shading over 40% increases the firmness of the fruits and decreases the Total Solids Soluble. In the present study, shading levels over 30% decreased the TSS and increased firmness in regards to the control treatment. In the study of the intensity of the red color of the fruit, there was a reduction in regards to the treatment control of the shaded treatments. Fruit pH was not affected by shading treatments. This was also described by other authors applying moderate [24; 25] and/or high [30] shading rates. [PERSON] et al. [48] asseverate that tomato fruit quality was highly affected by the environment (radiation intensity and duration, temperature, irrigation, fertilization, etc.).