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- Author or Editor: Irvin E. Widders x
Systemic acquired disease resistance (SAR) involves the activation of the natural physiological defense mechanisms against pathogen infection in a systemic manner within plants. A series of experiments were conducted in which `Beit Alpha' cucumber seed were treated with 2,6-dichloro-isonicotinic acid (INA) during osmotic conditioning or imbibition in water. Following germination and emergence, seedlings were challenge inoculated at the two-leaf stage with Colletotrichum lagenarium and incubated as a bioassay for expression of SAR. INA (289 μm) treatment of imbibed seeds reduced the number of anthracnose lesions per leaf by >90% as compared to untreated controls. Interactions were observed between INA treatment and the solute used for osmotic conditioning. Priming in 0.5 m KH2PO4 or 0.49 m sorbitol in combination with INA resulted in rapid seedling emergence and a 65% to 80% reduction in disease lesion incidence relative to primed controls. Increases in solute concentration, so as to lower the osmotic potential of the priming solution and thus extend the period of exposure to INA, did not result in enhanced SAR expression. Maximum SAR induction occurred when imbibed seeds were exposed to INA at the onset of expansive radicle growth. This study demonstrates that SAR can be abiotically induced in cucumbers during seed germination.
Abstract
Tomato (Lycopersicon esculentum Mill.) seedlings, cultured under a moderately low mineral nutrient regime, were fertilized with 4 to 28 mm N and 0.5 to 8 mm P solutions for up to 10 days before transplanting into the field. The high N solution drenches (16 and 28 mm N) promoted accelerated seedling growth, higher relative growth rates during the initial 5 days after field setting, and an increased net accumulation of N and P in the shoot following transplanting into the field. Pretransplant treatments with P did not result in a shoot growth response. The total P concentrations and contents in shoot tissues, however, were significantly higher in the seedlings treated with 4 or 8 mm P than in those receiving 0.5 mm P. Since the rates of P uptake and accumulation in the shoot relative to shoot biomass were enhanced by both the N and P pretransplant treatments, root initiation and growth may have been stimulated.
Ion concentrations in pericarp tissue, 3.2% K+, 0.9% Ca2+, 0.57% Cl- 550 ppm NO3 --N, 0.51% PO4 3--P and 1.2% SO4 2--S on a dry wt basis, differed from that in endocarp tissue, 4.1%, 0.3%, 0.46%, 96 ppm, 0.72% and 1.3%, respectively, in 5 cm diam. cucumber fruit. Controlled environment experiments tested the hypothesis that xylem and phloem differentially supply pericarp and endocarp tissues with nutrients. Application of 6 mM Ca (NO3)2 during fruit development resulted in the pericarp having higher contents of Ca2+ and NO3 - but not the endocarp. Shade (50% full sunlight) caused Cl- and NO3 - contents to decrease in both tissue. It was determined that xylem import could account for 73% and 100% of the total Ca2+ in pericarp and endocarp, respectively, but only approximately 19% and 8% of the K+. The role of xylem and phloem in supplying nutrients to developing cucumber fruits, as related to differences in transport of various ions within these two vascular systems, will be discussed.
Seedlings of processing tomato `H 2653' (Lycopersicon esculentum Mill.) were cultured in 288-cell (< 6 cm3 volume) plug trays in a soilless growing medium. Pretransplant fertilization with nutrient solutions containing 10 or 20 mm N and 2 or 5 mm P for 10 days altered the total ammoniacal-N and P, and the soluble NO3-N and PO4-P concentrations in the shoot tissue at transplanting. Post-transplanting shoot and root growth were more rapid in late May plantings than in earlier plantings. The 20-mm N and 2-mm P pretransplant treatment caused the most rapid shoot growth following early season plantings in the field. Rapid seedling establishment after transplanting was generally not a good indicator of potential fruit yield. The 5-mm P pretransplant treatment produced higher marketable fruit yields in early plantings but not in later. Culture of seedlings under a low fertilization regime (5.4 mm N, 1.0 mm P, and 1.6 mm K) before pretransplant treatment produced as high or higher fruit yields than did seedlings from a higher regimen. Withholding fertilizer temporarily before transplanting resulted in a depletion in tissue N and P concentrations, slow post-transplanting shoot growth, and lower yields.
Systemic acquired resistance (SAR) is a physiological defense response in plants conferring broad spectrum resistance to pathogens. SAR is inducible through infection by necrotizing pathogens or chemical inducers and involves the systemic activation of defense related genes. Our objectives were to evaluate resistance expression to phytophthora soft rot fruit in cucumber in response to increasing concentrations of 2,6 dichloroisonicotinic acid (INA) and benzo (1,2,3)thiadiazole-7-carbothioc acid S-methyl ester (BTH) by foliar applications. Excised leaves exhibited a resistance response to foliar applications of all concentrations of INA and BTH tested when challenge inoculated with Colletotrichum lagenarium. There was increasing benefit with increasing concentration of each chemical applied. Harvested cucumber fruit, 3.4 to 4.5 cm in diameter, were challenge inoculated with Phytophthora capsici; there were no significant chemical and rate interactions in terms of internal lesion measurements. Overall, INA consistently reduced lesion size in cucumber fruit. A bioassay conducted on fruit of different maturity levels, as defined by fruit diameter, revealed that larger sized fruit (4 to 5 cm) were more resistant to fruit rot. Fruit with diameters of 3 to 4 cm from plots treated with BTH showed little resistance as compared to the control and fruit from the same treatment with diameters of 2 to 3 cm. Fruit from plots treated with INA had at least 50% reduction in lesion size than the control. It is unclear if these differences were attributable to changes in physiological or anatomical factors. The true importance of these results should be interpreted with caution. Yield studies have not been conducted, and thus, with the experienced stunting, treatment with 100 ppm INA would be expected to lower yield and perhaps fruit quality. Determination of the optimal application regime and other cultural factors will provide broad control of plant diseases.
The effects of water deficit and fruiting on leaf gas exchange and dry-matter production and partitioning in cucumber (Cucumis sativus L.) plants were evaluated in greenhouse and field experiments. Fruiting plants had higher photosynthetic rates (15.8 μmol·m-2·s-1) than defruited plants (12.7 μmol·m-2·s-1). Although stomatal conductance was lower in defruited plants, it accounted for only ≈35% of the assimilation rate (A) reduction. Under water deficit, defruiting caused a similar response in A, even though A was only ≈50% of that in watered plants. Fruiting and water deficits limited vegetative plant dry weight and total leaf area. In field experiments, removing flowers from the first four or eight nodes resulted in a higher count and fresh weight at harvest of only those pickling cucumber fruit that were irrigated.
Abstract
Pickling cucumbers (Cucumis sativus L. cvs. Tamor and Castlepik) were direct-seeded at six plant densities (in thousands, 44, 77, 97, 121, 152, 194) using two between-row spacings (71 and 36 cm) and three within-row spacings (29, 14, and 11 cm between plants). Compared with the 29 cm within-row spacing, the 11- and 14-cm spacings resulted in significantly lower total above-ground plant dry weights, growth rates, and total leaf areas for both cultivars as early as 21 to 27 days after planting. The between-row spacing effects on plant growth were similar, but were of a lower magnitude and appeared later in plant development than for the within-row spacing effects. Leaf lamina and fruit tissue exhibited the largest reduction in tissue dry weights per plant compared to stem and petiole tissue when plant density was increased from ≈4.5 to 20 plants/m−2 (45,000 to 200,000 plants/ha). Lower fruit productivity per plant at higher plant densities resulted from fewer fruit set per plant and lower fruit : shoot ratios. Unit leaf rate (g dry weight/day per g of lamina dry weight) was not affected by plant spacing during the fruit development period. Increased densities resulted in significantly higher leaf area indexes, and vegetative and total above-ground dry weights/m−2. Total fruit yield with a single harvest did not increase above ≈77,000 plants/ha for both cultivars. A high correlation (r = 0.877) between leaf lamina dry weight and fruit growth rate indicates that net photosynthetic capacity might be limiting fruit productive potential in pickling cucumbers.
Processing tomato seedlings cultured in 288 cell plug trays were fertilized with solutions containing either 75-32-62 or 150-64-124 ppm N-P-K until the 4 to 5 true leaf stage (12 cm tall). At this developmental. stage, the seedlings were nutrient conditioned with 0-0-0, 75-32-62, 150-64-124, 300-128-248 or 450-194-374 ppm N-P-K for up to 12 days. Within 3 days of initiation of pretransplant fertilization treatments, both the contents and mean concentrations in shoot tissue of total N, P, K and soluble NO3 - were significantly altered. The maximum effects on tissue concentrations were observed within 5 days. Shoot growth rate of seedlings was affected within 5 to 8 days by a modification of elemental concentration within fertilizer solutions. The benefits of nutrient conditioning on tomato seedlings will be discussed.
Abstract
K concentrations in xylem exudate and K concentrations and contents in stem, petiole, leaf lamina, and fruit tissues were determined at 10-day intervals during the development of field-grown tomato plants (Lycopersicon esculentum Mill.). Four determinate-type processing tomato cultivars were compared. Xylem exudate K+ concentrations of 9 to 12 meq/liter were measured prior to anthesis but declined to 7 to 8 meq/liter with the onset of fruiting. Vegetative growth rates were highest at 70 days after planting. Maximum rates of K accumulation in the plants occurred at 90 days, which coincided with the period of maximum K accumulation in the fruit. The rate of K accumulation in the plant relative to the shoot dry weight was correlated with the relative growth rate. About 70% of the K accumulated in the plant at harvest was taken up during fruiting. K redistribution from vegetative tissues contributed a maximum of only 12% of the total accumulated K by the fruit. K content in leaf lamina, petiole, and stem tissue decreased by about 20 to 40%, 20 to 30%, and 15%, respectively, during fruiting, depending on the cultivar.
Cucumber plants were cultured in a greenhouse and subjected to either well-watered or water deficit conditions that reduced leaf water potential to-0.6 MPa. Leaf gas exchange measurements were conducted using an open gas exchange system. Carbon dioxide assimilation (A) attained saturation at a photon flux density (PFD) of 1000 μmol·m-2·s-1 (400-700 nm). There were no significant differences in A at ambient temperatures between 16 and 34C. Water use efficiency decreased rapidly with increasing vapor-pressure deficits to 2.5 kPa. Water stressed plants had lower stomata1 conductances and CO2 assimilation rates. The decrease in A was only partially due to stomata1 closure. The A vs. intercellular CO (Ci) relationship for stressed leaves revealed a change in the CO, compensation point, and that nonstomatal factors were contributing to the decrease in A in stressed plants. Thus, feedback inhibition of A may have occurred through photoassimilate accumulation. The concentrations of sucrose and raffinose were higher, and the concentration of stachyose was lower in leaves of stressed than of well-watered plants.