Effect of seawater irrigation on water relations and fruit quality in potted citrus trees

Seawater was applied to pot-grown citrus trees, namely, 2 cultivars of satsuma mandarin (Citrus unshiu Marcow.) and 'Miyauchi-iyokan' (Citrus iyo hort. ex Tanaka), to clarify the feasibility of salt stress to improve fruit quality and its physiological mechanism. The applications of undiluted seawater (1X) and half-strength-diluted seawater (1/2X) significantly increased soil electrical conductivity (EC) and decreased leaf water potential to different degrees compared with those in the control. The effect of seawater concentration on leaf water potential was more obvious when it was determined at predawn than at noon. Only a slight increase of leaf Na content and no significant leaf abscission occurred in 1/2X compared with those in the control, while significant increases of these parameters were observed in 1X. Seawater irrigation inhibited fruit growth, but did not affect fruit shape and flesh ratio. Soluble solids content (SSC) in the juice was significantly higher upon the seawater applications at about 0.8 to 2°Brix depending on the cultivar than in the control; reducing sugars and/or sucrose were also increased by the salinity treatments in satsuma mandarin. Seawater treatments tended to delay the decrease of titratable acidity, but no significant difference between 1/2X and the control was detected at harvest. These results indicate that the mild salt exposure induced by 1/2X at the extent of 0.3 to 0.5 MPa lower water potential at predawn than that in the unstressed control could improve citrus fruit quality without injury. The results also suggest that salt stress by seawater irrigation would affect fruit quality through a similar physiological process, osmoregulation, to that of drought stress.