B. Aloni

Influence of low night temperatures on sweet pepper flower quality and the effect of repeated pollinations, with viable pollen, on fruit setting

SummaryThe effects of night temperatures on the functioning and fertility of flower parts and, thereby, on fruit set and fruit shape in sweet pepper (Capsicum annum L.) were studied. In cv. Mazurka style length was positively, and the ovary diameter was negatively, correlated with the decrease in night temperatures. The functioning of the flower female organs was also affected by low (14°C or less) night temperatures, and the number of viable pollen grains per flower was markedly reduced and their germinability was impaired. Fruits which set under low night temperatures were deformed (and generally seedless), whereas under higher night temperatures most of the fruits were of the normal, blocky type. The male and female flower organs of cv. 899 were less sensitive to low temperatures than those of cv. Mazurka, so that all the fruits of cv. Mazurka which had set under low night temperatures were small and seedless (flattened) or pointed, while more fruits of cv. 899 were larger, seeded and of better shape. ...

Physiological and biochemical changes at the rootstock-scion interface in graft combinations between Cucurbita rootstocks and a melon scion

Summary Success in grafting depends on the identification of a compatible rootstock (RS) that promotes rapid formation of vascular connections between the RS and the scion, and rapid resumption of root and shoot growth. In this study, we used compatible and incompatible Cucurbita rootstocks, with a melon (Cucumis melo L. ‘Arava’) scion, to identify physiological and biochemical factors in the scion-RS interface that could be associated with graft compatibility. Anatomical characterisation of the grafting interface showed that the regeneration and differentiation of vascular elements was similar, at first, in both compatible and incompatible grafted seedlings although, in the latter, a protective layer was formed, but did not interfere with communication between the RS and the scion. Water uptake and sugar distribution between the plant canopy and the roots, measured 14 d after grafting (DAG) were not correlated with compatibility or incompatibility. At 24 DAG, both water uptake and root sugar concentrations decreased significantly in the incompatible RS, reflecting deterioration of the RS. At 24 DAG, part of the RS collapsed, but only in incompatible grafted seedlings. Histochemical staining revealed that superoxide, H2O2, peroxidase (POX) activity, and lignin deposits at the scion-rootstock interface were similar in both compatible and incompatible grafting combinations at 14 DAG; but, at 24 DAG, H2O2 and superoxide levels were higher in the incompatible grafted transplants. In addition, cell-wall POX and superoxide dismutase (SOD) activities were lower in the incompatible RS-scion interface at 14 and 24 DAG. These results suggest that a physical barrier is unlikely to be formed between the incompatible partners early after grafting, but that lower anti-oxidant enzyme activities and higher levels of reactive oxygen species (ROS) in the incompatible RS-scion interface may be responsible for degradation of the grafting zone.

CUTICULAR CRACKING IN PEPPER FRUIT. I. EFFECTS OF NIGHT TEMPERATURE AND HUMIDITY

SummaryThe objective of the present study was to characterize some of the anatomical and physiological characteristics of fruit cracking in bell pepper (Capsicum annuum L.). Scanning-electron micrographs show that initiation of fruit cracking in bell pepper is by formation of mini-cracks on the cuticle layer; these then enlarge into cracks and traverse the epidermal cells. Pepper cultivars differ in their sensitivity to cracking, partly because of differences in fruit pericarp thickness. Limitation of night transpiration by high humidity or low temperature increased the turgor potential of the fruit pericarp and enhanced cracking. Likewise, leaf pruning suppressed night transpiration and concomitantly increased cracking. Fruit cuticle transpiration was found to be low in the early stages of fruit development and to increase as the fruit became susceptible to cracking. Environmental conditions which enhanced cracking also increased cuticle transpiration in a sensitive pepper cultivar but not in a less sens...

Apoplastic anti-oxidants in pepper (Capsicum annuum L.) fruit and their relationship to blossom-end rot

Summary The aim of the present research was to study changes in the major anti-oxidative enzyme activities known to be associated with the apoplast, during the induction of blossom-end rot (BER) in bell pepper (Capsicum annuum L.) fruits grown under saline conditions. Pepper plants of the BER-sensitive cv. ‘Mazurka’ and less-sensitive cv. ‘Selica’ were grown under control, low- and high-salinity irrigation regimes. Fruits were harvested continuously and, after approx. 7 months under these conditions, BER symptoms started to appear (in the Spring season) and fruits were sampled for biochemical measurements. The intercellular washing fluid (IWF; apoplast fraction) of the fruit pericarp was prepared and enzyme activities in that fraction were determined. The results showed that production of reactive oxygen species (ROS) in the fruit apoplast was higher in ‘Mazurka’ than in ‘Selica’. Apoplastic peroxidase and superoxide dismutase (SOD) activities, and the concentration of ascorbic acid, increased during ‘Selica’ fruit development, whereas the concentration of H2O2 decreased. During the stage of rapid growth, when fruits are most susceptible to BER, apoplastic peroxidase activity was greatly enhanced in the healthy pericarp of BER-affected fruits compared to healthy fruits. Under saline conditions, apoplastic peroxidase and SOD activities, as well as ascorbic acid and H2O2 levels, increased in healthy fruits of the resistant cultivar ‘Selica’; whereas, in healthy fruit of the BER-sensitive cultivar ‘Mazurka’, apoplastic peroxidase activity decreased while the level of ascorbic acid increased, but to a much lesser extent than in ‘Selica’. SOD activity and H2O2 levels increased similarly in both cultivars with increasing levels of salinity. It is proposed that apoplast-associated peroxidase activity and ascorbic acid concentration are increased coordinately in response to salt stress and may be part of a protective anti-oxidation mechanism that determines cultivar sensitivity to BER.

Environmental factors affecting the cracking of greenhouse-grown bell pepper fruit

SummaryThe cracking of bell pepper fruit is one of the main physiological disorders that reduce marketable yield. Understanding the environmental and endogenous factors that influence this disorder may help to reduce its severity. Plants grown in three greenhouses were subjected to differing potential transpiration (Tp) at night, with identical daytime Tp. Measurements included climatic variables in the greenhouses, soil water content, sap flow, transpiration, leaf temperature, surface and internal fruit temperatures, fruit and stem diameters, stomatal conductance, and leaf area. The results show that at night pepper plants transpired at a rate proportional to the vapour pressure difference (leaves to air; VDPla) in the respective greenhouses. The percentage of cracked fruits was highest in the greenhouse with the lowest night vapour pressure deficit (VPD) and lowest in the greenhouse with the highest VPD. Both increased direct radiation reaching the fruit surface and decreased soil water content reduced ...

The susceptibility of pepper (Capsicum annuum) to heat induced flower abscission: Possible involvement of ethylene

SUMMARYHeat stress causes abscission of flowers in pepper plants and thus reduces yield. In the present study, we investigated the involvement of ethylene in heat-stress related flower abscission, by comparing the response of flowers of bell pepper, cv. Maor, known to be sensitive to flower abscission, and flowers of paprika, cv. Lehava, a resistant cultivar of Capsicum annuum. Their differing susceptibilities to high temperatures depend on light. Under high-temperature, high-light conditions, bell pepper was less susceptible than paprika, but, under high-temperature low-light, bell pepper was more susceptible. At high temperatures, flower explant abscission was much higher with bell pepper than paprika. Ethylene production (EP) by bell pepper explants reached a maximum at 34°C and decreased at higher temperatures (42 and 48 C), while in paprika EP rates were lower and reached the maximum at 42°C. Explants of bell pepper flowers were more susceptible to exogenous ethephon than paprika flowers. The sensiti...

Cuticular cracking in bell pepper fruit: II. Effects of fruit water relations and fruit expansion

SummaryThe objective of this research was to study the role of fruit expansion and turgor in the formation of cuticular cracking in bell pepper (Capsicum annuum L.). Fruit was artificially cracked by filling detached hollowed pepper fruits with distilled water. However, when isotonic mannitol solution was used, cracking did not occur. Similarly, immersion of intact red fruits in tap water caused formation of cuticular cracks following a delay of 30 h during which the turgor potential of the red pericarp and the expansion of the fruit diameter increased to critical threshold values of 0.65 MPa and 1±2%, respectively. On the other hand, in the greenhouse, attached red fruits underwent diameter change of only 0.2% and, when most expanded, the fruit had a turgor potential of only 0.3 MPa. In greenhouse grown plants, pepper fruits become susceptible to cracking at the stage of colour turning, a stage at which they also become susceptible to artificially induced cracking by immersion in water. Under greenhouse ...

Inhibition of heat induced pepper (Capsicum annuum) flower abscission and induction of fruit malformation by silver thiosulphate

SummaryFoliar spray of silver thiosulphate (STS) reduced abscission of flower buds, flowers and fruitlets in pepper plants, grown for four days under heat-stress conditions. Likewise, STS inhibited the abscission of flowers from explants (fixed on agar medium) under conditions of heat stress. Concomitantly, STS increased ethylene production by the flower explants. Ethephon, applied to STS-treated flowers was less effective in inducing flower abscission than when given to flowers which had not been treated with STS. The fruits which developed from the STS-treated plants were deformed. Similar fruit deformities were observed in ethephon, 2,3,5,-triiodo-benzoic acid (TIBA) and naphthalene acetic acid (NAA) treated plants. Ethephon, STS and TIBA inhibited the basipetal transport of [3H]-NAA in flower-bud peduncles. We suggest that STS reduced flower abscission by blocking ethylene action, and causes deformation of the developing ovary and fruit by inhibiting the export of auxin from the flower buds.

Water relations, photosynthesis and assimilate partitioning in leaves of pepper (Capsicum annuum) transplants: Effect of water stress after transplanting

SummaryWhen pepper (Capsicum annuum L., cv. Maor) transplants, grown in Speedling type trays, were transplanted into wet soil but not watered for up to 48 h (day/night temperature 30/25°C), their leaf water potential was reduced from —0.5 to nearly —2.0 MPa. This decrease was mainly due to loss of leaf tugor. At such low water potential, photosynthesis of the source leaves was completely inhibited. The leaf water potential returned to the control values 24 h after rewatering but the photosynthetic rate of the stressed plants did not fully recover and was dependent on the extent to which the water potential had decreased during the stress. Translocation of l4C-assimilates, following l4C02 feeding, was inhibited in stressed transplants with the upward flow towards the young leaves being more affected that the downward flow towards the roots. After rewatering, transport of 14C-assimilates from the source leaf of the stressed transplants to the root was recovered but transport to the young developing leaves r...

The effect of root restriction on the incidence of blossom-end rot in bell pepper (Capsicum annuum L.)

Summary Water and calcium uptake are important factors affecting the incidence of fruit blossom-end rot (BER) in tomato and pepper. In the present study an attempt was made to manipulate these factors by severe root pruning and to examine the effect on BER in greenhouse-grown bell pepper (Capsicum annuum L., cv. Mazurka). Pepper plants were transplanted, with the root system split into four separated compartments, each containing a single root quarter. Removal of half, or three quarters, of the root from fruit-bearing plants significantly reduced fruit BER incidence compared with plants with intact roots, especially in fruits which were at the rapid expansion stage at the beginning of the treatments. Removal of three quarters of the root reduced midday leaf water potential, stomatal conductance, and plant height. The number and weight of fruits were not affected by these treatments. Root pruning caused only a slight reduction in stem sap flow, as measured by the heat pulse technique. Calcium concentrations in the distal part of fruits from quarter-root-plants were higher than in fruits from non-pruned plants, whereas magnesium and potassium concentrations were not affected. On the other hand, in the leaves, calcium, magnesium and potassium concentrations were all reduced by root pruning. The K/Ca ratio decreased in the blossom-end of the fruits and increased in the leaves of root-pruned compared with control plants. The results suggest that root pruning did not affect the total uptake of calcium, apparently driven by transpiration, but did enhance calcium partitioning to the developing fruit. Root pruning also affected calcium distribution within the fruit and therefore attenuated BER incidence.