Microsoft word - 1st paper jovicich ishs pisa 2003.doc
Managing Greenhouse-grown Peppers in a Saline Environment
Indian River Research and Education Center
Keywords: Capsicum annuum
, transplant depth, irrigation, physiological disorder,
soilless culture, stem rot, Fusarium
Under certain conditions, greenhouse-grown pepper plants from various areas
of the world, including the Mediterranean and North America, exhibit symptoms
where the base of the main stem becomes swollen below the cotyledonary node level
and crack-like wounds develop at the base of the stem’s epidermis. Plants with this
disorder develop a localized rot which can result in sudden plant wilts. This
phenomenon has been observed in both, soil and soilless cultivated plants. We
proposed that deposits of salts on basal stem epidermal tissues may contribute to
localized injuries, which can predispose the plant to an infection by opportunistic
pathogens. In greenhouse studies with peppers grown in soilless culture, the
percentage of plants with epidermal wounds at the base of the stem was highest on
plants where seedling containers were placed on top of the media (perlite, pine bark,
or peat mix) surface of 12-L pots as compared with plants where the root ball was
transplanted directly into the media. A low percentage of plants transplanted with
the root ball to the cotyledonary node level exhibited epidermal damage, while
plants transplanted to the second leaf node did not exhibit epidermal damage. The
amount of salts deposited on the base of the stem was quantified and associated with
the level of epidermal damage. A positive linear relationship occurred between the
percentage of plants with epidermal wounds and the electrical conductivity of a
solution obtained from washing the epidermal tissues at the base of the stem. High
volumes of nutrient solution delivered close to the basal stem increased the amount
of deposited salts. Simple management practices, such as transplanting deep, using
cultivars with lower susceptibility to salt damage, and gradually moving back the
emitter from the base of the plant after transplanting (to reduce humid conditions
near the base of the stem) would help reduce the basal stem disorder in soilless-
Soilless-greenhouse-grown pepper (Capsicum annuum
) plants can present a
temporary or permanent plant wilt as a result of opportunist vascular-wilt fungi (i.e., Fusarium oxysporum
), which can enter the basal stem tissues through epidermal wounds. Similar descriptions of symptoms where epidermal wounds developed on a swollen region of the stem, just below the cotyledonary node, and which may be followed by a
sudden plant wilt, have been reported in greenhouse-grown pepper plants in Florida, USA (Jovicich et al., 1999; E. Lamb, personal communication), Italy (Cartia et al., 1988; Matta and Garibaldi, 1980), Spain (Morató, 1996), and Canada (Ferguson and Khosla, 1998). We also noticed basal stem epidermal wounds on pepper plants in commercial greenhouses in Florida, Murcia (Spain), and Albenga (Italy). In Italy, Matta and Garibaldi (1980) reported that temporary water logging caused the “foot corkiness” of soil-grown pepper. This same disorder was reported to be common in Sicily, with 60% of the total plants with basal stem rot (Cartia et al., 1988). Morató (1996), in Spain, reported a similar physiological disorder named bell pepper “basal stem rot,” developed as a plant response from a localized area with high humidity and lack of aeration around the stem base. Spanish and Canadian greenhouse growers have coined this disorder in pepper plants “Elephant’s Foot,” based on a description of the lower stem appearance (Ferguson and Khosla, 1998; Jovicich et al., 1999). In Florida, pepper plants grown in soilless media in both, greenhouses (Jovicich et al., 1999) and under conditions of high fertilizer and irrigation levels in the open field (E. Lamb, personal communication) had the “Elephant’s Foot” symptoms. Factors that may contribute to the cause of the disorder are not known. Morató (1996) reported that the basal stem rot can be caused by a constant or frequent drip of solution due to the proximity of the irrigation tubing to the stem base. Epidermal injuries on the base of the stem could be a plant response to salt accumulation and/ or excess of humidity. Even in soilless culture and with irrigation water of good quality, a localized saline environment could be created at the basal stem level as a consequence of irrigation and fertilization management, position of the irrigation emitter, and type of substrate. Identification and management of factors that avoid epidermal injuries at the basal stem level will prevent infections of opportunistic fungi and plant wilt.
We investigated the effects that different daily nutrient solution volumes, soilless
media, and transplant depths would have on salt accumulation and damage of the
epidermis at the stem base of pepper plants.
MATERIALS AND METHODS
The experiment was conducted in a passive-ventilated polyethylene-covered
greenhouse (Top Greenhouses Ltd., Rosh Ha’ayin, Israel) located at the Horticultural Sciences Protected Agriculture Center, University of Florida (http://www.hos.ufl.edu/protectedag/). Seedlings of bell pepper ‘Kelvin’ (De Ruiter Seeds Inc., Bergschenhoek, Holland) were grown in a 70% peatmoss : 30% vermiculite (v/v) substrate mix (Terra Asgrow, Apopka, FL) in polyethylene containers (475 cm3, Lerio Corp., Kissimmee, FL). In these containers, the cotyledonary nodes of the seedlings were at the surface of the substrate level.
On 29 June 1999, the seedlings in the containers were transplanted into cylindrical
polyethylene pots (12-dm3; Lerio Corp., Kissimmee, FL) at three transplant depths: a) at half of the container height (3.8 cm), cutting and discarding only the bottom of the container, b) at the cotyledonary node level, and c) at the second leaf node, about 3.0 cm above the cotyledonary node.
Twenty one days after transplanting (DAT), five irrigation treatments were started.
Plants were irrigated during daylight, with the same frequency but with different irrigation periods in order to provide daily volumes of 2, 2.5, 3, 3.5, and 4 L of nutrient solution per plant. At transplanting, the stake of the irrigation emitter (2 L·h-1, Netafim, Altamonte Springs, FL) was placed 2.5 cm away from the base of the stem. In plants transplanted with the bottomless polyethylene container, the emitter stake was moved from the top to the bottom of this container 21 DAT at time when roots from these plants were growing into the media.
Irrigation and transplant depth treatments were evaluated in four soilless media: a)
perlite of “horticultural grade” (Airlite Processing Corporation of Florida, Vero Beach, FL), b) coconut coir (Scotts-Sierra Horticultural Products Company, Marysville, OH), c) peat-vermiculite-perlite mix (60:20:20 % by volume, respectively) (Speedling, Bushnell, FL), and d) nonaged pine bark (Pinus
spp.), with particles size smaller than 6.3 cm2 (Elixon Wood Products Inc., Starke, FL).
Plants were irrigated with a complete nutrient solution with nutrient concentration
levels developed for greenhouse-grown hydroponic tomatoes (Hochmuth, 1991) and adapted for pepper plant developmental stages in this experiment (Jovicich, 2001). After fruit set in lower nodes, nutrient concentration levels of NO -
the irrigation solution were: 120, 50, 174, 143, 48, and 66 mg·L-1, respectively. The electrical conductivity (EC) in the solution was 2.0 mS·cm-1 and the pH 6.2.
Plants at a population density of 4.2 plants per m2 were not pruned and their
canopies were vertically supported as it is normally done in southern Spain (Nuez et al., 1996; Jovicich et al., 2002). The experimental design was arranged as split-split plot with 3 blocks. Irrigation volume was the main plot, transplant depth was the subplot, and medium type was the sub-subplot.
On 25 November (149 DAT), the percentage of plants with “Elephant’s Foot”
symptoms (crack-like wounds on the stem epidermis at the surface of the soilless media level) was determined. The lower 25 mm lengths of the stems were severed and their cylindrical-shaped epidermal tissues were peeled. The amount of salts deposited at the base of the stems was estimated by washing the epidermal sample in deionized water and measuring the EC (Twin Cond B-173, Horiba Ltd., Kyoto, Japan) in the recovered solution. Values of EC were expressed on an epidermal tissue area basis (ECA, units in mS·cm-1·mm-2) in order to relate accumulated salts to the epidermal area exposed at the surface of the media level.
Analyses of variance (ANOVA) were performed on the ECA measurements and on
the percentage of plants with basal stem swelling and epidermal damage symptom observations (SAS, 1999). Percentages of plants with wounds on the epidermis were arcsin-transformed prior to conducting the ANOVA. Correlation analysis was performed between ECA and percentage of plants with basal stem swelling and epidermal damage symptom. Volume of irrigation response was analyzed for its polynomial effects (SAS, 1999). Treatment means were compared using least significant (LSD) mean separation method.
RESULTS AND DISCUSSION
Transplant depth affected both the percentage of plants with epidermal wounds and
the amount of salts deposited at the base of the plant stems (Figure 1). Plants transplanted to half of the container height had the highest percentage of plants with wounded epidermis (83%) followed by significantly lower values in the plants transplanted to the cotyledonary node level (6%), while plants transplanted to the depth of the second leaf node did not exhibit any damage on the epidermis at the surface of the media level. Values of ECA were twice as high in plants transplanted to half of the container height (1.230 mS·cm-1·mm-2), with no difference in ECA between plants transplanted to the cotyledonary node level and plants transplanted to the depth of the second leaf node (0.626 and 0.625 mS·cm-1·mm-2, respectively) (Figure 1).
Irrigation with a nutrient solution affected the amount of salts at the base of the
stem but did not affect significantly the percentage of plants with epidermal wounds. The ECA increased linearly with increased deliveries of nutrient solution (2 to 4 L·day-1 per plant) (Figure 2A).
The treatments with a high percentage of plants with basal stem epidermal damage
corresponded with high values of ECA in the solution of water washed from the stem base. A positive linear relationship (r = 0.81) was obtained between ECA values and the percentage of plants with epidermal damage when all treatment data were considered (Figure 2B).
The type of media used in our study did not affect percentage of plants with basal
stem epidermal damage and did not affect the salt concentrations at the base of the stem in plants transplanted to the cotyledonary node level and plants transplanted to the depth of the second leaf node. Although less salts were deposited at the base of the plant stem when plants transplanted to half of the container height were grown in pine bark instead of the other media, the types of media did not appear to have an effect in the salt accumulation nor on the appearance of epidermal wounds.
Positioning the irrigation emitter at the bottom of the seedlings transplanted to half
of the container height, creating a greater evaporation at the top of the container, might have concentrated a greater amount of salts at the surface as compared with those transplants where the root ball was planted directly in the soilless media. The level of salts deposited around the base of the stem might depend on the volume and concentration of nutrient solution applied and on the placement of the emitter with respect to the transplant, as salts usually concentrate on the wet and dry boundary area of the soilless media.
A lower saline environment around the base of the stem was created by
transplanting the rootball directly into the pot media. Moreover, transplanting the pepper seedlings with the cotyledonary node under the soilless media surface may be a simple management practice to elude the development of the “Elephant’s Foot” disorder in hydroponic greenhouse bell pepper without any subsequent problems associated with plant growth or fruit production. In following pepper crops, we observed that transplanting to the first leaf node had the same positive response as had transplanting to the second node, and it was also noted that pepper cultivars could have a differential susceptibility to this disorder (Jovicich, 2001). In addition, gradually moving back the
emitter from the base of the plant after transplanting (to reduce humid conditions near the
base of the stem) helped to reduce the appearance of the disorder in soilless-grown
peppers (Jovicich et al., 1999). ACKNOWLEDGMENTS
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population, transplant depth, soilless media and irrigation. M.S. Thesis, Univ. of Fla., Gainesville.
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Figure 1. Percentage of plants with epidermal damage (A) and salts deposited at the base
of the pepper plant stems at different transplant depths (B). ECA: electrical conductivity of the solution used to wash a known area of epidermal sample at the base of the stems. Bars with different letters are significantly different based on LSD test (α = 0.05).
Figure 2. Responses of salts deposited at the base of the pepper plant stems (measured as
ECA) to increased volumes of nutrient solution per day (A), and correlation between percentage of plants with epidermal damage and salts deposited at the base of the stem (B).
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