How do salinity and water stress affect transport of water, assimilates and ions to tomato fruits?

Abstract

The study was conducted in order to determine whether water stress affects the accumulation of dry matter in tomato fruits similarly to salinity, and whether the increase in fruit dry matter content is solely a result of the decrease in water content. Although the rate of water transport to tomato fruits decreased throughout the entire season in saline water irrigated plants, accumulation rates of dry matter increased significantly. Phloem water transport contributed 80–85% of the total water transport in the control and water‐stressed plants, and over 90% under salinity. The concentration of organic compounds in the phloem sap was increased by 40% by salinity. The rate of ions transported via the xylem was also significantly increased by salinity, but their contribution to fruit osmotic adjustment was less. The rate of fruit transpiration was also markedly reduced by salinity. Water stress also decreased the rate of water transport to the tomato fruit and increased the rate of dry matter accumulation, but much less than salinity. The similar changes, 10–15%, indicate that the rise in dry matter accumulation was a result of the decrease in water transport. Other parameters such as fruit transpiration rates, phloem and xylem sap concentration, relative transport via phloem and xylem, solutes contributing to osmotic adjustment of fruits and leaves, were only slightly affected by water stress. The smaller response of these parameters to water stress as compared to salinity could not be attributed to milder stress intensity, as leaf water potential was found to be more negative. Measuring fruit growth of girdled trusses, in which phloem flow was inactive, and comparing it with ungirdled trusses validated the mechanistic model. The relative transport of girdled as compared to ungirdled fruits resembled the calculated values of xylem transport.

Relationship between Fruit Soluble Solid Content and the Sucrose Concentration of the Phloem Sap at Different Leaf to Fruit Ratios in Tomato

“…Sucrose concentration of the phloem sap ranged from 3 to 7% in 'House Momotaro' and 1 to 10% in 'Saturn', depending on the treatments. These values are approximately equivalent to the estimation by Ho et al (1987) and Plaut et al (2004).…”

“…Moreover, there are no reports which describe the successful collection and quantification of phloem sap that enters tomato fruits. The only method available is based on the estimation of the rate of water import through the phloem and that of carbohydrates (Ho et al, 1987;Plaut et al, 2004). Water import through the phloem could be estimated by the reduction in fruit growth after heat-girdling (Plaut et al, 2004); however, these reports discuss changes in the phloem sugar as a result of saline stress and do not focus on the relationship of phloem sugar and fruit solids.…”

“…Soluble sugars and organic acids, which contribute 2/3 of the fruit total solids (Bertin et al, 2000;Guichard et al, 2001), influence the taste and flavour of the fruits (Ho, 2003;Salunkhe et al, 1974). Solid contents may be manipulated by modifying growing conditions (Bertin et al, 2000;Ho et al, 1987;Islam et al, 1996;Plaut et al, 2004) or the leaf to fruit ratio (LFR) (Heuvelink, 1997;Hewitt and Stevens, 1981;Marcelis, 1993a, b, c), as well as genetically (Baxter et al, 2005;Petreikov et al, 2009).…”

“…The phloem was estimated to be the primary route for water transport into tomato fruits (Guichard et al, 2005;Ho et al, 1987;Plaut et al, 2004) at a later stage of fruit development. Low contribution of the xylem to the transport may be due to the high hydraulic resistance to xylem flow at the pedicel (Lee, 1989;Rancic et al, 2010;Van Ieperen et al, 2003), or within the pericarp (Malone and Andrews, 2001).…”

Measurement of Sap Flow Dynamics Through the Tomato Peduncle Using a Non-Invasive Sensor Based on the Heat Field Deformation Method

“…Tomato fruit growth results from the influx of water, carbon and minerals through the xylem and the phloem, which is driven by a gradient in hydrostatic and osmotic potential between the stem and the fruit. It is generally acknowledged that the phloem accounts for 80-90% of the influx, whereas the xylem only has a minor contribution (Ehret and Ho, 1986;Ho et al, 1987;Plaut et al, 2004;Guichard et al, 2005). Recently, Windt et al (2009) used magnetic resonance imaging (MRI) to study water flow into developing tomato fruits and demonstrated that xylem influx accounted for at least 75% of the net influx.…”