ISHS Acta Horticulturae 632: XXVI International Horticultural Congress: Citrus and Other Subtropical and Tropical Fruit Crops: Issues, Advances and Opportunities
ROOTSTOCKS AND PLANT WATER RELATIONS AFFECT SUGAR ACCUMULATION OF 'VALENCIA' SWEET ORANGE VIA OSMOTIC ADJUSTMENT |
| Authors: | G.H. Barry, W.S. Castle, F.S. Davies |
| Keywords: | Citrus sinensis, juice quality, soluble solids concentration, water-deficit stress |
| Abstract:
Citrus rootstocks have well-known effects on tree vigor and size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger, and lower in soluble solids concentration (SSC) and titratable acidity compared with fruit from trees budded on non-invigorating rootstocks. Though it is unclear how rootstocks exert their influence on juice quality of Citrus spp., plant water relations is thought to play a central role. To determine how rootstock type affects sugar accumulation in fruit of Citrus spp., controlled water-deficit stress was applied to ‘Valencia’ sweet orange trees on Carrizo citrange or rough lemon rootstocks. Withholding water from the root zone of citrus trees during stage II of fruit development decreased midday stem water potential and increased the concentrations of primary osmotica, fructose and glucose, without affecting sucrose concentration, suggesting that sucrose hydrolysis took place. Increased concentrations of sugars and SSC in fruit from moderately water-stressed trees occurred independently of fruit size and juice content, indicating that passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation among irrigation treatments. Controlled water deficit-stress caused active accumulation of solutes of ‘Valencia’ sweet orange by osmotic adjustment. The evidence presented supports the hypothesis that differential sugar accumulation of Citrus fruit from trees on rootstocks of contrasting vigor is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars via osmotic adjustment. Therefore, inherent rootstock difference affecting plant water relations is proposed as a primary cause of differences in juice quality among citrus rootstocks. This mechanism provides an explanation of how rootstock type affects juice SSC in fruit of Citrus spp. | |
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