Although the principle of mass balance is well-understood, few people understand how Hoagland and Arnon used it to develop their famous nutrient solution recipes. Here I review: 1) the application of mass balance in deriving unique hydroponic solution recipes, 2) the dangers of dumping and replacing hydroponic solutions, 3) the need to alter the silicon and chloride concentrations in Hoagland's solution based on recent advances in our understanding of plant nutrient requirements.
There is an increasing need to recirculate and reuse nutrient solutions to reduce environmental and economic costs. However, one of the weakest points in hydroponics is the lack of information on managing the nutrient solution. Many growers and research scientists dump out nutrient solutions and refill at weekly intervals. Some authors have recommended measuring the concentrations of individual nutrients in solution as a key to nutrient control and maintenance. Dumping and replacing solution is unnecessary. Monitoring ions in solution is unnecessary; in fact the rapid depletion of some nutrients often causes people to add toxic amounts of nutrients to the solution. Monitoring ions in solution is interesting, but it is not the key to effective maintenance. During the past 18 years, we have managed nutrients in closed hydroponic systems according to the principle of “mass balance,” which means that the mass of nutrients is either in solution or in the plants. We add nutrients to the solution depending on what we want the plant to take up. Plants quickly remove their daily ration of some nutrients while other nutrients accumulate in the solution. This means that the concentrations of nitrogen, phosphorous, and potassium can be at low levels in the solution (<0.1 mM) because these nutrients are in the plant where we want them. Maintaining a high concentrations of some nutrients in the solution (especially P, K, and Mn) can result in excessive uptake that can lead to nutrient imbalances.
Christopher Parry and Bruce Bugbee
Maize (Zea mays) is increasingly grown in controlled environments to facilitate phenotypic analysis. Even with ample chelated iron (Fe), maize often develops interveinal chlorosis in soilless substrates or hydroponics because of inadequate bioavailable Fe in the plant. We hypothesized that the excessive phosphorus (P) in standard greenhouse fertigation solutions would accentuate the chlorosis. Here, we report that reducing the P concentration from 0.7 to 0.07 mmol·L−1 (22 to 2 mg·L−1) provided adequate P for rapid growth and increased chlorophyll concentration from 263 to 380 µmol·m−2. Restricted root-zones in containers require frequent watering and are often watered to excess, which flushes the root-zone with a high P solution. In a separate study, minimizing the leaching fraction increased leaf chlorophyll concentration from 123 to 508 µmol·m−2. The use of a ceramic substrate typically improves the green leaf color of maize plants. Consistent with this observation, we found no effect of high P concentration in the irrigation solution on growth or chlorophyll density in ceramic substrates because it strongly absorbs P from solution. These findings can significantly improve maize growth and nutrition in controlled environments.
Paul Kusuma and Bruce Bugbee
Phytochrome, a well-studied photoreceptor in plants, primarily absorbs in the red (R) and far-red (FR) regions and is responsible for the perception of shade and subsequent morphological responses. Experiments performed in controlled environments have widely used the R:FR ratio to simulate the natural environment and used phytochrome photoequilibrium (PPE) to simulate the activity of phytochrome. We review why PPE may be an unreliable metric, including differences in weighting factors, multiple phytochromes, nonphotochemical reversions, intermediates, variations in the total pool of phytochrome, and screening by other pigments. We suggest that environmental signals based on R and FR photon fluxes are a better predictor of plant shape than the more complex PPE model. However, the R:FR ratio is nonintuitive and can approach infinity under electric lights, which makes it difficult to extrapolate from studies in controlled environments to the field. Here we describe an improved metric: the FR fraction (FR/R+FR) with a range from 0 to 1. This is a more intuitive metric both under electric lights and in the field compared with other ratios because it is positively correlated with phytochrome-mediated morphological responses. We demonstrate the reliability of this new metric by reanalyzing previously published data.
Stephen P. Klassen and Bruce Bugbee
Kevin R. Cope and Bruce Bugbee
Light-emitting diodes (LEDs) are a rapidly developing technology for plant growth lighting and have become a powerful tool for understanding the spectral effects of light on plants. Several studies have shown that some blue light is necessary for normal growth and development, but the effects of blue light appear to be species-dependent and may interact with other wavelengths of light as well as photosynthetic photon flux (PPF). We report the photobiological effects of three types of white LEDs (warm, neutral, and cool, with 11%, 19%, and 28% blue light, respectively) on the growth and development of radish, soybean, and wheat. All species were grown at two PPFs (200 and 500 μmol·m−2·s−1) under each LED type, which facilitated testing the effect of absolute (μmol photons per m−2·s−1) and relative (percent of total PPF) blue light on plant development. Root and shoot environmental conditions other than light quality were uniformly maintained among six chambers (three lamp types × two PPFs). All LEDs had similar phytochrome photoequilibria and red:far red ratios. Blue light did not affect total dry weight (DW) in any species but significantly altered plant development. Overall, the low blue light from warm white LEDs increased stem elongation and leaf expansion, whereas the high blue light from cool white LEDs resulted in more compact plants. For radish and soybean, absolute blue light was a better predictor of stem elongation than relative blue light, but relative blue light better predicted leaf area. Absolute blue light better predicted the percent leaf DW in radish and soybean and percent tiller DW in wheat. The largest percentage differences among light sources occurred in low light (200 μmol·m−2·s−1). These results confirm and extend the results of other studies indicating that light quantity and quality interact to determine plant morphology. The optimal amount of blue light likely changes with plant age because plant communities balance the need for rapid leaf expansion, which is necessary to maximize radiation capture, with prevention of excessive stem elongation. A thorough understanding of this interaction is essential to the development of light sources for optimal plant growth and development.
Oscar A. Monje and Bruce Bugbee
Two types of nondestructive chlorophyll meters were compared with a standard, destructive chlorophyll measurement technique. The nondestructive chlorophyll meters were 1) a custom built, single-wavelength meter, and 2) the recently introduced, dual-wavelength, chlorophyll meter from Minolta (model SPAD-502). Data from both meters were closely correlated with destructive measurements of chlorophyll (r2 = 0.90 and 0.93; respectively) for leaves with chlorophyll concentrations ranging from 100 to 600 mg·m-2, but both meters consistently overestimated chlorophyll outside this range. Although the dual-wavelength meter was slightly more accurate than the single-wavelength meter (higher r2), the light-scattering properties of leaf cells and the nonhomogeneous distribution of chlorophyll in leaves appear to limit the ability of all meters to estimate in vivo chlorophyll concentration.
Jonathan M. Frantz and Bruce Bugbee
Cloudy days cause an abrupt reduction in daily photosynthetic photon flux (PPF), but we have a poor understanding of how plants acclimate to this change. We used a unique 10-chamber, steady-state, gas-exchange system to continuously measure daily photosynthesis and night respiration of populations of a starch accumulator [tomato (Lycopersicon esculentum Mill. cv. Micro-Tina)] and a sucrose accumulator [lettuce (Lactuca sativa L. cv. Grand Rapids)] over 42 days. All measurements were done at elevated CO2 (1200 μmol·mol-1) to avoid any CO2 limitations and included both shoots and roots. We integrated photosynthesis and respiration measurements separately to determine daily net carbon gain and carbon use efficiency (CUE) as the ratio of daily net C gain to total day-time C fixed over the 42-day period. After 16 to 20 days of growth in constant PPF, plants in some chambers were subjected to an abrupt PPF reduction to simulate shade or a series of cloudy days. The immediate effect and the long term acclimation rate were assessed from canopy quantum yield and carbon use efficiency. The effect of shade on carbon use efficiency and acclimation was much slower than predicted by widely used growth models. It took 12 days for tomato populations to recover their original CUE and lettuce CUE never completely acclimated. Tomatoes, the starch accumulator, acclimated to low light more rapidly than lettuce, the sucrose accumulator. Plant growth models should be modified to include the photosynthesis/respiration imbalance and resulting inefficiency of carbon gain associated with changing PPF conditions on cloudy days.
Tracy A.O. Dougher and Bruce Bugbee
Blue light (320 to 496 nm) alters hypocotyl and stem elongation and leaf expansion in short-term, cell-level experiments, but histological effects of blue light in long-term studies of whole plants have not been described. We measured cell size and number in stems of soybean (Glycine max L.) and leaves of soybean and lettuce (Lactuca sativa L.), at two blue light fractions. Short-term studies have shown that cell expansion in stems is rapidly inhibited when etiolated tissue is exposed to blue light. However, under long-term light exposure, an increase in the blue light fraction from <0.1% to 26% decreased internode length, specifically by inhibiting soybean cell division in stems. In contrast, an increase in blue light fraction from 6% to 26% reduced soybean leaf area by decreasing cell expansion. Surprisingly, lettuce leaf area increased with increasing blue light fraction (0% to 6%), which was attributed to a 3.1-fold increase in cell expansion and a 1.6-fold increase in cell division.