Abstract
The fruit crop industry is continuously seeking for new technologies able to improve the overall sustainability of the production systems together with an enhancement of quality and safety of the products. Over the last 10 years, plant biostimulants have emerged as new and promising production tool able to increase the use efficiency of other agronomical inputs (i.e. irrigation water and fertilizers), to promote crop tolerance toward different abiotic stressors (i.e. drought, salinity, extreme temperature and radiation) and to enhance the final quality of food products. Despite the considerable amount of researches that have been conducted to elucidate the mode of action of plant biostimulants and their agronomical performances, the number of studies focusing on fruit crops is relatively limited and the current knowledge on the interaction of these substances with the fruit tree growth and physiology is still largely incomplete. This review provides a complete scientific overview of the available literature on the experimental application and the physiological mechanisms of plant biostimulants on the most relevant fruit crops, including berry crops, grapevine, olive, citrus fruits, pome fruits, stone fruits and other fruit species. Following the functional claims currently used for the biostimulants definition at European level, the review is articulated in sections focusing on i) the evidences of the biostimulants implication for fruit tree resistance toward abiotic stresses (drought, salinity and thermal extremes); ii) the evidences of the biostimulants capacity to enhance nutrients use efficiency in fruit crops; iii) the evidences of the biostimulants effects on tree yield and final fruit quality. Overall, the potentiality of plant biostimulants to be successfully integrated within the current management of modern orchards and vineyards emerges as clear. There are anyway aspects typically linked to the perennial nature of fruit trees (i.e. the carry-over effect of previous seasons on the tree internal reserves) or the different environmental conditions imposed by the open-field cultivation systems that need to be further investigated and that represent the future challenges for the research applied on the use of plant biostimulants for fruit crops.