Effects of amino acids and nitrogen on plant regrowth or recovery from drought stress remain largely unknown. The objectives of this study were to examine how gamma-aminobutyric acid (GABA) or proline, alone and in combination, or inorganic nitrogen [ammonium nitrate (NN)] may differentially affect turf performance during drought stress and rewatering, and to determine which specific endogenous amino acids regulated by GABA, proline, or NN priming were associated with plant tolerance to drought stress and postdrought recuperation in cool-season grass species. Creeping bentgrass (Agrostis stolonifera cv. Penncross) planted in porous ceramic fritted clay medium were exposed to well-watered conditions or drought stress by withholding irrigation for 21 days in growth chambers. Plants were treated with water (untreated control), GABA, or proline alone and in combination, or NN through foliar spray before drought stress and every 7 days during the 21-day stress period. For postdrought recovery, at 21 days of drought treatment, plants were rewatered for 14 days to return soil water content to prestress levels. Plants treated with GABA or proline alone or in combination maintained higher turf quality (TQ), dark green color index (DGCI), and stolon length by 21 days of drought stress, whereas proline-treated plants also maintained higher leaf relative water content (RWC) during drought as well as longer stolon length during rewatering. Plants treated with NN maintained higher TQ and leaf RWC during drought and had improved percent canopy cover, DGCI, and stolon length during postdrought rewatering. Accumulation of endogenous amino acids under drought stress, including proline and alanine, for proline-treated creeping bentgrass may have contributed to the enhancement of drought tolerance and postdrought regrowth. Nitrogen-enhanced accumulation of GABA, proline, and glutamic acid may have played a role in active amino acid assimilation and subsequent postdrought regrowth. Results from this study indicate that GABA or proline were mainly effective in promoting the tolerance of creeping bentgrass to drought stress while inorganic NN was effective in promoting rapid postdrought recovery and regrowth potential through the activation of amino acid metabolism. Endogenous amino acids, including GABA, proline, alanine, and glutamic acid, may be used as biomarkers to select for drought-tolerant plants and biostimulant components for improvement of drought stress tolerance and poststress recovery in cool-season turfgrass species.
Cathryn Chapman, Stephanie Rossi, Bo Yuan, and Bingru Huang
Lisa Tang and Carol J. Lovatt
Effects of water-deficit stress and foliar-applied gibberellic acid (GA3) on ‘Washington’ navel orange (Citrus sinensis) floral gene expression and inflorescence number were quantified. Trees subjected to 8 weeks of water-deficit stress [average stem water potential (SWP) −2.86 MPa] followed by 3 weeks of re-irrigation (SWP recovered to > −1.00 MPa) produced more inflorescences in week 11 than trees well-irrigated (SWP > −1.00 MPa) for the full 11 weeks (P < 0.001). After 8 weeks of water-deficit stress, bud expression of flowering locus t (FT), suppressor of overexpression of constans1 (SOC1), leafy (LFY), apetala1 (AP1), apetala2 (AP2), sepallata1 (SEP1), pistillata (PI), and agamous (AG) increased during the re-irrigation period (weeks 9 and 10), but only AP1, AP2, SEP1, PI, and AG expression increased to levels significantly greater than that of well-irrigated trees. Foliar-applied GA3 (50 mg·L−1) in weeks 2 through 8 of the water-deficit stress treatment did not reduce bud FT, SOC1, or LFY expression, but prevented the upregulation AP1, AP2, SEP1, PI, and AG expression that occurred during re-irrigation in water-deficit stressed trees not treated with GA3. Applications of GA3 to water-deficit stressed trees reduced inflorescence number 95% compared with stressed trees without GA3. Thus, GA3 inhibited citrus (Citrus sp.) floral development in response to water-deficit stress through downregulating AP1 and AP2 expression, which likely led to the failed activation of the downstream floral organ identity genes. The results reported herein suggest that bud determinacy and subsequent floral development in response to water-deficit stress in ‘Washington’ navel orange are controlled by AP1 and AP2 transcript levels, which regulate downstream floral organ identity gene activity and the effect of GA3 on citrus flower formation. The water-deficit stress floral-induction pathway provides an alternative to low-temperature induction that increases the potential for successful flowering in citrus trees grown in areas experiencing warmer, drier winters due to global climate change.
Koichi Nomura, Eriko Wada, Masahiko Saito, Hiromi Yamasaki, Daisuke Yasutake, Tadashige Iwao, Ikunao Tada, Tomihiro Yamazaki, and Masaharu Kitano
In horticultural leafy vegetable production, continuously monitoring crop size indicators such as the leaf area index (LAI), leaf fresh weight (LFW), and leaf length (LL) is of practical value because these indicators are related to crop yields and harvest timing. The aim of this study was to develop a method that enables the continuous, automatic estimation of the LAI, LFW, and LL of a Chinese chive (Allium tuberosum) canopy by combining timelapse photography with allometric equations. LAI was estimated based on the gap fractions of nadir photographs (i.e., the fractions of nonleaf area), which were retrieved using the deep learning framework DeepLabv3+ with satisfactory accuracy (mean intersection over union, 0.71). This photographically estimated LAI (LAIphoto) corresponded well with the destructively measured LAI (LAIdest) (LAIphoto = 0.96LAIdest, R 2 = 0.87). LAIphoto was then used as the input of allometric regression equations relating LAIphoto with LFW and LL. A power function (y = axb) fit the observed LAIphoto–LFW and LAIphoto–LL relationships well (R 2 = 0.89 and 0.74, respectively). By combining nadir timelapse photography with the allometric equations, changes in the LFW and LL of a Chinese chive canopy were estimated successfully for a 9-month cultivation period. Our approach can replace time-consuming, labor-intensive manual measurements of these crop size indicators for Chinese chive and may be applicable to other crops with different parameter sets.
Jeff B. Million and Thomas H. Yeager
Fabric containers (FAB), due to their root-pruning properties, can be used as an alternative to conventional plastic containers (PLA) in container nurseries. Because sidewall evaporation in FAB has been shown to reduce container substrate temperatures, our objective was to determine if FAB would reduce the release rate of controlled-release fertilizer (CRF), resulting in less leachate loss of nitrogen (N) and phosphorus (P) and greater CRF longevity. Dwarf Burford holly were grown in 36-cm-diameter (18-L substrate) FAB or PLA in a bark-peat substrate with incorporated CRF. Spray stake irrigation was routinely adjusted to a target leaching fraction of 25%. Maximum daily substrate temperature, measured 3 cm from southwest-facing container wall, averaged 6 °C lower in FAB than in PLA. For two 31-week experiments where leachate was continuously collected and sampled weekly, FAB reduced leachate N loss by 30% and P loss by 47% despite requiring 66% more irrigation water and collecting 31% more leachate than with PLA. FAB reduced average N loss from 114 to 78 kg·ha−1 and average P loss from 16.0 to 8.6 kg·ha−1. FAB increased plant size by 8% and shoot dry weight by 12% for one experiment but had no effect in the other. We concluded that compared with PLA, the use of FAB can decrease leachate loss of N and P but require considerably more irrigation water to offset water loss via sidewall evaporation.
Azeezahmed Shaik and Sukhbir Singh
Recent studies suggest that arbuscular mycorrhizal fungi (AMF) have the potential to improve the growth and yield of eggplant (Solanum melongena L.) under soil-based organic production systems. However, the application of AMF in organic soilless vegetable production in a greenhouse has not been well studied, creating an important knowledge gap. Therefore, two greenhouse experiments [Experiment 1 (E1) and Experiment 2 (E2)] were conducted to investigate the effect of AMF (Glomus spp.) on the growth, gaseous exchange, and yield of eggplant fertilized with various liquid organic fertilizers (OFs) and inorganic fertilizers (IFs) in a soilless greenhouse production system. The experiment was conducted in a split-plot design with four replications in which liquid OFs [OF1 (5N–1P–1K), OF2 (0N–5P–5K and 3N–3P–3K), OF3 (3N–1P–1K), OF4 (5N–1P–2K), OF5 (3.7N–2.7P–3.7K), and OF6 (3N–3P–5K)], and IFs [IF1 (6N–4P–4K) and IF2 (4N–0P–1K and 1N–3P–5K)] were randomized as main plot factor, and AMF [inoculated and uninoculated (control)] as a subplot factor. Results indicate that AMF inoculation had no significant effect on the growth, gaseous exchange, and yield parameters of eggplant. Among different OFs, the eggplant fertilized with OF6 resulted in a 4.3% and 3% reduction of leaf area compared with top-performing IF1 treatment in E1 and E2, respectively. Further, the OF6 treatment resulted in a 12% and 15% reduction in total yield per plant compared with IF1 in E1 and E2, respectively. The differences in plant gaseous exchange parameters were also nonsignificant for eggplants fertilized with different OF and IF treatments in both E1 and E2. These results conclude that Glomus spp. were not associated with a significant increase in the yield of eggplant in the soilless production system. However, OFs were performing similar to IFs in terms of growth and yield, which could be due to a higher nutrient availability of these OFs, which are highly useful for the production of eggplant in greenhouse soilless production systems.
S. Alan Walters and Bryan G. Young
Preemergence (PRE) herbicides are an important part of the overall weed management plan in no-till (NT) pumpkin (Cucurbita pepo) production. A field evaluation was conducted in an NT production system using PRE herbicides labeled for pumpkins to determine the benefits of specific herbicide combinations and the economic returns on investment associated with their use. The PRE herbicide treatments evaluated were 1) s-metolachlor (1360 g⋅ha–1 a.i.), 2) clomazone (350 g⋅ha–1 a.i.) and ethalfluralin premix (1120 g⋅ha–1 a.i.), 3) s-metolachlor + clomazone and ethalfluralin premix, 4) s-metolachlor + halosulfuron (35 g⋅ha–1 a.i.), 5) clomazone and ethalfluralin premix + halosulfuron, and 6) none. The primary weed species present were tall waterhemp (Amaranthus tuberculatus), redroot pigweed (Amaranthus retroflexus), giant foxtail (Setaria faberi), and large crabgrass (Digitaria sanguinalis). The best weed control option for full-season broadleaf and grass weed control was s-metolachlor + clomazone and ethalfluralin. This herbicide combination also provided the greatest economic return on investment, ranging from a 20% to 40% improvement (depending on the year) compared with the next closest PRE herbicide treatment. Those with the lowest returns on investment were s-metolachlor combined with halosulfuron, and clomazone and ethalfluralin combined with halosulfuron. Besides providing the highest returns on investment, the PRE application of s-metolachlor with clomazone and ethalfluralin also produced the largest pumpkin fruit for the weed species present. Although growers often look for ways to reduce input costs in NT pumpkin production systems, the proper selection of PRE herbicides that considers the weed species present is clearly an important investment that improves pumpkin yields and revenues.
Leynar Leyton Naranjo and Carol D. Robacker
Cindy Rouet, Joseph O’Neill, Travis Banks, Karen Tanino, Elodie Derivry, Daryl Somers, and Elizabeth A. Lee
Field winterhardiness is a critical trait in rose cultivars (Rosa ×hybrida) grown in northern climates. Although the molecular basis of cold hardiness has been well documented in model organisms such as Arabidopsis thaliana, little is known about the genetics and mechanisms underlying winterhardiness in roses. This research aims to explore the genetic control of winterhardiness for application in breeding programs using quantitative trail loci (QTL) analysis in two biparental rose populations derived from cold-hardy roses of the Canadian Explorer Series Collection. Field winterhardiness was assessed as a complex trait with winter damage and regrowth recorded in multiyear and multilocation trials in Ontario and Saskatchewan, Canada. In addition, this research explored the relationship between field measurements and electrolyte leakage recorded under artificial conditions. Electrolyte leakage had limited utility for application in rose breeding programs as a substitute for field evaluation, but did enable identification of QTL associated with potential cold hardiness candidate genes. A QTL for electrolyte leakage mapped to a genomic region that harbors a CBF1-like transcription factor. A total of 14 QTLs associated with field winter damage and regrowth were discovered, and they explained between 11% and 37% of the observed phenotypic variance. Two QTL associated with winter damage and regrowth overlapped with a known QTL for black spot (Diplocarpon rosae) disease resistance, Rdr1, in an environment under high disease pressure. Due to the complexity of field winterhardiness and its direct reliance on intertwined factors, such as overall plant health, moisture status, snow cover, and period of prolonged sub-zero temperatures, field trials are the ultimate measurement of field winterhardiness. Transgressive segregation was observed for all traits, and it was most likely due to complementary gene action. Field winter damage and regrowth were highly heritable in single environments, but they were subject to genotype × environment interaction resulting from pest pressure and severe climatic conditions.