Aspectos fisiológicos del déficit hídrico en el jinjolero (zizifus jujuba mill.). Efectos en la calidad comercial y compuestos bioactivos del fruto

Resumen

The purpose of the present study was to analyse some physiological aspects of the response of jujube (Zizyphus jujuba Mill) plants to deficit irrigation during fruit maturation. In this sense, the resistance mechanisms (avoidance and tolerance) developed in response to a water stress and during recovery, the effect of deficit irrigation during fruit maturation on jujube yield and fruit characteristics, including amino acids and procyanidins content were studied. Jujube trees exposed to water stress depend strongly on stress avoidance and stress tolerance mechanisms. From the beginning of water stress to the time of maximum water stress, leaf turgor was maintained allowing substantial gas exchange levels and, as a consequence, good leaf productivity. This leaf turgor maintenance was mainly due to two simultaneous and complementary mechanisms: decreased leaf conductance and a shorter period of maximum stomatal opening in order to control water loss via transpiration (stress avoidance mechanisms). The gradual recovery of leaf conductance (gl) after rewatering can also be considered as a mechanism for promoting leaf rehydration. In addition, from the beginning of the stress period, active osmotic adjustment operated, which could have contributed to the maintenance of leaf turgor (stress tolerance mechanism). The high leaf apoplastic relative water content (RWCa) levels and the possibility of increasing the accumulation of water in the apoplasm in response to water stress, supporting a steeper gradient in water potential between the leaf and the soil, which can be considered another drought tolerance characteristic in jujube leaves. Jujube yield and fruit characteristics can be clearly modified by water deficit imposed during fruit maturation. One essential (cystine (Cys-cys)) and seven non-essential (4-hydroxyproline (p-Hyp), a-aminoadipic acid (AADA), ornithine (orn), ß-aminoisobutyric acid (BAIB), a-amino-n-butyric acid (AABA), cystathionine (Cysta), and homocystine (Hcys-cys)) amino acids were identified for the first time. Fruits from plants exposed to moderate water deficit during the maturation stage initiated the ripening phase earlier than control fruits and had an improved eating quality. Fruits subjected to severe water deficit showed changes in their physical characteristics and reached a more advanced degree of ripening than T0 and T1 fruits, with not only most of the fruit chemical characteristics that determine taste being improved but also the nutritional value. The decrease in the asparagine (Asn) content of the fruit as a result of severe water deficit is a positive aspect, which prevents acrylamide formation during heat-processing of the fruit. The current work demonstrates the occurrence of novel procyanidins in jujube. To date, only two procyanidins [(epi)catechin and its dimer] have been described. In the present study, two trimers, two tetramers, and six procyanidin pentamers have been tentatively identified and quantified for the first time in jujube. The results confirm that proanthocyanidins in jujube fruits consist exclusively of B type procyanidins, whose levels are increased by water deficit during the fruit maturation stage. The fact that the total procyanidin content of the edible portion of fruits under water deficit is based mainly on an increase in the low molecular mass compounds leads us to conclude that jujube fruits from trees exposed to water deficit increase procyanidin bioavailability and enhance the potential physiological effects on human health. The tendency of these molecules to selfaggregate does not change with the portion of the fruit or the irrigation treatment and is similar to that observed in other fruits. Additionally, fruits from well watered trees may increase their procyanidin content during fruit cold storage, whereas fruits from trees that were exposed to severe water stress decrease their procyanidins content during cold storage.