To be useful for indicating plant water needs, any measure of plant stress should be closely related to some of the known short- and medium-term plant stress responses, such as stomatal closure and reduced rates of expansive growth. Midday stem water potential has proven to be a useful index of stress in a number of fruit tree species. Day-to-day fluctuations in stem water potential under well-irrigated conditions are well correlated with midday vapor-pressure deficit, and, hence, a nonstressed baseline can be predicted. Measuring stem water potential helped explain the results of a 3-year deficit irrigation study in mature prunes, which showed that deficit irrigation could have either positive or negative impacts on tree productivity, depending on soil conditions. Mild to moderate water stress was economically beneficial. In almond, stem water potential was closely related to overall tree growth as measured by increases in trunk cross-sectional area. In cherry, stem water potential was correlated with leaf stomatal conductance and rates of shoot growth, with shoot growth essentially stopping once stem water potential dropped to between −1.5 to −1.7 MPa. In pear, fruit size and other fruit quality attributes (soluble solids, color) were all closely associated with stem water potential. In many of these field studies, systematic tree-to-tree differences in water status were large enough to obscure irrigation treatment effects. Hence, in the absence of a plant-based measure of water stress, it may be difficult to determine whether the lack of an irrigation treatment effect indicates the lack of a physiological response to plant water status, or rather is due to treatment ineffectiveness in influencing plant water status. These data indicate that stem water potential can be used to quantify stress reliably and guide irrigation decisions on a site-specific basis.

Sorbitol and sucrose are major products of photosynthesis distributed in apple trees (Malus domestica Borkh. cv. ''Greensleeves'') that affect quality in fruit. Transgenic apple plants were silenced or up-regulated for sorbitol-6-phosphate dehydrogenase by using the CaMV35S promoter to define the role of sorbitol distribution in fruit development. Transgenic plants with suppressed sorbitol-6-phosphate dehydrogenase compensated by accumulating sucrose and starch in leaves, and morning and midday net carbon assimilation rates were significantly lower. The sorbitol to sucrose ratio in leaves was reduced by Ϸ90% and in phloem exudates by Ϸ75%. The fruit accumulated more glucose and less fructose, starch, and malic acid, with no overall differences in weight and firmness. Sorbitol dehydrogenase activity was reduced in silenced fruit, but activities of neutral invertase, vacuolar invertase, cell wall-bound invertase, fructose kinase, and hexokinase were unaffected. Analyses of transcript levels and activity of enzymes involved in carbohydrate metabolism throughout fruit development revealed significant differences in pathways related to sorbitol transport and breakdown. Together, these results suggest that sorbitol distribution plays a key role in fruit carbon metabolism and affects quality attributes such as sugar-acid balance and starch accumulation.gene silencing ͉ sugar-acid balance ͉ translocation ͉ starch accumulation A pples are in the family Rosaceae, which includes temperate species with fleshy fruit. The family produces sorbitol, sucrose, and starch as primary products of photosynthesis ( Fig. 1). Sorbitol is synthesized via reduction of glucose-6-phosphate to sorbitol-6-phosphate by aldose-6-phosphate reductase (EC 1.1.1.200), also called sorbitol-6-phosphate dehydrogenase (S6PDH). Sorbitol is the main sugar present in apple leaves and is transported in phloem with sucrose ( Fig. 1) (1). Sugars are distributed through a network of sieve elements to sink tissues such as developing fruit, seed, and leaves in a complex process regulated by photosynthetic rate, phloem loading, long-distance translocation and unloading, postphloem transport, and metabolism within sink tissues (2, 3). Transporters of sugar alcohols like sorbitol and other structural components involved in phloem unloading of sorbitol into the apoplast of apple fruit have been identified (4-6).In sink tissues such as fruit, sucrose is metabolized by invertases and sucrose synthase, whereas sorbitol is converted to fructose by sorbitol dehydrogenase (SDH, EC 1.1.1.14) (7,8). Expression of SDH in developing apple fruit is highest 2-3 weeks after bloom, a period sensitive to carbon availability (9). Additionally, sorbitol and other sugars may regulate expression of SDH mRNA and protein in pear fruit slices (10).In this article, we describe how sorbitol distribution affects fruit quality using transgenic apple plants with altered levels of S6PDH. Understanding the relationship between sorbitol accumulation and fruit quality is important not only for a...

The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.

During spring, bud growth relies on long-distance transport of remotely stored carbohydrates. A new hypothesis suggests this transport is achieved by the interplay of xylem and phloem. During the spring, carbohydrate demand of developing buds often exceeds locally available storage, thus requiring the translocation of sugars from distant locations like limbs, stems and roots. Both the phloem and xylem have the capacity for such long-distance transport, but their functional contribution is unclear. To address this ambiguity, the spatial and temporal dynamics of carbohydrate availability in extension shoots of Juglans regia L. were analyzed. A significant loss of extension shoot carbohydrates in remote locations was observed while carbohydrate availability near the buds remained unaffected. This pattern of depletion of carbohydrate reserves supports the notion of long-distance translocation. Girdling and dye perfusion experiments were performed to assess the role of phloem and xylem in the transport of carbohydrate and water towards the buds. Girdling caused a decrease in non-structural carbohydrate concentration above the point of girdling and an unexpected concurrent increase in water content associated with impeded xylem transport. Based on experimental observations and modeling, we propose a novel mechanism for maintenance of spring carbohydrate translocation in trees where xylem transports carbohydrates and this transport is maintained with the recirculation of water by phloem Münch flow. Phloem Münch flow acts as a pump for generating water flux in xylem and allows for transport and mobilization of sugars from distal locations prior to leaves photosynthetic independence and in the absence of transpiration.