In previous work no difference was found in leaf water potential or solute potential between young guttating leaves and older non-guttating leaves of the same plant. This suggested that the absence of guttation in older leaves was associated with a plant resistance component in the hydathodes. Hydathodes of young, folded leaves contained water pores with various apertures and no signs of occlusion.. In expanded, young leaves, production of epicuticular waxes and excretion of some substance through the pores was observed in the hydathode region. By the time leaves had fully expanded the hydathodes had become brownish. The combination of wax deposition and excreted substance had formed plates of solid material covering water pores. These observations suggest that deposition of substances on top of pores contribute to occlusion of water pores in old leaves.
Hydathodes of young, folded strawberry (Fragaria × ananassa Duch.) leaves had unoccluded water pores With various sized apertures, as observed by low-temperature scanning electron microscopy. Hydathodes of fully expanded leaves were brownish and the water pores within the hydathodes were covered with a solid material, presumably comprised of epicuticular waxes and substances excreted through the hydathodes. The entire water pore area of the hydathode was occasionally covered with a shield-like plate. The shield-like plate over the hydathode water pores impeded water flow even with an induced positive pressure. Mechanical scraping of the hydathode area eliminated impedance to water conduction. These observations suggest that external occlusion of water pores in the hydathodes is the resistance component associated with the absence of guttation in older strawberry leaves.
Foliar anatomical comparisons were made between in vitro-grown plantlets and greenhouse-grown plants of ‘Silvan’ blackberry (Rubus sp.) using scanning and light microscopy. Each apex and marginal serration of in vitro- and greenhouse-grown leaves had a terminal hydathode region composed of a scattered, primarily adaxial, group of sunken water pores. Water pores and stomata of plantlet leaves were open, while greenhouse-grown plant leaves had closed water pores and stomata or comparatively small apertures. Internally, the hydathodes of both cultured plantlets and greenhouse-grown plants were delimited by a bundle sheath that flanked the vascular tissues and extended to the epidermis. Between the vascular tissues and the epidermis were loosely arranged epithem cells. The hydathodes of plantlet leaves were simpler than those of greenhouse-grown plants, with fewer water pores and reduced epithem. Water loss from detached leaves of plantlets occurred through both leaf surfaces, although more water was lost from the abaxial surface. In contrast, foliar water loss from severed leaf blades of greenhouse-grown plants was primarily abaxial.
Foliar anatomical comparisons were made between in vitro-grown plantlets and greenhouse-grown plants of ‘Queen Elizabeth’ rose (Rosa sp.) using scanning and light microscopy. Each acuminate leaf apex and marginal serration had a terminal hydathode region composed of a glandular tip and a subterminal, adaxial group of sunken water pores. Leaf apices of greenhouse-grown plants had up to 35 water pores per hydathode, while cultured plantlets had < 20. Hydathodes of leaf serrations had up to 10 water pores in both sample groups. Water pores and stomata of plantlet leaves were open, while those of greenhouse-grown plants had smaller apertures or were completely closed. Internally, hydathodes were delimited by a bundle sheath extending below the vascular tissues and approaching the adaxial epidermis on each side of the water pore zone. Files of tracheary elements extended various distances into the leaf teeth. Small, irregularly shaped parenchyma cells (epithem) abutted on the xylem parenchyma cells, filling the space between the files of tracheary elements and the adaxial epidermis. The hydathodes of plantlet leaves were smaller with fewer water pores and reduced epithem than those of greenhouse-grown plants. Ex vitro guttation probably occurs as a result of increased water potential and high relative humidity when plantlets are transferred from culture to soil.
Leaves of in vitro-grown plantlets and greenhouse-grown plants of ‘Totem’ strawberry (Fragaria × ananassa Duch.) were compared using scanning and light microscopy. Each apex and marginal serration of in vitro- and greenhouse-grown leaves had a terminal hydathode region. The leaf teeth were composed of an acuminate-mucronate tip, obscured in greenhouse-grown plants by an abaxial cluster of thick-walled unicellular trichomes, and a subterminal, adaxial group of sunken water pores. Water pores and stomata of plantlet leaves were open, whereas greenhouse-grown plant leaves had closed water pores and stomata or comparatively small apertures. Internally, the hydathodes of greenhouse-grown plants and cultured plantlets were delimited by a bundle sheath that extended below the vascular tissues, approaching the adaxial epidermis on each side of the zone of water pores. Between the epidermis and the vascular tissues were loosely arranged epithem cells. The hydathodes of plantlet leaves were smaller than greenhouse-grown plants, with fewer water pores and reduced epithem.
Infection of Anthurium andraeanum Andre by Xanthosomas campestris pv. dieffenbachiae appears to be through hydathodes. Hydathodes occur around the entire margin of the leaves. A small vein that runs parallel to the margin delimits the outer edge of the mesophyll chlorenchyma and the inner edge of the hydathode. External to the vein, epithem cells and intercellular spaces occur adjacent to the xylem vessel elements. Hydathode pores mostly occur on the adaxial surface of the leaf margin. Hydathode structure is consistent and does not appear to differ among cultivars. Glutamine may occur in the guttation liquid with higher levels of nitrogen nutrition. At the same level of nitrogen, more susceptible cultivars appear to produce more glutamine than cultivars more tolerant to the blight. More glutamine appears to increase the rate of blight infection.
The water status of strawberry (Fragaria х ananassa Duchesne) was indicated by the occurrence of guttation. Guttation was present when pre-dawn leaf water potential (PLWP) was greater than -0.07 MPa and absent when PLWP was below – 0.11 MPa. Plants exhibiting guttation had greater stomatal conductivity and lower leaf – air temperature at midday, indicating a greater transpiration rate. Hydathodes on older leaves did not consistently express guttation; thus, the occurrence of guttation must be evaluated on young leaves.
The effects of leaf age, guttation, stomata and hydathode characteristics, and wounding on the symptom development of gummy stem blight [Didymella bryoniae (Auersw.) Rehm] of cucumber (Cucumis sativus L.) were studied to develop a useful germplasm screening method. Older cucumber leaves were more susceptible than younger leaves in field, greenhouse, and detached-leaf tests. Compared to seedlings with true leaves, seedlings at the cotyledon stage were less susceptible, had a smaller variance for ratings, and were more likely to escape infection. Stomata density and hydathode counts were not correlated with field ratings; but, stomata length on older leaves was highly correlated with susceptibility y. In greenhouse and field tests, susceptibility y increased as guttation increased and actively guttating plants were more susceptible than nonguttating plants. Phylloplane moisture and/or nutrition were more important in the infection process than was stomata] opening. Although important, guttation was not necessary for infection. Dawn inoculation of field or greenhouse tests increased leaf symptoms compared with dusk inoculation. The increase was likely due to the free water and nutrients provided by guttation. Genotype ranks and ratings for detached-leaf tests were not correlated with field results. A useful method, highly correlated (r = 0.82 to 0.96) with field ratings. for screening germplasm in the greenhouse was developed.
Resistance to Black rot was studied in B. oleracea, B. campestris and B. napus, using three different inoculation procedures. The results indicated that hydathode inoculation without wounding and the wound suspension technique were useful for differentiating levels of resistance found in B. oleracea and B. campestris, but not in B. napus. Only the wound colony method allowed differentiation between high and moderate resistance in B napus. B. napus, PI 199947 and PI 199949, exhibited the highest resistance found in cultivated Brassica species. In B. campestris, two chinese cabbage accessions showed quantitative inheritance for moderate levels of resistance. In B. napus, the high level of reistance was conferred by one dominant gene, to which the symbol Br was assigned, whereas the moderate resistance was due to one recessive gene bm.
Paull, 1990 ). Stomatal density, stomatal size, and hydathode density were determined on three randomly selected spathes per cultivar. Four 1.5 × 0.5-cm pieces were cut from each spathe and placed in a clearing solution in a petri dish to remove