Search Results

Sapote mamey is a sweet and aromatic tropical fruit that is very perishable. It is a climacteric fruit and has high rates of respiration and ethylene production. Maturity indices for this commodity are difficult to define because fruit show few changes in external appearance as they ripen. The fruit flesh, however, shows large changes in color, firmness, and sugar content measured as soluble solids content (SSC). The objective was to model fruit ripeness from measurements of SSC. We selected SSC because it is easy to measure and because sweetness is an important quality attribute in sapote mamey. Typical values of SSC range from 12% (immediately after harvest) to 30% to 35% (ripe fruit). A linear-plateau model was used to describe the changes in SSC over time of ripening fruit kept at different temperatures. The model assumed that, as fruit ripened, SSC increased at a linear rate reaching a maximum of 30% SSC at the ripe stage after which SSC changed little. From the model we calculated the rate of fruit ripening and the time to reach the ripening stage (30% SSC). The rate of ripening showed a quadratic relationship with storage temperature. Fruit kept at 27, 25, or 20 °C ripened 3.5, 5, or 7 days after harvest. The model can be used to estimate when fruit will reach the ripe stage, as long as we know the initial SSC and storage temperature. This model was constructed from data obtained over 2 years from fruit grown in the state of Morelos, Mexico. It is still to be tested for its applicability on fruit from other growing regions.

Tomato spotted wilt (TSW) is a serious constraint to tomato production in various regions of the world. The effect of TSW on tomato yield is largely influenced by time of infection. Early infection usually results in severe stunting of the seedling and even death of the plant. Plastic mulches have been found to affect both the incidence of TSW, and plant growth and yield of tomato. The objective of this study was to determine the effect of root zone temperature (RZT) as affected by plastic mulch on the timing of appearance of first TSW symptoms, vegetative top fresh weight (FW) and fruit FW in tomato. Under natural TSW infection as well as in artificially inoculated plants, vegetative top FW and fruit FW were both linearly related with number of days after transplanting (DAT) the plants were free from TSW symptoms. The plants grown on black mulch during the fall showed earlier appearance of TSW symptoms, and had significantly reduced vegetative growth and fruit yields. The mean RZT during the first 42 DAT—the time before the plants reached full canopy closure—significantly affected the timing of first appearance of TSW symptoms, tomato plant growth and fruit yield. Vegetative top FW (optimal RZT = 24.1 °C) and fruit FW (26.9 °C) fitted the quadratic relationships with the mean RZT during the first 42 DAT. In conclusion, utilization of plastic mulches that created conditions of RZT stress, particularly heat stress, resulted in reduced plant growth and yield and may predispose the plants to earlier expression of TSW symptoms compared to plants grown under RZTs more favorable to tomato plant growth.

Tomato spotted wilt virus (TSWV) can cause serious damage to tomato, pepper, lettuce, and other crops. The virus is transmitted by several species of thrips. The objective of this study was to determine the effect of the time (t, days after transplanting) when TSWV symptoms first appeared on tomato plant size and fruit yield. Tomato (`Florida-47') plants were drip-irrigated and planted over black plastic mulch. The experiment was carried out in Tifton, Ga., during Spring 1999. High populations of thrips were detected since early stages of plant development, which resulted in a high incidence of TSWV. Plant fresh weight was significantly higher (r ² = 0.632, n = 216) the later in plant development TSWV symptoms first appeared (i.e., with increasing t values). Total fruit production of individual plant linearly increased with increasing t values (r ² = 0.664, n = 216). As with total fruit production, fruit marketable yield was also higher with increasing t values; however, marketable yield was significantly reduced even when plants were infected later in the season (t > 55 days after transplanting). Our results suggest that it is important to keep tomato plants free from TSWV as long as possible. This applies to both developing and developed plants.

Soil warming is one of the benefits associated with use of plastic film mulches. However, under high temperature conditions during the summer, especially in the southeastern United States, some mulches warm the soil to temperatures that might be deleterious to plant growth. Tomato (Lycopersicon esculentum Mill.) plants grown in the field were exposed to a range of root-zone temperatures (RZTs), resulting from growing the plants in different seasons and by using colored mulches that differed in their soil-warming ability. The objective was to determine the relationship of mean seasonal RZT, as affected by different colored plastic film mulches, with plant growth and fruit yield. The study consisted of experiments carried out in three seasons: Fall 1999 (five mulches, one cultivar), Spring 2000 (eight mulches and three cultivars), and Fall 2000 (four mulches and three cultivars). Treatments were black (n = 2), gray, red, silver (n = 3), and white (n = 2) mulches, and bare soil. Over the season, mean RZT decreased in the fall (from 32 to 24 °C) and increased in the spring (from 20 to 29 °C). Daily mean values of RZT over the season under plastic mulches were higher (1 to 5 °C) than those of air temperature. The highest RZT at midday occurred under black mulch, and the lowest under bare soil and white mulch. Bare soil showed the largest diurnal RZT fluctuation. RZT at midday was up to 4 °C higher under black or gray mulch compared to the other mulches or bare soil. The degree of soil warming was correlated with reflectivity of the mulch. Black mulch had the lowest light reflectance [10% photosynthetically active radiation (PAR)] while silver mulch had the highest (55% PAR). There were, however, differences in reflectance among mulches of the same color depending on the manufacturer. RZT affected vegetative top fresh weight (FW), fruit yield, fruit number, and individual fruit FW. All these growth attributes fitted a quadratic relationship with mean RZT for the season, with an optimal that ranged between 25.4 and 26.3 °C. The effects of colored mulches on plant response depended on the impact of the mulch on RZT. Plant growth and yield were highest as RZT approached the optimal RZT for the plants.

Bolting causes significant economic losses in sweet onion (Allium cepa L.) production. Although temperature and photoperiod are considered to be the main factors that initiate bolting in onions, preliminary results suggested that low N fertilization rates increased bolting. The objective of our study was to determine the relationships of bolting, yield and bulb decay with N fertilization rates. The N fertilization rates applied ranged from the infraoptimal to the supraoptimal (from 102 to 302 kg·ha^-1 N). Shoot and bulb N content increased with increasing N rates, but there were no differences in the respective shoot and bulb N contents among cultivars. Bolting incidence declined steadily with increasing N fertilization rates up to 197 kg·ha^-1 N. Bolting incidence was among the highest in the cultivar Pegasus. The percent of decayed bulbs also increased at a steady rate with the rate of N applied. Total (14.7 t·ha^-1) and marketable (0.8 t·ha^-1) yields at the lowest N rate (102 kg·ha^-1 N) were lower (P ≤ 0.01) than those at higher N rates. Rates of N ≥145 kg·ha^-1 had no significant effect on either total (mean = 33.6 t·ha^-1) or marketable (mean = 21.6 t·ha^-1) yields. Losses in marketable yield were primarily a combination of bolting and bulb decay and were minimized at 162 kg·ha^-1 N. Yield losses at low N rates were mostly due to bolting while yield losses at high N rates were mostly due to decay. Thus, excess applications of N fertilizer should be avoided since they have little effect on yields or bolting but they increase bulb decay.

Little is known about the physiological changes that occur during acclimatization and how these changes influence plant survival and growth in the new environment. In particular, it is unclear to what extent in vitro-formed roots are functional in water uptake, particularly when the plantlet is exposed to conditions of increasing evaporative demand. Tissue-cultured shoots and plantlets (shoots with roots) were acclimatized by exposing them to a linear reduction in relative humidity (RH) from 99 % to 75%over 4 days. When conductance was measured at 95% RH (21 C), in vitro shoots and plantlets showed a very high initial conductance, followed by a gradual decline, reaching steady state in 12 hours. Acclimatized shoots and plantlets had a 50% lower initial conductance compared to nonacclimatized ones, and reached steady state in 4 hours. The reduction in conductance as a result of acclimatization most likely contributes to a reduced transpiration under conditions of increased evaporative demand. Roots formed in vitro were associated with a higher plant water status, suggesting that these roots were functional in water uptake. Relative water content of the shoot was positively correlated with leaf conductance and net photosynthesis. We suggest that tissue-cultured plantlets behave as hydraulically integrated units, in which there must be a coordination between control of water loss by the shoot and uptake of water by the root to maintain a favorable plant water balance. Our results also indicate that methods that use excised shoots or leaves to determine transpiration gravimetrically may not accurately represent the stomatal water loss characteristics of tissue-cultured plants.

Tomatillo (Physalis ixocarpa Brot. ex Hornem) is a popular crop in Mexico and other Latin American countries. There is an increasing demand for this vegetable in the United States, particularly from the growing Latino population. However, there is limited information about tomatillo production. The objectives of this study were to determine the effects of plastic mulches on plant growth, yield, and root zone temperature in two cultivars of tomatillo. The study was conducted in Spring and Summer 2000. The design was a randomized complete block with a split plot arrangement, where plastic film mulch (black, gray, and silver mulches, and bare soil) was the main plot and cultivar (`Toma Verde' and `Verde Puebla]) the subplot. In the spring, mulch treatments had little effect on plant growth during the first 30 days after transplanting and there were no significant differences in fruit yields. In the summer planting, both early growth and fruit yields were greatest with the silver and gray mulch treatments and lowest on bare soil. Plant growth during the establishment was related with subsequent plant growth and yield. In mature plants, vegetative top fresh weight and total fruit yield were higher (P < 0.01) in the spring than in the summer. Total fruit yield (both seasons), marketable yield (spring) and cull yield (spring) were lower in `Toma Verde' than in `Verde Puebla'. Root zone temperatures (RZTs) in the spring (mean = 26.4 °C) were lower than in the summer (mean = 29.3 °C). In both seasons, mean RZT was highest under black mulch and lowest in bare soil. In the summer, plant growth and fruit yields tended to decrease with increasing RZTs. Tomatillo plants grown on mulches with a mean seasonal RZT of 30 °C had fruit yields that were 65% (`Toma Verde') or 50% (`Verde Puebla') lower respectively than those of plants on mulches with a RZT of 27 °C. There were no significant differences in foliar concentrations of N, Ca, Mg, S, B, Zn, Cu and Na among mulches. Foliar concentrations of the majority of mineral nutrients were not related with the mean RZT for the season.

Tomato spotted wilt virus (TSWV) is a serious constraint to tomato production worldwide. Losses are significant because the disease is difficult to control and most of the commercially available tomato cultivars are susceptible to TSWV. This study was intended to provide information that could be used to design more appropriate disease management strategies. The objective was to determine the relationship of tomato plant growth and fruit yield with the time of TSWV symptom appearance. Experiments were carried out during Spring 1999 and 2000, using drip irrigation and plastic film mulched beds with black plastic mulch alone (1999) or different colored mulches (2000). The mulches used were black, black-on-silver, gray-on-black, red, silver-on-black, silver (painted) and white-on-black, and bare soil. The 1999 experiment included a single TSWV-susceptible cultivar (Florida-47), while the 2000-experiment included two TSWV-susceptible (Florida-91 and Sun Chaser) and one TSWV-resistant cultivars (BHN-444). Colored mulches and tomato cultivars affected the time between transplanting and appearance of first symptoms of TSWV. For all tomato cultivars, vegetative top fresh weight (FW), fruit number and total fruit yield increased linearly with the time the plants remained free from TSWV symptoms. Marketable fruit yield also increased as the time from transplanting to the first appearance of symptoms increased. When data for cultivars were pooled, vegetative top FW and total fruit yield were reduced by 2.1% and 2.3%, respectively, for each day prior to harvesting that plants showed TSWV symptoms.

There are very few postharvest studies about the mamey sapote fruits. The lack of appropriate harvest indexes for this crop result in fruits having a wide variability in maturity after harvest. Fruit skin shows no apparent changes in color as maturity progresses. Another complication results from harvesting the fruit with long poles, which restricts the harvester from touching the fruit to evaluate fruit softening. The objective of this study was to evaluate the effect of exogenous ethylene applications to fruits on increasing the uniformity of fruit maturity. Fruits were harvested every 2 weeks over a 4-month period. Fruit harvest was initiated 8 weeks before the estimated ripening day. Fruits were treated by immersion for 1 min in an ethephon solution at 0, 500, or 1000 mg·liter^–1 and stored at 20°C (65% RH) for 4 or 8 days. After the storage period, fruits were analyzed for fruit firmness, color (external and internal), pH, titratable acidity, soluble solids content, ascorbic acid, and starch. Postharvest exogenous applications of ethylene stimulated postharvest ripening of the mamey sapote fruits. Ripening was associated to fruit softening, a change in pulp color from a pale pink to an intense pink color, and an increase in SSC. Fruit response to exogenous ethylene applications was small in immature fruits and increased as fruits approached the ripe stage, and decreased again in over-ripe fruits. In conclusion, postharvest applications of ethylene increased both fruit earliness and maturity uniformity in fruits.

Sunnhemp (Crotalaria juncea L.) is a tropical legume that could be an important summer cover crop in the southeastern United States, but it has the potential for suppressing both crops and weeds. Allelopathic effects of sunnhemp on weeds, vegetable crops, and cover crops were evaluated in greenhouse and growth chamber experiments. In the greenhouse, ground dried sunnhemp residues (applied mixed with the soil at 1.6% w/w) reduced percent germination of lettuce (Lactuca sativa L.) and smooth pigweed (Amaranthus hybridus L.) to a similar degree as that caused by cereal rye (Secale cereale L. subsp. cereale) residues (applied at 1.5% w/w). The allelopathic activity of sunnhemp was greater in the leaves than in the roots or stems. In growth chamber studies, the mean reduction in germination (relative to the control) caused by sunnhemp leaf aqueous extracts was: bell pepper (100%), tomato (100%), onion (95%), turnip (69%), okra (49%), cowpea (39%), collard (34%), cereal rye (22%), sweet corn (14%), Austrian winter pea (10%), crimson clover (8%), cucumber (2%), and winter wheat (2%). In lettuce, carrot, smooth pigweed, and annual ryegrass, sunnhemp aqueous leaf extract reduced seedling length to a degree similar as that produced by rye aqueous leaf extract. Sicklepod [Senna obtusifolia (L.) H.S. Irwin & Barneby CA] germination was not inhibited by any of the sunnhemp or rye aqueous extracts. In conclusion, sunnhemp reduced the germination percentage and seedling growth of various crop species. The allelochemical activity in sunnhemp was primarily in the leaves and remained active at least 16 d after harvest under dry conditions. Sunnhemp's allelochemical effect may be a useful attribute for weed management in sustainable production systems. However, plant growth in the field in crops such as bell pepper, tomato, onion, and turnip may be impacted as a result of allelopathic activity of sunnhemp residues. Thus, weed management may be more effective when sunnhemp is grown in rotation with crops that tolerate the allelochemicals from sunnhemp, resulting in optimization of the rotation effects.