Genetic differences in fruit-set patterns are determined by differences in fruit sink strength and a source : sink threshold for fruit set

Background and Aims

Fruit set in indeterminate plant species largely depends on the balance between source and sink strength. Plants of these species show fluctuations in fruit set during the growing season. It was tested whether differences in fruit sink strength among the cultivars explained the differences in fruit-set patterns.

Methods

Capsicum was chosen as a model plant. Six cultivars with differences in fruit set, fruit size and plant growth were evaluated in a greenhouse experiment. Fruit-set patterns, generative and vegetative sink strength, source strength and the source : sink ratio at fruit set were determined. Sink strength was quantified as potential growth rate. Fruit set was related to total fruit sink strength and the source : sink ratio. The effect of differences observed in above-mentioned parameters on fruit-set patterns was examined using a simple simulation model.

Key Results

Sink strengths of individual fruits differed greatly among cultivars. Week-to-week fruit set in large-fruited cultivars fluctuated due to large fluctuations in total fruit sink strength, but in small-fruited cultivars, total fruit sink strength and fruit set were relatively constant. Large variations in week-to-week fruit set were correlated with a low fruit-set percentage. The source : sink threshold for fruit set was higher in large-fruited cultivars. Simulations showed that within the range of parameter values found in the experiment, fruit sink strength and source : sink threshold for fruit set had the largest impact on fruit set: an increase in these parameters decreased the average percentage fruit set and increased variation in weekly fruit set. Both were needed to explain the fruit-set patterns observed. The differences observed in the other parameters (e.g. source strength) had a lower effect on fruit set.

Conclusions

Both individual fruit sink strength and the source : sink threshold for fruit set were needed to explain the differences observed between fruit-set patterns of the six cultivars.     

Cellulase elicitor induced accumulation of capsidiol in Capsicum annumm L. suspension cultures

When growth-phase cell suspension cultures of Capsicum annuum were treated with cellulase-elicitor preparation at 3 μg/ml, the level of capsidiol was transiently increased in the culture media rather than in the cells reaching its maximum approx 24 h after treatment. With methyl jasmonate it took 18 h. Elicitor treatment doubled phospholiphase A₂ (PLA₂) activity but simultaneous treatment with aristolochic acid, a PLA₂ inhibitor, inhibited sesquiterpenoid accumulation as well as PLA₂ activity. Mastoparan, a G protein activator, treatment also increased PLA₂ activity and capsidiol production. Taken together, the present study shows that induction of capsidiol production in the C. annuum is mediated by PLA₂ activation.     

A natural recessive resistance gene against potato virus Y in pepper corresponds to the eukaryotic initiation factor 4E (eIF4E)

We show here that the pvr2 locus in pepper, conferring recessive resistance against strains of potato virus Y (PVY), corresponds to a eukaryotic initiation factor 4E (eIF4E) gene. RFLP analysis on the PVY-susceptible and resistant pepper cultivars, using an eIF4E cDNA from tobacco as probe, revealed perfect map co-segregation between a polymorphism in the eIF4E gene and the pvr2 alleles, pvr2(1) (resistant to PVY-0) and pvr2(2) (resistant to PVY-0 and 1). The cloned pepper eIF4E cDNA encoded a 228 amino acid polypeptide with 70-86% nucleotide sequence identity with other plant eIF4Es. The sequences of eIF4E protein from two PVY-susceptible cultivars were identical and differed from the eIF4E sequences of the two PVY-resistant cultivars Yolo Y (YY) (pvr2(1)) and FloridaVR2 (F) (pvr2(2)) at two amino acids, a mutation common to both resistant genotypes and a second mutation specific to each. Complementation experiments were used to show that the eIF4E gene corresponds to pvr2. Thus, potato virus X-mediated transient expression of eIF4E from susceptible cultivar Yolo Wonder (YW) in the resistant genotype YY resulted in loss of resistance to subsequent PVY-0 inoculation and transient expression of eIF4E from YY (resistant to PVY-0; susceptible to PVY-1) rendered genotype F susceptible to PVY-1. Several lines of evidence indicate that interaction between the potyvirus genome-linked protein (VPg) and eIF4E are important for virus infectivity, suggesting that the recessive resistance could be due to incompatibility between the VPg and eIF4E in the resistant genotype.     

QTLs involved in the restriction of cucumber mosaic virus (CMV) long-distance movement in pepper

Partial restriction of cucumber mosaic virus (CMV) long-distance movement originating from the Capsicum annuum inbred line ’Vania’ was assessed in a doubled-haploid progeny using two screening methods: the first allowed one to assess the resistance of adult plants decapitated above the fourth leaf and inoculated on the third leaf using a common CMV strain, and the second allowed one to assess CMV resistance to long-distance movement on seedlings inoculated using an atypical CMV strain. For both resistance tests, the behavior of the F₁ hybrid between ’Vania’ and the susceptible line ’H3’ indicated that partial resistance is inherited as a dominant trait. Phenotypic data from the two screening methods were correlated but the one performed on seedlings was much more severe. A subset of 184 molecular markers well-distributed over the pepper genome was selected for QTL mapping using the composite interval mapping (CIM) method. A total of seven genomic regions, including one major effect and several minor effect QTLs, were shown to be associated with partial restriction of CMV long-distance movement. These results are compared with those already obtained in pepper and also in other solanaceous crops, potato and tomato. Received: 22 March 2001 / Accepted: 9 July 2001     

Archaeobotanic evidence of the preincaic Chiribaya culture. Determination of capsaicinoids in archaeological samples of Capsicum frutescens and votive foods

The lack of Capsicum species or paprika (a basic ingredient of Peruvian foodstuff) in preincaic archaeological samples and votive foods, as evidenced by archaeobotanic studies, has stimulated the chemical analyses of these samples by HPLC methods. The results confirmed the absence of capsaicinoids in these samples whereas they were detected in more ancient fossil fruit.     

Early steps of deoxyxylulose phosphate pathway in chromoplasts of higher plants

1-Deoxy-D-xylulose 5-phosphate and 2C-methyl-D-erythritol 4-phosphate have been shown as intermediates of the deoxyxylulose phosphate pathway used for terpenoid biosynthesis in plants and many microorganisms. In plants this non-mevalonate pathway is located in plastids. In order to investigate the formation of five carbon intermediates, chromoplasts from Capsicum annuum and Narcissus pseudonarcissus were incubated with isotope-labeled 1-deoxy-D-xylulose 5-phosphate or 2C-methyl-D-erythritol 4-phosphate. The downstream metabolites were detected and separated by reversed-phase ion-pair radio-HPLC and their structures elucidated by mass spectroscopy. Here we report the isolation and structural identification of 4-diphosphocytidyl-2C-methyl-D-erythritol and 2C-methyl-D-erythritol 2,4-cyclodiphosphate from chromoplasts; the genes of the corresponding enzymes had been previously identified from Escherichia coli and Arabidopsis.     

Seasonal differences in mineral content, distribution and leakage of sweet pepper seeds

Pepper seed quality is sensitive to variations in climatic conditions during seed development, which might be associated with accumulation, distribution and leakage of mineral elements from the seeds. This was examined in hybrid seeds of sweet pepper (Capsicum annuum L. cv.‘Hazera’ 1195) in two experiments during two growing seasons. The mean daily temperature (day/night) and daily total radiation receipt during seed development were 27.9/23.2°C and 8.63 kW m^?2 in the summer and 18.3/ 14.9°C and 3.18 kW m^?2 in the winter, respectively. Seeds developed in the summer had lower percentage of seedling emergence and leaked a larger portion (45%) of their K content into the water medium than in winter‐developed seeds. Summer seeds accumulated more K and Cl, but less P, Mg, Ca and the weight ratio of linoleic acid to oleic acid was lower than in the winter seeds. The season did not significantly affect N, S and total fatty acids. The most abundant element on the seed coat surface was K in the summer and Ca in the winter seeds. The cotyledon and endosperm of the summer seeds contained relatively higher ratios of K and Ca and lower ratios of P and Mg than the winter seeds. Transportation of mineral nutrients appeared to be involved in the effect of heat and moisture stresses on emergence quality of the pepper seeds.     

Screening Capsicum germplasm for resistance to tomato spotted wilt virus (TSWV)

Resistance conferred by the Tsw locus from Capsicum chinense against Tomato spotted wilt virus (TSWV) has been widely used in breeding programmes. Nevertheless, this resistance depends on inoculation conditions, and isolates able to overcome it have already been detected. In this work 29 accessions of several Capsicum species have been mechanically inoculated with TSWV to identify new sources of resistance. Five accessions showed variable percentages of resistant plants, two of which did not show local lesions on inoculated leaves, suggesting that the response was not mediated through hypersensitivity. Two of these accessions also had a remarkable reduced viral accumulation compared to susceptible control. ECU‐973., a C. chinense accession, showed the best performance against TSWV, with 100% resistant plants. This response was confirmed after mechanical inoculation with three different TSWV isolates. The resistance was maintained when the accession was inoculated with TSWV using a high pressure of viruliferous thrips. These results open new possibilities in the development of a durable resistance to TSWV in pepper.