Characterization of leaf physiology is an important step for understanding the ecophysiology of a crop as well as for developing a process-based crop simulation model. We determined photosynthetic and transpiration responses to photosynthetic photon flux (PPF), carbon dioxide concentrations, and temperature, and parameterized a coupled leaf gas-exchange model for hardneck garlic (Allium sativum). The parameterized model performed with high accuracy and precision in predicting photosynthetic responses [r 2 = 0.95, bias = 1.7 μmol·m−2·s−1, root mean square error (RMSE) = 2.4 μmol·m−2·s−1] when tested against independent data that were not used for model calibration. The model performance for transpiration rates was less satisfactory (r 2 = 0.49, bias = –0.14 mmol·m−2·s−1, RMSE = 0.94 mmol·m−2·s−1). In addition, we characterized the relationships among chlorophyll meter readings, leaf photosynthetic capacity (A max), and leaf nitrogen content in garlic leaves. The chlorophyll meter readings were a reasonable indicator of both A max (r 2 = 0.61) and leaf nitrogen (N) status (r 2 = 0.51) for garlic leaves we studied. The garlic leaf gas-exchange model developed in this study can serve as a key component in ecophysiological crop models for garlic. Similarly, the quantitative relationship identified between chlorophyll meter readings and A max in this study can provide useful information for non-destructively assessing leaf photosynthetic capacity in garlic.
Soo-Hyung Kim, Jig Han Jeong, and Lloyd L. Nackley
Lloyd L. Nackley, Elias Fernandes de Sousa, Bruno J.L. Pitton, Jared Sisneroz, and Lorence R. Oki
Potted poinsettia (Euphorbia pulcherrima) is an important commercial commodity for the U.S. floriculture industry. The production of poinsettia demands intensively managed light control, heat, fertilizer, and water; inhibiting elongation with plant growth regulators, and protecting plants from diseases and pests with pesticide applications. Excessive irrigation creates pollution, promotes disease, and is expensive. Sensor-based control systems can optimize irrigation schedules. Irrigation management is crucial in nursery production of poinsettias because water is a limited resource and agricultural runoff is monitored in many states across the United States. By pairing environmental sensors with sensors that continuously monitor plant transpiration, we can determine how plant water use and water stress fluctuate with environmental and physiological demands. We hypothesized that continual measurements of sap flow could be correlated with environmental sensors to develop a new water stress index (WSI), which can deliver the benefits of detecting water stress that might affect the quality of potted poinsettias. To test this hypothesis, rooted cuttings of poinsettia (E. pulcherrima cv. Prestige Red) were individually potted into twelve 11-L black plastic nursery pots. Potted plants were grown in a naturally illuminated temperature-controlled glasshouse. The 12 plants were randomly assigned one of three watering treatments: weekly, biweekly, and triweekly irrigation. From the data collected, we were able to create a WSI that correlated available soil moisture with the difference between the expected transpiration with actual transpiration rates. Our results suggest that the plants in the weekly treatment group did not experience water stress until 0.3 m3·m–3 volume water content indicated by <0.2 WSI. These results support previous research that found 0.1 to 0.3 m3·m–3 can be stressful soil moisture conditions for greenhouse-grown crops. Results also show that for substrates with similar substrates that irrigation set points can be reduced to 0.2 m3·m–3 for improved irrigation efficiency.
Lloyd L. Nackley, Jig Han Jeong, Lorence R. Oki, and Soo-Hyung Kim
Garlic (Allium sativum) is a commercially and culturally important crop worldwide. Despite the importance of garlic, there have been few studies investigating how garlic growth and development will be affected by the atmospheric enrichment of carbon dioxide (CO2). A split-plot experiment with CO2 concentrations as main plot and nitrogen (N) fertilization as subplot was carried out to examine the effects of elevated CO2 at (mean ± sd) 745 ± 63 µmol·mol−1 across three levels of N: high-N (16.0 mm), mid-N (4.0 mm), and low-N (1.0 mm). Three hypotheses were tested: 1) garlic plants will allocate proportionally more biomass to bulb when grown in elevated CO2 compared with the plants grown in ambient CO2; 2) plants will sustain improved photosynthesis without downregulation in elevated CO2, irrespective of N; and 3) elevated CO2 will improve plant water use efficiency (WUE) across N fertilization levels. We found that proportional biomass allocation to bulb was not significantly enhanced by CO2 enrichment in garlic. Overall biomass accumulation represented by leaf, stem, and bulb did not respond significantly to CO2 enrichment but responded strongly to N treatments (P < 0.001). Contrary to our hypothesis, photosynthetic downregulation was apparent for garlic plants grown in elevated CO2 with a decrease in Rubisco capacity (P < 0.01). Instantaneous leaf WUE improved in response to elevated CO2 (P < 0.001) and also with increasing N fertilization (P < 0.001). Finally, our results indicate that bulbing ratio is likely to remain unchanged with CO2 or N levels and may continue to serve as a useful nondesctructive metric to estimate harvest timing and bulb size.
Lloyd L. Nackley, Brent Warneke, Lauren Fessler, Jay W. Pscheidt, David Lockwood, Wesley C. Wright, Xiaocun Sun, and Amy Fulcher
To optimize pesticide applications to the canopies of deciduous perennial crops, spray volume should be adjusted throughout the year to match the changes in canopy volume and density. Machine-vision, computer-controlled, variable-rate sprayers are now commercially available and claim to provide adequate coverage with decreased spray volumes compared with constant-rate sprayers. However, there is little research comparing variable- and constant-rate spray applications as crop characteristics change throughout a growing season. This study evaluated spray volume, spray quality (e.g., coverage and deposit density), and off-target spray losses of variable- and constant-rate sprayers across multiple phenophases in an apple (Malus domestica) orchard and a grape (Vitis vinifera) vineyard. The variable-rate sprayer mode applied 67% to 74% less volume in the orchard and 61% to 80% less volume in the vineyard. Spray coverage (percent), measured by water-sensitive cards (WSC), was consistently greater in the constant-rate mode compared with the variable-rate mode, but in many cases, excessive coverage (i.e., over-spray) was recorded. The variable-rate sprayer reduced off-target losses, measured by WSC coverage, up to 40% in the orchard and up to 33% in the vineyard. Spray application deposit densities (droplets per square centimeter) on target canopies were typically greater in variable-rate mode. However, the deposit densities were confounded in over-spray conditions because droplets coalesced on the WSC resulting in artificially low values (i.e., few, very large droplets). Spray efficiencies were most improved early in the growing season, when canopy density was lowest, demonstrating the importance of tailoring spray volume to plant canopy characteristics.