Relative growth rate (RGR), the relative increase in weight per day, can analyze the effect of environment and nutrition on growth. I examined which of the parameters responding to plant growth scaled according to RGR for lettuce and spinach grown in heated greenhouses in hydroponics with control of the nutrient solution. The experiments for lettuce in 2006–08 included all times of year, high vs. low temperature, and effect of withdrawal of nitrogen. There were four parameters that were significant in multiple linear regression vs. RGR; irradiance divided by leaf area index if it was greater than one, or normalized daily light integral (NDLI), solution temperature, electrical conductivity (EC), and logarithm solution nitrate when it was between 3 and 55 mg·L−1 N. NDLI had the most significant coefficient, but the other parameters had regression coefficients more than three times se. For experiments on spinach in 2009–10, all the parameters mentioned previously were significant in multiple linear regression vs. RGR, except EC. The coefficient for NDLI in spinach was about half the value in lettuce. The coefficients for solution temperature and low nitrate were two and three times that in lettuce. In a third set of experiments on lettuce in 1996–98, solution temperature was the only significant parameter among those mentioned previously. The coefficient for solution temperature was similar to that for regression of lettuce in 2006–08.
Martin P.N. Gent*
Shading a greenhouse increased the fraction of tomatoes that were marketable, and the marketable yield, in a comparison of greenhouse tomato yields across years, in some of which the greenhouses were shaded. In 2003, the yield and quality of greenhouse tomatoes were compared directly when grown in spring and summer in Connecticut in identical greenhouses that differed only in the degree of shade. Each half of four greenhouses was either unshaded or shaded using reflective aluminized shade cloth rated to reduced light transmission by 15%, 30%, or 50%. Each shade treatment was repeated in two houses. Tomatoes were germinated in February and transplanted in March The houses were shaded when fruit began to ripen in early June. Picking continued through August. The effect of shade on total yield developed gradually. Yields in June were unaffected by shade, but in August yield under no shade was about 30% higher than under 50% shade. In contrast, there was an immediate effect of shade on fruit size. Fruit picked in June from plants under 50% shade was 16% smaller than from plants grown under no shade. This difference declined later in the season, to 6 and 9%, in July and August respectively. The highest yield of marketable fruit in 2003 was picked from houses under no shade, but this was only 10% more than picked from the houses under 50% shade. Shade increased the fraction of marketable fruit, from 54% under no shade to 63% under 50% shade. Certain defects were decreased by shade. For instance the fraction of fruit with cracked skin was decreased from 33% to 25%. In general, effects on fruit quality varied linearly with the degree of applied shade.
Martin P.N. Gent
Some amount of shade may be optimal to produce high-quality tomatoes in a greenhouse during summer months in the northeast United States. Simultaneous comparisons were made among greenhouse sections that were either not shaded or covered with reflective aluminized shadecloth that attenuated 15%, 30%, or 50% of direct sunlight. The shadecloth was applied at the start of warm weather in June. The houses were shaded for the rest of the summer, and fruit was picked until late August. Total yield decreased linearly with increasing shade, but there was no significant difference among shade treatments in marketable yield. The fraction of fruit that was marketable was greatest for plants grown under 50% shade. This fraction was 9% greater than in a greenhouse with no shade in 2003 and 7% greater in 2004 and 2005. Cracked skin was the defect most affected by shade. Among sensitive cultivars, up to 35% of the fruit produced in greenhouses with no shade had cracked skin, whereas in greenhouses covered with 50% shade, only 24% to 26% of the tomatoes had cracked skin. There was no consistent trend for shade density in the fraction of fruit with green shoulder, blossom end rot, or irregular shape. The effect of shade increased with duration of shading. There was no effect of 50% shade compared with no shade on total yield within 20 days, but yield decreased by 20% in the interval from 25 to 45 days after shading and by 30% after 50 or more days of shading in 2005. Marketable yield only decreased after more than 45 days of shading for cultivars that were not sensitive to cracked skin or uneven ripening. Shade decreased fruit size over the entire season only in 2003. In general, shading increased the fraction of marketable tomato fruit without affecting fruit size.
Martin P.N. Gent
Strawberry (Fragaria × ananassa Duchesn.) cultivars differ in response to removal date of row covers when they are used for winter protection and to accelerate fruit development and production. In 1986-87 and 1987-88, eight cultivars were overwintered under either spun-bonded polypropylene row cover or under straw. The straw was removed from control plots in late March. Row covers were removed on four dates beginning in late March and separated by about 2-week intervals. The time of flowering, fruit set, and fruit ripening was advanced in direct relation to the time that row covers remained over plants in spring. The differences in time of fruit ripening were less than those of time of flowering, however. The mid-harvest date was advanced as much as 8 days for `Earlidawn' and `Midway', but only 4 days for `Redchief' and `Scott'. Weight per fruit and percentage of marketable fruit were reduced when plants remained under row cover until mid-May. This effect was most noticeable for `Earlidawn', `Guardian', and `Redchief'. The fruit quality of `Midway' and `Jerseybelle' was not significantly affected by date of row cover removal. These cultivar-specific responses were probably not related to the stage of fruit development when row covers were removed, as both early and late-flowering cultivars were sensitive (and insensitive) to the date of row cover removal.
Martin P.N. Gent
Serial plantings of hydroponic lettuce were grown throughout the year in the northeast United States to determine how sunlight intensity and solution nitrate affect nitrate in leaf tissue. Two nutrient solutions were used. All essential elements were supplied at the same concentration, except nitric acid was added to the high-N treatment to increase nitrate to 5.7 mm (352 ppm), compared to 4.0 mm (248 ppm) in the low-N treatment. A feedback control system maintained a constant conductivity and volume in the recirculating nutrient solution. The actual nitrate concentration in solution was higher in winter than in summer. In winter, it rose to 800 ppm in the high-N solution, while it remained below 200 ppm in the low-N solution. In summer, nitrate was 200 to 400 ppm in the high-N solution, compared to 40 to 120 ppm in the low-N solution. Concentration of other mineral elements remained at levels similar to the original formulation. Nitrate concentration in leaf tissue when the lettuce plants reached a marketable size was sensitive to sunlight and nitrate supply. In spring and summer, tissue nitrate was as low as 1100 ppm. It increased to about 4000 ppm in lettuce grown in mid-winter in a shaded greenhouse and fed high-N solution, while low-N plants had less than 3000 ppm nitrate. Tissue nitrate was related to solution nitrate. Tissue nitrate increased in proportion to solution nitrate, up to about 400 ppm nitrate in solution, then leveled off at a concentration of about 4000 ppm in the leaves, a relation that was the same under all sunlight intensities. The accumulation of nitrate in the nutrient solution was one cause of the high concentration of nitrate in lettuce leaves.
Martin P.N. Gent
Leaf tissue composition may depend on season and method of fertilization. Lettuce and spinach were grown in spring, fall, and winter in unheated high tunnels using organic or conventional fertilization. The root medium was either perlite fertilized with a nitrate-based complete soluble fertilizer, or leaf compost/perlite 1:1 v/v fertilized with cottonseed meal. Growth rate did not differ due to the method of fertilization, but specific leaf area was 10% greater with compost. Growth in compost raised the concentrations of total reduced nitrogen, phosphate, and potassium in both species. Effects of season were factored with a 3rd-order polynomial in Julian day. Nitrate, total phosphorus and potassium varied with season. Interaction of effects of season and fertilization were only significant for total reduced nitrogen and phosphate in leaves of lettuce. The difference in nitrogen due to fertilization was larger in fall harvests than at other times of year. There was a similar, but nonsignificant, trend with time for total reduced nitrogen in spinach. Differences in nitrate due to fertilization were small, compared to those of reduced nitrogen. The same seasonal trend in potassium was seen in both species, and for both methods of fertilization. Concentrations were highest in spring and lowest in fall.
Martin P.N. Gent
Nutrient availability may depend on method of fertilization particularly when the root medium is cool. The salad greens, arugula, lettuce, and spinach, were grown in spring, fall, and winter using organic or conventional fertilization to test this hypothesis. Field plots were mineral soil fertilized with 10N-10P-10K, or soil was amended with leaf compost and cotton-seed meal. Unheated high-tunnel plantings plots contained either perlite fertilized with a complete soluble fertilizer or a 1 leaf compost: 1 perlite mixture fertilized with cotton-seed meal. There was no consistent difference in growth due to the method of fertilization, either in the field or in high tunnels. Over all plantings in field and high-tunnel plots, concentrations of nitrogen and phosphorus were higher in leaves of plants grown with leaf compost. The time of year did not affect the difference in composition between plants grown in compost and perlite in a manner that could be related to the environment or rate of growth. Although relative growth rates were only 5% per day in high tunnels in winter compared to 10% to 18% per day in other seasons, the difference in reduced nitrogen among plants grown in compost and perlite was similar in winter and summer. The changes in composition due to method of fertilization were similar in all three plant species under study.
Martin P. N. Gent
Tomatoes were grown in spring and summer in Connecticut in greenhouses covered with a double layer of 4-mL clear polyethylene film. Some sections were covered with reflective aluminized shadecloth that provided 85%, 70%, or 50% transmittance of direct radiation, respectively. This shading was applied in mid-June, after fruit began to ripen, and remained for the rest of the summer. Fruit was picked through August. A similar experimental protocol was used in 2003 and 2004. The maximum shading only decreased daily integrated solar radiation to 69% of that without shade, as measured by PARsensors set at a 2-m height in each greenhouse. Shading reduced yield of ripe fruit from 16.6 and 13.1 kg·m-2, proportional to the measured decrease in radiation. Neither fruit size nor weight fraction of marketable fruit was affected by shading in 2004. Nutrient content was analyzed in tissues of ripe fruits, and uppermost expanded leaves harvested in early August. As shading decreased transmittance, it increased the concentration of most elements in leaves. Total N, P, and K concentrations followed this trend; however, Ca was not affected by shading. Fruit dry matter content declined slightly, from 5.9% to 5.7% of fresh weight, for plants grown with no shade or shade with 50% transmittance, respectively. However, there was no significant effect of shading on K, Ca, Mg, or on minor elements in fruit tissue, whether expressed on a fresh weight or dry weight basis. Thus, shading a greenhouse to improve fruit quality had no effect on the value of ripe tomatoes as a dietary source of mineral nutrients.
Martin P.N. Gent
The concentrations of metabolites in plants are affected by sunlight integral and other factors such as plant size, water content, and time of day. Tissue composition was measured for various sizes of hydroponic lettuce (Lactuca sativa L.) grown under seasonal variation in sunlight in a greenhouse and harvested in the morning or afternoon. Daily sunlight integral varied from 4 to 14 mol·m−2·day−1 photosynthetically active irradiance, and plant size varied from 2 to 260 g fresh weight (FW)/plant in this study. Much of the variation in tissue composition on a FW basis could be explained by the increase in dry matter content with irradiance normalized per unit area. Except for nitrate, metabolite concentrations on a FW basis increased with irradiance, and the changes resulting from irradiance were greater when harvested in the afternoon than in the morning. Nitrate concentration decreased with normalized irradiance, and the trend was the same whether measured in morning or afternoon. Malic acid increased with irradiance but not enough to counter the decrease in nitrate on a charge equivalence basis. Irradiance normalized per unit leaf area explained many effects of light and plant size on dry matter content and soluble metabolite concentrations. Lettuce for human consumption is best harvested in the afternoon after growth under high light, when it has the least nitrate and more of other nutrients.
Martin P.N. Gent
Solution electrical conductivity (EC) and the supply of nitrate in proportion to other elements (nitrate supply ratio) should effect tissue composition of lettuce (Lactuca sativa L.) grown in hydroponic solution. These parameters were varied in several series of successive plantings in greenhouses in the northeast United States. In 1996, when the treatments differed only in EC, 0.65 and 0.9 dS·m-1, but not in nitrate supply ratio, leaf tissue had more nitrate and total reduced-N and lettuce grew faster in the solution with higher EC. Over four series of plantings in 1997 and 1998, the nitrate supply ratio of a low-N treatment was only 60% of that for a high-N treatment, and EC was varied from 1.2 to 2.0 dS·m-1. In 1997 and 1998, tissue nitrate was lower in the low-N treatment only when EC was less than in the high-N treatment. However, under irradiance greater than 10 MJ m-2 per day, the lower EC also slowed growth. Stepwise regression over data from all experiments showed leaf nitrate was primarily a function of EC, and a term that described the interaction between irradiance and EC. Due to selective uptake by the plants, the ratio of elements in the recirculating solution differed from the ratio in which they were supplied. Under irradiance less than 10 MJ m-2 per day and solution EC greater than 1.5 dS·m-1, nitrate accumulated in solution to a concentration greater than expected from simple dilution of the concentrates. Tissue nitrate was also related to solution nitrate, increasing by 0.08-0.09 mg·g-1 dry weight per 1 mg·L-1 increase in solution nitrate. To prevent a rise in tissue and solution nitrate under low irradiance, both solution EC and nitrate supply ratio had to be reduced by about one-third, compared to the conditions required for rapid growth under high irradiance.