Estimation of an effective soil water potential at the root surface of transpiring plants

Abstract. A simple method is described for estimating an average of 'effective' soil water potential at the root surface for transpiring plants. The method is based on measurements of leaf water potential and leaf conductance to water vapour in stressed plants and in well-watered controls, and uses the simple Ohm's law analogue for water flow in the soil-plant system. The technique is applied to data for field-grown apple trees and to previously published data for wheat and cowpea.     

Variation in fatty acid composition of apples in relation to soft scald

The distribution of fatty acids within individual fruits was uneven early in storage. When soft scald developed in the fruit, the affected tissue contained less linoleic acid than sound tissue. Differences in fatty acid composition were also found between freshly harvested fruit from different trees within an orchard. Apples that were stored at O° had a low linoleic acid content during the early weeks of storage when the fruit are susceptible to the disorder but the content then increased substantially during subsequent storage. It seems that a low linoleic acid content renders the fruit more susceptible to soft scald.     

Metabolism of added gibberellic acid in Malus pumilla in relation to cool storage breakdown

Gibberellic acid (GA₃) that was injected into the core of apples to reduce internal breakdown, was found to accumulate in the cortical tissue during cool storage only to a limited extent and never exceeded 0.5 % of the added dose. Limitations on the commercial use of GA₃ would appear to be associated with the low level of incorporation into susceptible tissue.     

Expression ofMbR4, a TIR-NBS type of appleR Gene, confers resistance to bacterial spot disease inArabidopsis

A single disease resistance gene candidate,MbR4, was isolated from the wild-type apple speciesMalus baccta. This gene was predicted to encode motifs characteristic of the Toll Interleukin 1 Receptor (TIR) — Nucleotide Binding Site (NBS) of theR gene. Starting with an isolated cDNA clone, genomic clones were obtained via inverse polymerase chain reaction (IPCR). TheMbR4 gene has a single open reading frame (ORF) of 2178 nucleotides, a 41-b untranslated 5’ region, a 21-b untranslated 3’ region, and a predicted protein of 726 amino acids (82 kDa). Its deduced amino acid sequence resembles the N protein of tobacco and the NL25 protein of potato. Ectopic expression ofMbR4 induced enhanced resistance in transgenicArabidopsis plants against the virulent pathogen,Pseudomonas syringae pv.tomato DC3000. Microarray analysis confirmed the induction of defense-related gene expression in pathogen-free 35S::MbR4 heterologousArabidopsis plants, thereby indicating that theMbR4 gene likely activates a pathogen-independent resistance pathway, rather than a gene-for-gene pathway. Our results suggest thatMbR4 plays a role in theR gene, and may be a source of resistance for cultivated apple species.     

In situ rheology of yeast biofilms

The aim of the present work was to investigate the in situ rheological behavior of yeast biofilms growing on stainless steel under static and turbulent flow. The species used (Rhodototula mucilaginosa, Candida krusei, Candida kefyr and Candida tropicalis) were isolated from a clarified apple juice industry. The flow conditions impacted biofilm composition over time, with a predominance of C. krusei under static and turbulent flow. Likewise, structural variations occurred, with a tighter appearance under dynamic flow. Under turbulent flow there was an increase of 112 μm in biofilm thickness at 11 weeks (p < 0.001) and cell morphology was governed by hyphal structures and rounded cells. Using the in situ growth method introduced here, yeast biofilms were determined to be viscoelastic materials with a predominantly solid-like behavior, and neither this nor the G’₀ values were significantly affected by the flow conditions or the growth time, and at large deformations their weak structure collapsed beyond a critical strain of about 1.5–5%. The present work could represent a starting point for developing in situ measurements of yeast rheology and contribute to a thin body of knowledge about fungal biofilm formation.     

Field Evaluation of Some Models Estimating the Seasonal Pattern of Airborne Ascospores of Venturia inaequalis

A 6-year study was carried out to evaluate the accuracy of some models in estimating airborne ascospores of Venturia inaequalis . The proportion of the season's ascospores trapped on each discharge event was compared with the proportion of mature ascospores, estimated by the New Hampshire model or by some related models. The models differed from each other in the degree-day cumulation, accounting or not for the leaf litter wetness caused by rainfall or by deposition of atmospheric humidity. The New Hampshire model did not fit spore trappings well: 59% of the actual values fell outside the range of the estimates, and 83% of them were overestimates. The wide discrepancy between reality and estimates resulted from the effect of dryness: when many consecutive rainless days occurred, the proportion of ascospores trapped was constantly lower than the model estimates, due to a slowed spore maturation. The effect of dryness was evident during the greater part of the ascospore maturity season, irrespective of the proportion of the season's ascospores that had just matured when the dry period began. Models accounting for leaf litter wetness significantly improved estimates. Therefore, in the Po Valley, the accuracy of the New Hampshire model can be improved by accumulating degree-days only when leaf litter is wet.     

Artificial injury experiments on the damaging effect of Leucoptera malifoliella on apple trees

In 1986 and 1987 defoliation experiments on the apple cultivar Herma were performed to simulate the occurrence of Leucoptera malifoliella Costa. Those model experiments were based on investigations on the mine size of Leucoptera malifoliella and yield loss on infested and tagged trees. Defoliation carried out by hand showed that heavy leaf loss reduced fruit number. Insufficient nutrient supply due to reduced leaf area and a consequently smaller leaf-fruit ratio led to a decrease in mean fruit size and total yield. Heavy leaf loss had also an adverse effect on fruit quality components (dry matter substance, viscosity, total sugar and acid content of fruit juice). This was finally reflected in sensory evaluation criteria. Heavy leaf losses also influenced blossom bud differentiation in the blossom set of the following year. Both number of inflorescences and number of blossoms per inflorescence were reduced. The evaluation of the extent of damage of L. malifoliella was based on previous investigations on reduced leaf area. For L. malifoliella a mean mine size of 0.96 cm² was found. This corresponds to a 4.2% loss of leaf area.The leaf-fruit ratio is of special importance for the evaluation of the extent of damage of leaf-damaging pests and was used to derive injury threshold ranges for L. malifoliella. Previous investigations on L. malifoliella infestations and the results from the artificial defoliation experiments enabled the determination of preliminary flexible injury thresholds. Depending on leaf-fruit ratio, yield level, and leaf miner generation, the thresholds vary between 0.1–2.5 eggs and mines per leaf.