Beny Aloni

Effects of climatic factors and water stress on the diurnal ¯uctuation in diameter of bell pepper fruit

Summary Cracking of bell pepper fruits is one of the most severe problems affecting their marketable yield. Repeated shrinkage and expansion weakens the fruit cuticle and causes cracking. This study investigated the effects of climatic factors and water stress on the diurnal fluctuation in pepper fruit diameter. Two experiments were conducted in three controlled greenhouses with different air temperatures and relative humidities. The results indicated that the factors affecting leaf transpiration, such as solar radiation, vapour pressure deficit (VPD), air temperature and water stress, affect the timing and amplitude of diurnal fluctuations in diameter of bell pepper fruit. Earlier fruit shrinkage start time (FSST) in the morning, earlier fruit shrinkage ending time (FSET) in the afternoon and larger shrinkage amount were observed in fruits grown in the greenhouse with high night air temperature and low night vapour pressure deficit. Plant root water stress resulted in earlier FSST. Higher incoming radiation increased fruit shrinkage. Sunlit fruits shrank more than those in shade under the same environmental conditions, which is a possible mechanism for more cracking in sunlit fruits. However, no differences of FSST and FSET were observed between fruits in sun and in shade.

Antagonistic effects of paclobutrazol and gibberellic acid on growth and some biochemical characteristics of pepper (Capsicum annuum) transplants

Abstract Spraying pepper ( Capsicum annuum L.) transplants (30 days old) with paclobutrazol (PP333) at concentrations higher than 1 mg l −1 inhibited their growth. At 0.5 and 1 mg l −1 , seedling growth was slightly enhanced, whereas at 50 mg l −1 stem elongation was completely inhibited. At all PP333 concentrations tested (0.5–50 mg l −1 ), root development was stimulated and leaf chlorophyll was increased. Spraying 50-mg l −1 PP333-treated seedlings with gibberellic acid (GA 3 ) after their growth had been inhibited caused a resumption of stem elongation, reduction in chlorophyll content of the newly developed leaves, reduced root weight gain, and concomitantly stimulated root exudation, which had been almost totally abolished in the PP333-treated seedlings. Fourteen days following application of PP333 (50 mg l −1 ) there was a slight increase in soluble sugars in both leaves and roots. However, most noticeably the free proline content was 3- and 4-fold higher in the roots and leaves, respectively, of treated plants than in untreated plants. The changes in the free amino acids level were small, suggesting that proline accumulation was unique. Four days following treatment of these transplants, treated transplants returned to the levels found in the untreated transplants. On the other hand, when given in solution to leaf discs prepared from PP333-treated plants, GA 3 did not change proline content, nor did GA 3 have any effect on proline levels of discs which were pre-loaded with exogenous proline or which accumulated proline during disc incubation in PEG solution. It is suggested that GA 3 causes the disappearance of the accumulated proline and sugars in the PP333-treated seedlings by influencing the growth of the young sink leaves and not by an in situ effect on metabolization of the substrates.

Interaction with salinity of GA3-induced leaf elongation, petiole pithiness and bolting in celery

Abstract GA 3 sprayed on celery plants, which received a continuous supply of 0, 0.2, 0.4, 0.6 or 1.0% NaCl solutions to the roots, caused a pronounced increase in leaf elongation. Along with enhanced elongation, GA 3 increased breakdown of parenchymatous tissue in the petiole, but this effect was drastically suppressed in the salt-treated plants. In a similar fashion, the effect of GA 3 on the elongation of the flowering-stem (bolting) was significantly diminished in salt-treated plants. In no case did the salinity treatment totally abolish the bolting-process induced by GA 3 . Changing the temperature of the lower part of the root system altered the magnitude of GA 3 effect on bolting. As the temperature was lowered from 30 to 6° C, the ability of GA 3 to cause bolting was greatly reduced. The results are discussed in view of the possible interaction between salinity and the GA 3 effect on petiole elongation, cellular breakdown and bolting.

Varietal differences in the susceptibility to pointed fruit malformation in tomatoes: histological studies of the ovaries

Differences were found among three tomato cultivars (cvs.) in their susceptibility to pointed-fruit malformation. Pointed fruits are characterized by a conical, rather than a round shape. This type of fruit malformation is often found in winter-grown plants and is due to an uneven development of the locules. Transverse cross-sections of the fruit revealed that one or more of the locules had not developed, forming a flattened side, while the other side of the fruit was round (normal) in shape, because of the presence of developed placenta(s), with seeds and jelly filling their cavity(ies). Histological examination of the ovaries showed that under high night temperatures all the locules, of all the examined cvs., were normal and contained placentas with attached ovules. Ovaries that developed under lower night temperatures contained deformed locules: in the less severe cases the placenta was retarded, with fewer attached ovules than normal; in the most severe ones the locules were severely deformed and the placentas and ovules were absent. The number of deformed ovules per fruit and the degree of deformation were cultivar- and night temperature-dependent. Artificial pollination of the affected ovaries and exogenous auxin application did not improve the fruit shape. It is, therefore, concluded that the absence of placentas, seeds and jelly from one or more locules brings about the formation of a malformed, pointed (lop-sided) fruit.

Effects of salts on NADH-malate dehydrogenase activity in celery leaves during their maturation and senescence

Abstract The effect of several inorganic salts on the activity of NADH-malate dehydrogenase (MDH) from the petiole and the blade of celery leaves of different age, was investigated. KCl and NaCl, in a concentration range of 100–180 mM, activated the enzyme similarly when given in the assay medium. At higher concentrations, both salts became inhibitory. CaCl2 and MgCl2 activated the enzyme at a much lower concentration, with MgCl2 being the weaker activator. The activation of celery leaf MDH by KCl was found to vary with the leaf age. As the leaf matured, the extracted enzyme exhibited increased activation by KCl. However, the transition from maturity to senescence was accompanied by a decline in the activation of the enzyme by the salt. During senescence of detached leave blades in the dark, the MDH activation by KCl undergoes the same changes found in the intact plant. The changes in MDH activity due to either age or KCl, were a result of changes in the Vmax and not of the Km-value for oxaloacetate. Acceleration of senescence by transient water stress or flooding induced a marked increase in the MDH activation by KCl in vitro. It is suggested that the change in the enzyme response to ions is part of a regulation mechanism by which MDH activity is maintained during various developmental stages of the leaf. However, in accelerated senescence situations, this regulation mechanism is disrupted.

Dehydrogenase activity in dehydrated parenchyma slices: Probing with 2,3,5-triphenyl tetrazolium chloride

Abstract The reduction of 2,3,5-triphenyl tetrazolium chloride (TTC) in slices, prepared from petioles of celery leaves ( Apium graveolens L.) and potato tubers ( Solanum tuberosum ), was found to be highly sensitive to dehydration. Incubation of the slices in mannitol or sucrose solutions with an osmotic potential slightly higher than isotonicity, inhibited the TTC reduction by about 70%. The inhibition was instantaneous and reached a maximal value as the tissue reached equilibrium with the dehydrating solution. The inhibition of TTC reduction was reversed if the slices were rehydrated in distilled water and regained the initial water content. However, under conditions at which the dehydrated state could not be fully reversed, the inhibition of the TTC reduction was not reversed. Incubation of tissue slices in non-dehydrating concentrations of KCl, NaCl, CaCl 2 and MgCl 2 for 60 min caused marked inhibition of TTC staining, with the divalent salts being inhibitory at much lower concentrations than the monovalent salts. Soluble MDH from celery petiole slices was affected similarly by the monovalent and the divalent salts. The results suggest that the changes in TTC reduction under mild dehydration conditions are caused by in vivo reversible inactivation of dehydrogenases activity and that these enzymatic changes are related to alteration of the ionic concentration in the cytoplasm.