Apoplasmic and simplasmic phloem unloading mechanisms: Do they co-exist in Angeleno plums under demanding environmental conditions?

Abstract

Biophysical fruit growth depends on a balance among the vascular and transpiration flows entering/exiting the fruit via phloem, xylem and through the epidermis. There is no information on vascular flows of Japanese plums, a species characterized by high-sugar content of its fruit at harvest. Vascular flows of Angeleno plums were monitored by fruit gauges during late fruit development, under the dry environment of the Goulburn Valley, Victoria, Australia. Phloem, xylem flows and skin transpiratory losses were determined, as well as diurnal leaf, stem and fruit pressure potentials. Fruit seasonal development, skin conductance and dry matter accumulation were also monitored. Fruit grew following a double-sigmoid pattern, but fruit size increased only 3.1 g over the last 3 weeks of development. Fruit grew very little in the morning, primarily due to phloem inflows (0.05 g fruit -1 hr -1 ), while water left the fruit via the xylem. Negligible skin transpiration was recorded for vapor pressure deficit (VPD) values below 3 kPa. This growth pattern, in the absence of skin transpiration, suggests apoplastic phloem unloading. However, at VPD values over 3 kPa (e.g. from early afternoon to a peak around 18:00 hour), transpiratory losses through the skin (up to 0.25 g fruit -1 hr -1 ) caused fruit to shrink, leading to enhanced phloem and xylem inflows (ca. 0.15 g fruit -1 hr -1 ), a scenario that would correspond to symplastic phloem unloading. Over 24 hours the fruit showed a slightly negative total growth, consistent with fruit growth measured in situ during the season at weekly intervals. A few fruit species are known to alter their phloem unloading mechanism, switching from symplastic

Sugar Metabolism in Stone Fruit: Source-Sink Relationships and Environmental and Agronomical Effects

“…In sweet cherry, small xylem backflows occasionally occur around midday during stage III (Brüggenwirth and Knoche, 2016). In ripening Japanese plum, some xylem backflow occurs during the morning (Corelli Grappadelli et al, 2019). The low or non-existent backflows in peach and cherry are significant because they exclude the possibility that excess water is recycled back to the axial transpiration stream, as proposed in grape (Zhang and Keller, 2017).…”

“…In prunes (Prunus domestica), liquid input via the xylem accounts for a large proportion of the total liquid imported during stage II; this proportion decreases during stage III and then increases again later in stage III (Matthews and Shackel, 2005). In Japanese plum, during stage III, input can occur via both the xylem and phloem, and the relative contributions are dependent on the time of day (Corelli Grappadelli et al, 2019). In peach, during both stages I and III, liquid inputs to the fruit via phloem (30% of the total imported) or the xylem (70%) are comparable.…”

“…A recent work in the ripening fruits of Japanese plum has shown that rapid switches between the apoplastic and symplastic routes can occur over the course of each day (Corelli Grappadelli et al, 2019). The high hydrostatic pressure of the phloem is thought to contribute to the symplastic movement of materials unloaded from the phloem (Morandi et al, 2010;Patrick et al, 2013); nevertheless, it has been recently shown that the hydrostatic pressure of the phloem from which unloading occurs could potentially be low (Milne et al, 2018).…”

Non-structural Carbohydrate Metabolism in the Flesh of Stone Fruits of the Genus Prunus (Rosaceae) – A Review

“…Whatever, the loading mechanism used, sugars then move to sink tissues by bulk flow through the phloem, and this is driven by a pressure gradient that is largely produced by an inflow of water into the phloem of source tissues by osmosis (Voitsekhovskaja et al, 2006). In the fruit sugars can then travel from the phloem to sink cells by either an intracellular (symplastic) route via plasmodesmata, an extracellular route (apoplastic) or a combination of both routes (Grappadelli et al, 2019). Both sucrose and sorbitol are transported in the phloem of stone fruits.…”

“…One alternative explanation is that cell wall acid invertase increases the solute concentration in the apoplast by hydrolysing sucrose, which reduces the turgor pressure of the sink cells and facilitates symplastic flow from the phloem. In stone fruits, such as plum, it appears that apoplastic movement via diffusion makes only a very small contribution to apoplastic transport of sugars, and further, there is evidence that phloem unloading can rapidly switch between apoplastic and symplastic (Grappadelli et al, 2019). Thus, it is possible that in stone fruit flesh the relative actions of vacuolar and cell wall invertase can contribute to altering turgor pressure that modifies the contributions of the symplastic and apoplastic pathways to post-phloem transport.…”