Irrigation control systems that irrigate container-grown plants based on crop water needs can reduce water and fertilizer use and increase the sustainability of ornamental crop production. The use of soil moisture sensors to determine when to irrigate is a viable option. We tested a commercially available irrigation controller (CS3500; Acclima, Meridian, ID), which uses time domain transmissometry (TDT) sensors to measure soil volumetric water content (θ). The objectives of this study were: 1) to test the accuracy of TDT sensors in soilless substrate; 2) to quantify the ability of the Acclima CS3500 irrigation controller to maintain stable θ readings during the production of container-grown begonia (Begonia semperflorens L.) by turning a drip irrigation system on and off as needed; and 3) to study the growth and photosynthetic physiology of begonia at six θ levels. Calibration of the TDT sensors in pots filled with substrate (but without plants) showed that the θ determined by the TDT sensors had a very close relationship (R 2 = 0.99) with the gravimetrically determined θ, but the TDT sensors underestimated θ by ≈0.08 m3·m−3. Therefore, a custom calibration of the TDT sensors for the soilless substrate was necessary to get accurate θ data. The irrigation controller was programmed to maintain six θ thresholds, ranging from 0.136 to 0.472 m3·m−3 (based on our own sensor calibration), and was able to maintain θ readings within 0.008 m3·m−3 of the threshold. Theta and Sigma probes were used to collect comparative θ and bulk electrical conductivity (EC) data, respectively. The results showed a strong correlation with TDT sensor measurements of θ (R 2 = 0.92) but a moderate relationship for bulk EC (R 2 = 0.53). The begonias had similar dry weight at θ levels of 0.348 m3·m−3 and higher, whereas total evapotranspiration increased linearly with the θ threshold. The lowest θ threshold reduced leaf size, net photosynthesis (Pn), and stomatal conductance (g S). Overall, the TDT sensors can provide accurate measurements of θ in soilless substrate but need substrate-specific calibration. The Acclima CS3500 controller, using TDT sensors, was able to maintain stable θ readings throughout a production cycle. These results suggest that this irrigation controller may be suitable for production of greenhouse crops as well as in drought stress research.
Julián Miralles-Crespo and Marc W. van Iersel
Julián Miralles, Raquel Valdes, Juan J. Martínez-Sánchez and Sebastián Bañón
The appropriate management of crop conditions can reduce the salt damage suffered by ornamental species and produce high-quality plants even when saline irrigation water is used. The aim of this study was to determine whether the pot-in-pot (PIP) cultivation system can improve the saline irrigation tolerance of Euonymus japonicus compared with aboveground potting (AGP) in terms of growth and development, aesthetic quality, ion accumulation, and leaf potentials. A 5-month experiment started on 6 Mar., and the interaction between the cultivation system (PIP or AGP) and water quality (fresh water and saline water, with 1.76 and 9.04 dS·m−1, respectively) was assessed. The substrate used was a mixture of white peat, coconut fiber, and perlite (40/40/20, v/v/v). A soil moisture sensor-controlled system was used to irrigate all the treatments when the AGP treatment irrigated with fresh water reached a volume water content (θ) of 0.33–0.35 m3·m−3. An interaction effect reduced the salinity effects in PIP and saline irrigation (PIP-s) compared with AGP and saline irrigation (AGP-s) in terms of damaged leaf area, plant dry weight (DW), and the compactness index. The PIP-s plants showed a survival rate of 93% compared with 57% in AGP-s. The substrate temperatures were milder in PIP regardless of the irrigation water, and the pore water electrical conductivity (EC) was 36% lower in PIP-s than in AGP-s. PIP reduced the Cl− accumulated in leaves but did not influence Na+, Ca2+, Mg2+, or the K+/Na+ ratio. The lower amount of Cl− accumulated increased leaf water potential (Ψo) in PIP. Saline irrigation produced a general accumulation of Cl− and Na+ in leaves and decreased Ca2+, Mg2+, the K+/Na+ ratio, Ψo, the shoot to root ratio, and height. In general, PIP reduced the salinity damage to Euonymus japonicus, the main effect being the lower Cl− ion uptake, which improved its aesthetic value (less damage and greater compactness and growth).
Julián Miralles-Crespo, Juan Antonio Martínez-López, José Antonio Franco-Leemhuis and Sebastián Bañón-Arias
Physiological and biochemical indicators that reflect the responses of plants to chilling stress could be useful for identifying plant damage caused by freezing or other stresses. The objective of this study was to determine any relationship between changes in chlorophyll fluorescence and the appearance of visual symptoms resulting from freezing temperatures in two cultivars of oleander. In the least frost-sensitive cultivar (yellow oleander), freezing temperatures (–4 °C for 3 h) did not produce changes in the photochemical parameters. In the more frost-sensitive cultivar (pink oleander), non-photochemical quenching (NPQ) and the maximum photochemical efficiency of photosystem II (Fv/Fm) decreased after the same freezing treatment. The first of these potential indicators remained low, whereas the second steadily recovered during the 4 months after freezing simulation. The results suggest that measuring chlorophyll fluorescence may provide a rapid method for assessing freezing injury in oleander.
Raquel Valdés, Julián Miralles, Jesús Ochoa, Sebastián Bañón and María Jesús Sánchez-Blanco
Potted gerberas were grown in a greenhouse with one, two, or four emitters (1.2 L·h−1) per pot and irrigated with water of two levels of salinity (1.5 dS·m−1 and 3 dS·m−1). All pots received the same volume of water. The aim was to determine whether the number of emitters per pot affects the distribution of roots and salts in the substrate (100% coconut fiber). We determined the electrical conductivity (EC) distribution at three different heights (upper, middle, and lower). We also studied the roots and EC horizontal distribution in four quadrants (southeast, southwest, northwest, and northeast). Shoot growth, leaf damage, plant water status, and photochemical capacity of photosystem II were also studied. Two or four emitters per pot reduced the leaching fraction compared with that observed with one emitter, improving both the amount and homogeneity of substrate moisture. In the two saline conditions used, the salt concentration in the substrate was irregular both vertically and horizontally, and the presence of roots in the horizontal quadrant sides was heterogeneous. Both of these behaviors decreased as the number of emitters per pot increased. Root growth was weakly related with the soluble salt content in the root zone. When saline water is used, two emitters per pot are to be recommended because the difference between using two and four emitters was very slight. The use of only one emitter reduced shoot and root growth and encouraged salt damage to leaves.
Julián Miralles-Crespo, María J. Sánchez-Blanco, Alejandra Navarro G., Juan J. Martínez-Sánchez, Jose A. Franco L. and Sebastián Bañón A.
The dendrometer has been proposed as a sensitive plant water indicator based on stem growth. However, studies including dendrometers have been mainly focused on fruit trees and less attention has been paid to ornamental shrubs (small plants). In the study described here, stem dendrometers were used to ascertain whether there is any relationship between water status and dendrometric indices in potted ornamental shrubs (1 to 2 cm diameter). For this purpose, three Mediterranean shrubs (Pittosporum tobira, Callistemon citrinus, and Rhamnus alaternus) were studied under water stress recovery conditions in winter, spring, and early summer. At the end of the experiment, an extreme water stress treatment, which resulted in plant death (August) was also studied. Stem diameter variations [maximum and minimum daily stem values (MXDS and MNDS, respectively), maximum daily shrinkage (MDS), and stem growth rate (SGR)], daily evapotranspiration (daily plant ET), and leaf water potential (Ψleaf) parameters were considered throughout the experiment. A regression analysis between dendrometric indices and daily plant ET showed that MXDS and MNDS were sensitive under water stress recovery conditions, especially in severe environmental conditions (spring and summer). The SGR in C. citrinus, the MDS in P. tobira, and both indices in R. alaternus were seen to be sensitive during the stress to death period. Although more studies are needed, the results confirm that the use of dendrometers in small plants may be useful to provide continuous and automated registers of the plant water status under different substrate water content and climatic conditions. However, the response of these indices may imply moderate water stress.