Search Results

Long-term, whole-crop CO₂ exchange measurements can be used to study factors affecting crop growth. These factors include daily carbon gain, cumulative carbon gain, and carbon use efficiency, which cannot be determined from short-term measurements. We describe a system that measures semicontinuously crop CO₂ exchange in 10 chambers over a period of weeks or months. Exchange of CO₂ in every chamber can be measured at 5 min intervals. The system was designed to be placed inside a growth chamber, with additional environmental control provided by the individual gas exchange chambers. The system was calibrated by generating CO₂ from NaHCO₃ inside the chambers, which indicated that accuracy of the measurements was good (102% and 98% recovery for two separate photosynthesis systems). Since the systems measure net photosynthesis (P_net, positive) and dark respiration (R_dark, negative), the data can be used to estimate gross photosynthesis, daily carbon gain, cumulative carbon gain, and carbon use efficiency. Continuous whole-crop measurements are a valuable tool that complements leaf photosynthesis measurements. Multiple chambers allow for replication and comparison among several environmental or cultural treatments that may affect crop growth. Example data from a 2 week study with petunia (Petunia ×hybrida Hort. Vilm.-Andr.) are presented to illustrate some of the capabilities of this system.

The effect of increasing daily light integral (DLI; 5.3, 9.5, 14.4, and 19.4 mol·m^-2·d^-1) on photosynthesis and respiration of wax begonia (Begonia semperflorens-cultorum Hort.) was examined by measuring CO₂ exchange rates (CER) for a period of 25 d in a whole-plant gas exchange system. Although plant growth rate (GR, increase in dry weight per day) increased linearly with increasing DLI, plants grown at low DLI (5.3 or 9.5 mol·m^-2·d^-1) respired more carbohydrates than were fixed in photosynthesis during the early growth period (13 and 4 d, respectively), resulting in a negative daily carbon gain (DCG) and GR. Carbon use efficiency [CUE, the ratio of carbon incorporated into the plant to C fixed in gross photosynthesis (P_g)] of plants grown at low DLI was low, since these plants used most of the C fixed in P_g for maintenance respiration (R_m), leaving few, if any, C for growth and growth respiration (R_g). Maintenance respiration accounted for a smaller fraction of the total respiration with increasing DLI. In addition, the importance of R_m in the carbon balance of the plants decreased over time, resulting in an increase in CUE. At harvest, crop dry weight (DW_CROP) increased linearly with increasing DLI, due to the increased photosynthesis and CUE at high PPF.

As a result of the decreasing availability of high-quality irrigation water, salinity tolerance of greenhouse crops is of increasing importance. Saline irrigation water can have many negative effects on plants, but also has the potential to act as a growth regulator because of its ability to reduce plant height. To determine the effects of NaCl in the irrigation water on the growth, physiology, and nutrient uptake of chrysanthemums (Chrysanthemum ×morifolium Ramat.), plants were watered with solutions with different NaCl concentrations (0, 1, 3, 6, or 9 g·L⁻¹). Plants receiving 9 g·L⁻¹ NaCl had a 76% reduction in shoot dry weight, a 90% reduction in stomatal conductance (g _S), and a 4-day delay in flowering compared with control plants. Chrysanthemums receiving 1 g·L⁻¹ NaCl had a 4-cm reduction in height with only a small reduction in shoot dry weight. Stomatal conductance and transpiration were reduced by more than 60% by NaCl concentrations of 1 g·L⁻¹ as compared with control plants. The combination of a small reduction in dry weight and a large decrease in transpiration resulted in increased water use efficiency when plants received 1 g·L⁻¹ NaCl. Concentrations of 3 g·L⁻¹ NaCl or higher resulted in poor-quality plants either as a result of wilting of the leaves (3 g·L⁻¹) or severely stunted plants (6 and 9 g·L⁻¹). Our findings indicate that chrysanthemums can be grown successfully with 1 g·L⁻¹ NaCl in the irrigation water without negative impacts on plant quality. This has important implications for the greenhouse industry as the availability of nonsaline water decreases. Saline water may be more readily available and can have the added benefit of reduced plant height, which is an important quality characteristic for floriculture crops.

Physiological antitranspirants can reduce financial risks to growers by temporarily preventing drought stress, improving product quality, and extending the shelf life of ornamental bedding plants. Exogenous abscisic acid (ABA) is an effective antitranspirant that induces stomatal closure in a rate-dependent manner, reducing transpirational water loss in many species. However, it may also cause chlorosis, which reduces product quality. Synthetic ABA analogs have similar effects on stomatal conductance (g _S) but are not known to induce chlorosis. We studied the effects of ABA and its analog 8′ acetylene ABA methyl-ester (PBI 429) on g _S and net photosynthesis (P_n) in pansies (Viola ×wittrockiana), compared the efficacy and longevity of each compound, and quantified the resulting chlorosis. Plants were treated with spray solutions of ABA (0 to 2000 mg·L⁻¹) and PBI 429 (0 to 200 mg·L⁻¹) and irrigated daily. Gas exchange and leaf chlorophyll measurements were made twice weekly for 2 weeks. Additional measurements were taken once or twice weekly through 47 days. Abscisic acid reduced leaf chlorophyll content and P_n in a rate-dependent manner for 14 days after application but reduced g _S for only 11 days, whereas PBI 429 reduced P_n and g _S similarly for 7 days and did not reduce leaf chlorophyll content. Reductions in g _S and P_n were greatest on the first day after treatment for both compounds. Our results demonstrate that ABA is more effective than PBI 429 at 100 and 200 mg·L⁻¹, but also causes chlorosis, whereas PBI 429 is an effective antitranspirant without this phytotoxic effect.

Temperature-response curves for photosynthesis and respiration are useful in predicting the ability of plants to perform under different environmental conditions. Whole crop CO₂ exchange rates of three magnolia (Magnolia grandiflora L.) cultivars (`MGTIG', `Little Gem', and `Claudia Wannamaker') were measured over a 25 °C temperature range. Plants were exposed to cool temperatures (13 °C day, 3 °C night) temperatures before the measurements. Net photosynthesis (P<sub>net</sub>) of all three cultivars increased from 3 to 15 °C and decreased again at higher temperatures. `MGTIG' had the highest and `Little Gem' the lowest P<sub>net</sub>, irrespective of temperature. The Q<sub>10</sub> (relative increase in the rate of a process with a 10 °C increase in temperature) for P<sub>net</sub> of all three cultivars decreased over the entire temperature range. `MGTIG' had the lowest Q₁₀ at low temperatures (1.4 at 8 °C), while `Little Gem' had the lowest Q<sub>10</sub> for P<sub>net</sub> at temperatures >17 °C and a negative Q<sub>10</sub> > 23 °C. This indicates a rapid decline in P<sub>net</sub> of `Little Gem' at high temperatures. The decrease in P_net of all three cultivars at temperatures >15 °C was caused mainly by an exponential increase in dark respiration (R_dark) with increasing temperature. `Little Gem' had a lower R<sub>dark</sub> (per unit fresh mass) than `MGTIG' or `Claudia Wannamaker', but all three cultivars had a similar Q<sub>10</sub> (2.46). Gross photosynthesis (P<sub>gross</sub>) was less sensitive to temperature than P<sub>net</sub> and R<sub>dark</sub>. The optimal temperature for P<sub>gross</sub> of `MGTIG' was lower (19 °C) than those of `Little Gem' (21 °C) and `Claudia Wannamaker' (22 °C). The Q₁₀ for P_gross decreased with increasing temperature, and was lower for `MGTIG' than for `Little Gem' and `Claudia Wannamaker'. All three cultivars had the same optimal temperature (11 °C) for net assimilation rate (NAR), and NAR was not very sensitive to temperature changes from 3 to 17 °C. This indicates that the plants were well-adapted to their environmental conditions. The results suggest that respiration rate may limit magnolia growth when temperatures get high in winter time.

Light and temperature responses of whole-plant CO₂ exchange were determined for two cultivars of Angelonia angustifolia Benth., `AngelMist Purple Stripe' and `AngelMist Deep Plum'. Whole crop net photosynthesis (P_net) of `AngelMist Purple Stripe' and `AngelMist Deep Plum' were measured at eight temperatures, ranging from 17 to 42 °C. P_net for both cultivars increased from 17 to ≈20 °C, and then decreased as temperature increased further. Optimal temperatures for P_net of `AngelMist Purple Stripe' and `AngelMist Deep Plum' were 20.8 and 19.8 °C, respectively. There was no significant difference between the two cultivars, irrespective of temperature. The Q₁₀ (the relative increase with a 10 °C increase in temperature) for P_net of both cultivars decreased over the entire temperature range. Dark respiration (R_dark) of both cultivars showed a similar linear increase as temperature increased. As photosynthetic photon flux (PPF) increased from 0 to 600 μmol·m^-2·s^-1, P_net of both cultivars increased. Light saturation was not yet reached at 600 μmol·m^-2·s^-1. The light compensation point occurred at 69 μmol·m^-2·s^-1 for `AngelMist Purple Stripe' and at 89 μmol·m<sup>-2</sup>·s<sup>-1</sup> for `AngelMist Deep Plum'. The lower light saturation point of `AngelMist Purple Stripe' was the result of a higher quantum yield (0.037 mol·mol<sup>-1</sup> for `AngelMist Purple Stripe' and 0.026 mol·mol^-1 for `AngelMist Deep Plum'). The difference in quantum yield between the two cultivars may explain the faster growth habit of `AngelMist Purple Stripe'.

Dietary sources of selenium (Se) are associated with human health benefits, and Brassica species are good sources of Se in human diets. Selenium and S compete for absorption and accumulation in plant tissues; therefore, the ratios of Se to S in the growing environment determine the accumulation of selenium in plants. To determine responses for Brassica oleracea L., two levels of Na₂SeO₄ (96 mg·L⁻¹ SeO₄ ^2– and 0.384 mg·L⁻¹ SeO₄ ^2–) were added to nutrient solutions with or without MgSO₄·7H₂O (96 mg·L⁻¹ SO₄ ^2–). The highest plant fresh weight and S and SO₄ ^2– accumulation were found when plants were grown in the medium with a SeO₄ ^2– to SO₄ ^2– ratio of 1 : 250 (0.384 mg·L⁻¹ SeO₄ ^2– and 96 mg·L⁻¹ SO₄ ^2–). However, the highest accumulation of Se was found when a low level of selenate (0.384 mg·L⁻¹ SeO₄ ^2–) was added to nutrient solutions without S. The activity of glutathione peroxidase (GPx) was regulated by Se status; the highest GPx activity was measured when a high level of SeO₄ ^2– (96 mg·L⁻¹) was supplied to nutrient solutions without S supplementation. The lowest concentration of total glucosinolates was found when adding SeO₄ ^2– to nutrient solutions without S. We saw no difference in plant growth and mineral accumulation when plants were grown with K₂SeO₄ versus Na₂SeO₄, suggesting that the growth-inhibiting effect of Na₂SeO₄ was the result of the SeO₄ ^2– rather than potentially toxic effects of Na⁺.

An increase in nutrient solution concentration to produce high-quality fruit vegetables, such as tomatoes, may reduce growth and yield. One reason might be inhibition of photosynthesis, but results of photosynthesis studies in the literature are inconsistent. In this study, we investigated growth and photosynthesis of whole `Celebrity' and `Counter' tomato [Lycopersicon esculentum (L.) Mill.] plants in response to nutrient solution concentration, measured as electrical conductivity (EC). The effects of two levels of photosynthetic photon flux density (PPF = 400 or 625 μmol·m^-2·s^-1) on plant response to nutrient solution EC in a range between 1.25 to 8.75 dS·m^-1 in a series of four experiments in gas exchange chambers placed in larger growth chambers were examined. Increasing PPF enhanced tomato growth and photosynthesis but increasing EC diminished them. Reduction of dry weight was 1.9% to 7.3%, while plant photosynthesis was reduced between 1.7% and 4.5% for each 1 dS·m^-1. Increasing EC did not decrease dry matter content and leaf photosynthesis. Mean plant dry matter content ranged between 70 and 95 g·kg^-1, and net leaf photosynthesis on the last measurement day was between 7.5 and 11.3 μmol·m^-2·s^-1, depending on experiment. The decrease in whole plant photosynthesis with an increase in EC was caused by decreased leaf area but not by a decrease in leaf photosynthesis.

Pre- and posttransplant growth of plug seedlings is affected by the nutrition of the plants. The effects of weekly applications of nutrient solution with different N (8—32 mM) or P and K (0.25—1.0 mM) levels on the growth and nutrient composition of impatiens (Impatiens wallerana Hook. f.) and petunia (Petunia ×hybrida hort. Vilm.-Andr.) plug seedlings were quantified. Impatiens and petunia pretransplant seedling growth was most rapid with a NO₃ ^- concentration of 24 or 32 mM (N at 336 and 448 mg·L^-1), while P and K had little effect. Increasing the N concentration in the fertilizer also increased shoot tissue N levels of both impatiens and petunia and decreased shoot P level of impatiens and K level of petunia. Posttransplant growth was most rapid in plants that received N at 16 to 32 mM. Decreasing P and K from 1 to 0.25 mM in the pretransplant fertilizer reduced posttransplant growth. Shoot P level of impatiens 15 d after transplanting decreased from 6.9 to 4.8 mg·g^-1 as the pretransplant fertilizer N concentration increased from 8 to 32 mM, while N level increased from 18 to 28 mg·g^-1 as P and K fertilizer concentrations increased from 0.25 to 1 mM. Using posttransplant growth as a quantitative norm for plug quality, the sufficiency ranges for tissue N level are 28 to 40 mg·g^-1 for impatiens and 30 to 43 mg·g^-1 for petunia plugs. These results indicate that fertilization programs for high-quality plug production should focus on N nutrition, and that plugs can be grown with greatly reduced levels of P and K.

Good fertilizer management is important in plug seedling production of bedding plants to prevent nutrient deficiencies and toxicities. We determined the effect of N, P, and K nutrition on the growth of plugs of impatiens (Impatiens wallerana Hook. f.), petunia (Petunia ×hybrida Hort. Vilm.-Andr.), salvia (Salvia splendens F. Sellow ex Roem.& Schult.), and vinca (Catharanthus roseus L.). For all four species, shoot N concentration was correlated linearly with shoot dry mass of the seedlings at transplant. Phosphorus or K concentration in the nutrient solution or shoot tissue had little or no effect on the shoot growth of seedlings, but shoot P levels increased with P concentrations in the fertilizer solution (luxury consumption). Salvia was the only species that also exhibited luxury consumption of K. Results of this study indicate that seedling growth of these species is mainly determined by N and this should probably be the main focus of fertility programs in the plug industry, while P and K applications can be reduced.