Rootstocks and Plant Water Relations Affect Sugar Accumulation of Citrus Fruit Via Osmotic Adjustment

Abstract

<jats:p>Citrus rootstocks have well-known effects on tree size, crop load, fruit size, and various fruit quality factors. Fruit from trees budded on invigorating rootstocks are generally larger with lower soluble solids concentration (SSC) and titratable acidity compared to fruit from trees budded on less invigorating rootstocks. Although it is unclear how rootstocks exert their influence on juice quality of <jats:italic>Citrus</jats:italic> L. species, plant water relations are thought to play a central role. In addition, the larger fruit size associated with invigorating rootstocks and the inverse relationship between SSC and fruit size implies that fruit borne on trees on invigorating rootstocks have lower SSC due to dilution effects in larger fruit. To determine how rootstock type affects sugar accumulation in fruit of <jats:italic>Citrus</jats:italic> species, controlled water-deficit stress was applied to mature `Valencia' sweet orange [<jats:italic>C. sinensis</jats:italic> (L.) Osb.] trees on Carrizo citrange [<jats:italic>C. sinensis</jats:italic> × <jats:italic>Poncirus trifoliata</jats:italic> (L.) Raf.] or rough lemon (<jats:italic>C. jambhiri</jats:italic> Lush.) 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. Sucrose concentration was not affected, 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. Thus, passive dehydration of juice sacs, and concentration of soluble solids, was not the primary cause of differences in sugar accumulation. Controlled water-deficit stress caused active osmotic adjustment in fruit of `Valencia' sweet orange. However, when water-deficit stress was applied later in fruit development (e.g., stage III) there was no increase in sugars or SSC. The evidence presented supports the hypothesis that differential sugar accumulation of citrus fruit from trees on rootstocks of contrasting vigor and, hence, plant water relations, is caused by differences in tree water status and the enhancement of sucrose hydrolysis into component hexose sugars resulting in osmotic adjustment. Therefore, inherent rootstock differences affecting plant water relations are proposed as a primary cause of differences in sugar accumulation and SSC among citrus rootstocks.</jats:p>

Influence of deficit irrigation timing on the fruit quality of grapefruit (Citrus paradisi Mac.)

“…Passive dehydration of the juice sacs of citrus fruits is not the primary cause of differences in sugar accumulation among irrigation treatments (Barry, Castle & Davies, 2004;Yakushiji et al, 1998). The increase of TA found in T3 fruits could be related with a decrease in the acid catalysis that takes place throughout phase III.…”

Physicochemical changes and chilling injury disorders in ‘Tango’ mandarins stored at low temperatures

“…It has been established that the fibrous roots are higher in sandy soils than in loamy soils . The higher the fibrous root density, the higher the plant water status due to more effective water uptake and greater hydraulic conductivity, inducing higher TSS and TA ratios …”

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An apple sucrose transporter MdSUT2.2 is a phosphorylation target for protein kinase MdCIPK22 in response to drought

“…Sweetness plays an important role in fruit quality of apple and other fruit trees. Generally, moderate drought stress improves fruit quality by enhancing sugar accumulation in fruits (Barry et al, 2004). Therefore, MdCIPK22-MdSUT2.2 regulatory module shed light to the molecular mechanism by which plants respond to drought stress and drought improves fruit quality.…”