Induction and regression of early boron deficiency in grapevine in hydroponics: macro- versus micro-scale symptomatology

Boron (B) deficiency is widespread in viticultural areas especially in sandy and alkaline soils that are poor in organic matter. Two types of B deficiency have been identified in vineyards: the “early spring temporary deficiency” and the “late summer deficiency”. Symptoms reported include leaves that are distorted, cupped, chlorotic, with prominent veins and flecks. Less affected leaves have partially abnormal margin, and only a portion of the interveinal areas may show symptoms. Dwarfed stems, short and swollen internodes and tendril necrosis are frequent in severely affected grapevines. Flower clusters are twisted, malformed and abortive. Root apexes are necrotic and deformed. Aiming to study the symptoms of early B deficiency without the interference of biotic or abiotic factors, pest injury or adverse climatic conditions, we established a hydroponic culture of virus-free grapevine cuttings ('Pinot Gris'), in environmentally controlled conditions. The first symptoms appeared about 50-55 days after sprouting. The symptom were correlated to the B concentration in plant tissues and to ultrastructural changes observed with transmission electron microscopy. Symptoms in hydroponically-grown grapevines were identical to those described in the field, and agreed with the B-deficiency syndrome reported previously in grape. Moreover, new asymptomatic vegetation from symptomatic grapes was obtained by placing them in a complete medium, containing B. The main structural modifications, visible in both leaves and roots, were thickenings and undulations of the cell walls. These morphological changes have been proposed as a response to nutritional deficiency, as a way to increase the surface area for nutrient absorption and transport. Our findings support the hypothesis of the involvement of B-deficiency in grapevine springtime vegetative disorders observed in many viticultural areas and emphasize the importance of a correctly balanced B nutrition in grapevines.