## 1 Introduction  
Grapevine is a major agricultural crop that is often exposed to variable stress conditions during its lifespan and growing cycle [1, 2]. Among other husbandry techniques, fertilization has been shown to play a key role in grapevine cultivation, since it may affect berry and vine quality [3, 4], in addition to improving disease resistance [5, 6] and tolerance to other stress factors [7].  
Calcium is an essential nutrient for plants that plays key signaling and structural roles [8]. Roots absorb this element from the soil solution, and Ca is subsequently transported to the shoots in the xylem [8]. Calcium mobility in plants is generally low, this element being chiefly accumulated in mature organs despite the high Ca requirements of actively growing plant parts such as expanding leaves, flowers or fruits [9, 10].  
The delivery of Ca to developing fruits depends on xylem transport and transpiration [11, 12], but a contribution of pedicel phloem transport has also been reported [9]. The high stomatal densities of leaves compared to fruits lead to higher transpiration and Ca accumulation rates in foliar tissues compared to fruiting organs [12].  
Although Ca is normally available in soils in sufficient amounts, the occurrence of localized Ca deficiencies (i.e., Ca-related disorders) in fruits and vegetables is a common agricultural production problem, which can lead to major economic losses at harvest and postharvest [13]. Calcium-related disorders may involve an array of alterations including the deterioration of cell walls and membranes [13], with many physiological and anatomical symptoms in plant organs as recently described for grapevine [14]. Given the limited Ca translocation from leaves to fruits during fruit growth and development [10; 15], the supply of Ca as sprays onto fruit surfaces is recommended [16; 17], as also evaluated in some studies performed with grapevine [3].  
On the other hand, Si is a nonessential element that has been found to be beneficial for improving crop yield and quality and also plant tolerance to various abiotic and biotic stress factors [18]. The effect of a Si supply on grapevine has been assessed in a few investigations [19; 20; 21], which reported positive changes in grape quality and must composition. Silicon is often provided as foliar sprays that have been shown to induce positive plant responses in terms of improved quality, yield and pest and disease tolerance [22; 23].  
Foliar nutrient sprays are nowadays widely used in plant production, chiefly as a complementary strategy to root treatments [24; 25]. Foliar-applied nutrient solutions may be absorbed by plant surfaces via stomata, the cuticle and cuticular irregularities, trichomes, veins and other modified epidermal structures, such as lenticels [26]. Research in the last 50 years largely focused on the mechanisms of cuticular penetration [24], with limited focus on the permeability of other epidermal structures such as stomata or trichomes, which may have a major contribution to the foliar absorption process [26; 27]. Recent studies are providing evidence for their major physical and chemical heterogeneity of plant surfaces, which are generally covered with a cuticle [26]. Indeed, the occurrence of cell wall polysaccharides in [28; 29; 30] and even at the very surface of the cuticle [31] has been shown for few species and plant organs, and this may have major implications for understanding the mechanisms of agrochemical spray foliar permeability or pest and pathogen attack, among others [26].  
Aware of the positive results reported after the application of Ca and Si sprays to crop plants in the few studies available as reported above, the aim of this investigation was to evaluate the effect of Ca and Si supplied as either single elements or in combination, in terms of Tempranillo berry yield, quality and subsequent must chemical composition under the prevailing growing conditions in La Rioja (Spain).