Polarity of water transport across epidermal cell membranes in Tradescantia virginiana.

Using the automated cell pressure probe, small and highly reproducible hydrostatic pressure clamp (PC) and pressure relaxation (PR) tests (typically, applied step change in pressure = 0.02 MPa and overall change in volume = 30 pL, respectively) were applied to individual Tradescantia virginiana epidermal cells to determine both exosmotic and endosmotic hydraulic conductivity (L(p)(OUT) and L(p)(IN), respectively). Within-cell reproducibility of measured hydraulic parameters depended on the method used, with the PR method giving a lower average coefficient of variation (15.2%, 5.8%, and 19.0% for half-time, cell volume [V(o)], and hydraulic conductivity [L(p)], respectively) than the PC method (25.4%, 22.0%, and 24.2%, respectively). V(o) as determined from PC and PR tests was 1.1 to 2.7 nL and in the range of optically estimated V(o) values of 1.5 to 4.9 nL. For the same cell, V(o) and L(p) estimates were significantly lower (about 15% and 30%, respectively) when determined by PC compared with PR. Both methods, however, showed significantly higher L(p)(OUT) than L(p)(IN) (L(p)(OUT)/L(p)(IN) ≅ 1.20). Because these results were obtained using small and reversible hydrostatically driven flows in the same cell, the 20% outward biased polarity of water transport is most likely not due to artifacts associated with unstirred layers or to direct effects of externally applied osmotica on the membrane, as has been suggested in previous studies. The rapid reversibility of applied flow direction, particularly for the PR method, and the lack of a clear increase in L(p)(OUT)/L(p)(IN) over a wide range of L(p) values suggest that the observed polarity is an intrinsic biophysical property of the intact membrane/protein complex.

Impact of diurnal temperature variation on grape berry development, proanthocyanidin accumulation, and the expression of flavonoid pathway genes.

Little is known about the impact of temperature on proanthocyanidin (PA) accumulation in grape skins, despite its significance in berry composition and wine quality. Field-grown grapes (cv. Merlot) were cooled during the day or heated at night by +/-8 °C, from fruit set to véraison in three seasons, to determine the effect of temperature on PA accumulation. Total PA content per berry varied only in one year, when PA content was highest in heated berries (1.46 mg berry(-1)) and lowest in cooled berries (0.97 mg berry(-1)). In two years, cooling berries resulted in a significant increase in the proportion of (-)-epigallocatechin as an extension subunit. In the third year, rates of berry development, PA accumulation, and the expression levels of several genes involved in flavonoid biosynthesis were assessed. Heating and cooling berries altered the initial rates of PA accumulation, which was correlated strongly with the expression of core genes in the flavonoid pathway. Both heating and cooling altered the rate of berry growth and coloration, and the expression of several structural genes within the flavonoid pathway.

Fruit ripening in Vitis vinifera: spatiotemporal relationships among turgor, sugar accumulation, and anthocyanin biosynthesis.

This study reports the first observations indicating the spatiotemporal relationships among genetic and physiological aspects of ripening in the berry of Vitis vinifera. At the onset of ripening in the red flesh variety Alicante Bouschet, colour development began in the flesh at the stylar end of the fruit and progressed toward the pedicel end flesh and into the skin. Tissue solute potential and cell turgor also decreased first in the flesh. The decrease in flesh solute potential was due to accumulation of sugars, glucose and fructose, an accumulation that is integral to ripening. Expression of the anthocyanin biosynthesis-related genes VvMybA and VvUFGT was linearly related to the decrease in solute potential. Expression of VvMybA, and to a lesser extent VvUFGT, was correspondingly low in green tissue, higher in the red, stylar end flesh of berries beginning to ripen, and greatest in red berries. In contrast, expression of the abscisic acid biosynthesis-related genes VvNCED1 and VvNCED2 was not correlated with the other spatiotemporal aspects of the onset of ripening. These results, together with earlier work showing that sugar accumulation and acid loss also begin in the stylar flesh in other varieties, indicate that ripening in the grape berry originates in the stylar end flesh.

Vascular function in grape berries across development and its relevance to apparent hydraulic isolation.

During the latter stages of development in fleshy fruit, water flow through the xylem declines markedly and the requirements of transpiration and further expansion are fulfilled primarily by the phloem. We evaluated the hypothesis that cessation of water transport through the xylem results from disruption or occlusion of pedicel and berry xylem conduits (hydraulic isolation). Xylem hydraulic resistance (R(h)) was measured in developing fruit of grape (Vitis vinifera 'Chardonnay') 20 to 100 d after anthesis (DAA) and compared with observations of xylem anatomy by light and cryo-scanning electron microscopy and expression of six plasma membrane intrinsic protein (PIP) aquaporin genes (VvPIP1;1, VvPIP1;2, VvPIP1;3, VvPIP2;1, VvPIP2;2, VvPIP2;3). There was a significant increase in whole berry R(h) and receptacle R(h) in the latter stages of ripening (80-100 DAA), which was associated with deposition of gels or solutes in many receptacle xylem conduits. Peaks in the expression of some aquaporin isoforms corresponded to lower whole berry R(h) 60 to 80 DAA, and the increase in R(h) beginning at 80 DAA correlated with decreases in the expression of the two most predominantly expressed PIP genes. Although significant, the increase in berry R(h) was not great enough, and occurred too late in development, to explain the decline in xylem flow that occurs at 60 to 75 DAA. The evidence suggests that the fruit is not hydraulically isolated from the parent plant by xylem occlusion but, rather, is "hydraulically buffered" by water delivered via the phloem.

The effects of Pierce's disease on leaf and petiole hydraulic conductance in Vitis vinifera cv. Chardonnay.

In this study, we test the hypothesis that the symptoms of Pierce's Disease (PD) result from the occlusion of xylem conduits by the bacteria Xylella fastidiosa (Xf ). Four treatments were imposed on greenhouse-grown Vitis vinifera cv. Chardonnay: well-watered and deficit-irrigated plants with and without petiole inoculation with Xf. The hydraulic conductance of the stem-petiole junction (k(jun)) and leaves (k(leaf)) were measured, and Xf concentrations were established by quantitative polymerase chain reaction (qPCR). Leaf hydraulic conductance decreased with increasing leaf scorch symptoms in both irrigation treatments. The positive relationship between Xf concentration and symptom formation in deficit-irrigated plants suggests that water-stress increases susceptibility to PD. In field-grown vines, water relations of symptomatic leaves were similar to naturally senescing leaves but differed from green control leaves. Overall, these results suggest that the development of PD symptoms represents a form of accelerated senescence as part of a systemic response of the plant to Xf infection.