Bhaskar R. Bondada

Loss of rachis cell viability is associated with ripening disorders in grapes

Rachises of grape (Vitis vinifera L.) clusters that appeared healthy or displayed symptoms of the ripening disorders berry shrivel (BS) or bunch-stem necrosis (BSN) were treated with the cellular viability stain fluorescein diacetate and examined by confocal microscopy. Clusters with BS and BSN symptoms experienced a decrease of cell viability throughout the rachis, and their berries contained 70–80% less sugar than healthy berries. The xylem-mobile dye basic fuchsin, infiltrated via the cut base of shoots with one healthy and one BS cluster, moved to all berries on the healthy cluster but generally failed to move into the peduncle of the BS cluster. Peduncle girdling did not interrupt dye movement in the xylem, but stopped solute accumulation in berries and led to berry shrinkage. In contrast, surgically destroying the peduncle xylem at the onset of ripening did not affect berry growth and solute accumulation. These results indicate that cessation of sugar and water accumulation in BS and BSN is associated with phloem death in the rachis. Although xylem flow to the berries may also cease, a functional xylem connection to the shoot may not be required for normal ripening, while water loss from berries by transpiration and xylem efflux may explain the characteristic berry shrinkage that is associated with these ripening disorders. The similarity of internal tissue breakdown in BS and BSN rachises and the correlation observed here between the proportion of shrinking berries on a cluster and the severity of rachis necrosis suggest that there may be a gradual transition between the two ripening disorders. Seeds from healthy and BS clusters showed no differences in colour, morphology, weight, viability, and ability to germinate, which indicates that the disorder may not appear until seeds are mature.

Sugar demand of ripening grape berries leads to recycling of surplus phloem water via the xylem

We tested the common assumption that fleshy fruits become dependent on phloem water supply because xylem inflow declines at the onset of ripening. Using two distinct grape genotypes exposed to drought stress, we found that a sink-driven rise in phloem inflow at the beginning of ripening was sufficient to reverse drought-induced berry shrinkage. Rewatering accelerated berry growth and sugar accumulation concurrently with leaf photosynthetic recovery. Interrupting phloem flow through the peduncle prevented the increase in berry growth after rewatering, but interrupting xylem flow did not. Nevertheless, xylem flow in ripening berries, but not berry size, remained responsive to root or shoot pressurization. A mass balance analysis on ripening berries sampled in the field suggested that phloem water inflow may exceed growth and transpiration water demands. Collecting apoplastic sap from ripening berries showed that osmotic pressure increased at distinct rates in berry vacuoles and apoplast. Our results indicate that the decrease in xylem inflow at the onset of ripening may be a consequence of the sink-driven increase in phloem inflow. We propose a conceptual model in which surplus phloem water bypasses the fruit cells and partly evaporates from the berry surface and partly moves apoplastically to the xylem for outflow.

Surface structure and anatomical aspects of Chinese brake fern (Pteris vittata; Pteridaceae)

The objective of this study was to describe a wide spectrum of surface structural and anatomical details of the Chinese brake fern (Pteris vittata) using scanning electron microscopy (SEM). SEM revealed that the epidermal cells of the pinnae were elongated with raised periclinal and sinuous anticlinal walls. The pinnae were hypostomatous with randomly scattered anomocytic stomatal complexes positioned at the same level as the epidermis. Stomates were large and elliptical (27.4 μm × 10.2 μm). Cross sections from the central regions of the rachis and the stipe revealed V- and U-shaped vascular bundles, respectively. In each vascular bundle, the xylem strands were sea-horse shaped (hippocampus). In contrast, the pinnae possessed a triangular vascular bundle with uniform mesophyll organization comprising of homogenous lobed parenchyma cells. The indumentum consisted of trichomes and scales, which formed various types of vestiture. Trichomes were borne only on the pinnae and scales on the rachis and stipe. The roots developed a dense network of long root hairs averaging 244 μm long, and the xylem consisted of tracheids with scalariform pitting. Sori were submarginal; continuous along both margins of the pinna and were covered with a false indusium. The sporangia were oblong with a short thick stalk and the annulus was positioned vertically resulting in transverse dehiscence of the sporangium. The paraphyses were uniseriate, unbranched, septate and found to be intermixed with the sporangia. The exine of the globose spores was adorned with thick reticulum in which the areoles contained round tubercles. This study describes surface features in detail, which is essential to studies examining the issue of whether morphological characteristics are related to arsenic hyperaccumulation inP. vittata.

Cuticular penetration characteristics of urea in citrus leaves

Summary Recent interest in reducing nitrate levels in ground water has stimulated a re-examination of the potential use of foliar applications of urea on citrus trees. We examined the penetration characteristics of urea through isolated cuticles in relation to their thickness and surface wax content in citrus (Citrus paradisi Macf.) leaves using a finite dose-diffusion system. Isolated cuticles with the greatest integrity were mounted on diffusion half-cells with the inner morphological surface in contact with citrate buffer solution. 14 C-urea in citrate buffer was applied to the outer wax-rich morphological surface and the penetration of urea was monitored by repeated sampling of the receiver solution. Urea penetration through cuticles exhibited an asymptotic curve characterised by an initial lag phase of about 10 min, a quasi-linear phase of maximum penetration rate, averaging 2% of the urea applied h–1, occurred about 40 min after urea application, followed by a plateau phase during which the rate of penetration approached zero at 144 h. Total urea penetration averaged approx. 35% and tended to decrease with increased cuticle weight until the cuticles were 6 months-old. Re-hydration of the urea deposit, with water, stimulated additional penetration of about 1%; however, this was not significant. Epicuticular wax occurred as platelets that increased with leaf age. Dewaxing the cuticles significantly enhanced the maximum penetration rates by 30% of the amount applied h–1 and increased total penetration by 64%. This study provides valuable details on the transport of urea through isolated cuticles which is important in fine-tuning foliar urea fertilisation in citrus trees.

Plant hydraulic conductance adapts to shoot number but limits shoot vigour in grapevines

The rate of shoot growth (vigour) in grapevines tends to decrease as the number of shoots per plant increases. Because the underlying causes of this relationship remain unclear, they were studied by variable pruning of field-grown, deficit-irrigated Merlot grapevines (Vitis vinifera L.). Shoot number ranged from 11 to 124 per vine and was inversely correlated with shoot growth rate, leaf appearance rate, axillary bud outgrowth, internode length, leaf size, shoot leaf area, carbon partitioned to the fruit (Cfruit) per shoot, average daily maximum photosynthesis (Amax), stomatal conductance (gmax), and leaf-specific hydraulic conductance (Kl). Shoot number was positively correlated with canopy leaf area, whole-vine Cfruit, whole-plant hydraulic conductance (Kv), and canopy conductance (Kc). Higher shoot vigour was associated with higher Amax, gmax, predawn leaf water potential (Ψpd), shoot hydraulic conductance (Ks), Kl, and Kv. Vigorous shoots supported both more vegetative growth and more reproductive growth; thus fruit growth did not compete with shoot growth for photosynthates. These results indicate that the hydraulic capacity of grapevines adapts to varying shoot numbers to support leaf physiology, growth, and carbon partitioning, but adaptation may be limited, putting upper bounds on the growth of individual shoots and fruit.

Viscosity-temperature relationships of soybean oil emulsions and their implications for oil retention by apple and peach leaves

Summary The retention of spray droplets is influenced by properties of the spray solution such as viscosity, and by the morphology of the target surface. The objectives of this study were: (i) to determine the retention of emulsions of 1% (v/v) soybean oil with Latron B-1956®, K1™, or Telomer™; (ii) the retention of 1% (v/v) Ballistol™, 1% (v/v) Soy Gold 1000™ and 1% (v/v) Soy Gold 2000™ emulsions in water by ‘Golden Delicious’ apple (Malus pumila Mill.) and ‘Red Haven’ peach [(Prunus persica (L.) Batsch] leaves; and (iii) the influence of temperature on the viscosity of the emulsifiers used in soybean oil sprays. The viscosity of all emulsifiers, at all concentrations tested in soybean oil or in their pure form, ranging from 160 – 28,000 centipoise, declined with increasing temperature. Telomer had the greatest viscosity, followed by K1™, Latron B-1956®, and Ballistol™. Retention of soybean oil emulsions by apple and peach leaves was greatest for soybean oil emulsified with K1™ (80 and 74 μg cm−2, respectively), followed by Latron B-1956® (48 and 52 μg cm−2, respectively), and Ballistol™ (38 and 42 μg cm−2, respectively). Soy Gold 1000™ (24 and 28 μg cm−2, respectively) and Soy Gold 2000™ (9 and 14 μg cm−2, respectively) were retained least by apple and peach leaves. There were no significant differences in soybean oil retention between peach and apple leaves. This study demonstrated that the retention of soybean oil was related to the viscosity of the emulsifier (i.e., the higher the viscosity of the emulsifier, the greater the retention of the soybean oil). We suggest that soybean oil emulsions formulated for pest control would work most effectively with an emulsifier possessing high viscosity.