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In Pakistan, orchards are irrigated using flood irrigation system that is considered sub-economical regarding water use efficiency. Considering the importance of natural water resources, there was a need to devise the method of applying irrigation water that would save water consumption, environmentally friendly and easily adopted by the growers. Thus, a research trial was conducted at Citrus Research Institute, Sargodha, Pakistan on 0.41 ha area of Kinnow mandarin. The total area was divided into two parts, half of the area was used for furrow irrigation system and the remaining half was used for flood irrigation system. Tensiometers were installed in the soil in furrow and flood irrigation system to assess the moisture condition in the soil. Irrigations were applied when tensiometer readings reached 40 centibars. The quantity of water used for irrigation was measured with a flume meter and then percentage of water saving was calculated. The study revealed that furrow irrigation system significantly improved the number of fruits per tree (1153), fruit yield (179.89 kg per plant) and juice percentage (48.38%) compared with flood irrigation system where the number of fruits per tree were711, fruit yield was 110.61 kg per plant and juice percentage was 46.31%. However, TSS/acidity ratio, and peel and rag percentage were not affected by the different irrigation systems. In furrow irrigation system 24 irrigations were applied per year with average water saving of 46.14% and water use efficiency (WUE) was 4.58 kg m-3 whereas in flood irrigation system average numbers of irrigations were 15 per year and WUE was 2.34 kg m-3only. Therefore, furrow irrigation system is recommended for the citrus growers to improve the water use efficiency, yield and fruit quality of Kinnow mandarin.

Low temperature is a major environmental factor that severely impairs plant growth and productivity. Although the response to low temperature stress is well studied, the mechanisms of chilling tolerance are still not well understood. Here, we describe experiments that aimed to determine whether relative humidity (RH) contribute to chilling tolerance by regulating leaf water potential in watermelon seedlings. Plants exposed to chilling stress (10 °C/5 °C day/night) were severely wilted, and the water potential in their leaves was decreased. We found that maintaining high RH when plants were subjected to chilling-stress conditions attenuated the reduction in leaf water potential, reduced electrolyte leakage, improved the stability of photosynthesis, and alleviated chilling damage. Pretreatment with ABA increased chilling tolerance in low RH conditions but became ineffective in high RH conditions. Analysis of endogenous ABA content indicated that water potential mediated chilling tolerance was independent of ABA. Analysis of stomatal resistance indicated that the maintenance of water potential was related to stomatal resistance but that the balance between water absorption and loss is more important. In conclusion, high RH maintained leaf water potential and cell turgor, maintained better cell morphology, improved stomatal conductance and thus, ultimately improved the chilling tolerance of watermelon seedlings.

Boron (B) is an essential trace element that plays a vital role in metabolic and physiological functions of higher plants. The adequate supply of B is important for plant growth and development. Grafting is a technique used to improve the ion uptake and plant growth. In this study, a commercial watermelon cultivar “Zaojia 8424” [Citrullus lanatus (Thunb.) Matsum. and Nakai.] was grafted onto pumpkin (Cucurbita maxima × Cucurbita moschata) rootstock cv. “Qingyan Zhenmu No.1” with an aim to investigate the response of grafted plants to different levels of B supply (0.25 μM, 25 μM and 75 μM B) in the nutrient solution. Self-grafted watermelon plants were used as control. Pumpkin rootstock improved the plant growth, chlorophyll and carotenoid contents, photosynthetic assimilation, stomatal conductance, transpiration rate, B accumulation and up-regulated the expression of NIP5;1, NIP6;1 and B transporter (BOR2, BOR4) genes in the roots and leaves at 25 μM B compared with self-grafted watermelon plants. Moreover, pumpkin rootstock reduced the oxidative stress and cell damage by reducing H2O2 and MDA contents, and down-regulating the expression of PDCD2-1, PDCD2-2 genes. Moreover, it enhanced the antioxidant activity of watermelon by up-regulating the expression of SOD1, SOD2, CAT2-1, and CAT2-2 genes. Based on these observations, we concluded that pumpkin rootstock has ability to improve the plant growth of watermelon by enhancing the B uptake. This study may help adjust the B concentration in the nutrient medium for watermelon production where pumpkin grafted plants are utilized.

Vanadium (V) is a transition metal found in the Earth crust. V adversely affects plant growth and development. Besides several other management practices, grafting of scion cultivars onto appropriate rootstock provides a suitable solution. Grafting is an important agro-technical procedure utilized to enhance the capacity of plants to tolerate biotic and abiotic stresses. In this study, watermelon was grafted onto bottle gourd and pumpkin rootstock, and self-grafted watermelon plants were utilized as a control. V was applied at the rate of 50 mg/L under hydroponic conditions. The result showed that V application substantially reduces the growth of watermelon plants, however, grafting of watermelon onto bottle gourd and pumpkin rootstock improves V stress tolerance of watermelon by reducing the V concentration in leaf tissues, improving the relative chlorophyll content (SPAD index) and photosynthetic assimilation, up-regulating the expression of SOD (Cla008698, Cla0012125, Cla009820 and Cla001158), glutathione S-transferase (Cla013224) and glutathione peroxidase (Cla021039) genes in the leaves, and enhancing the activities of antioxidant enzymes (SOD, CAT). The scanning electron microscopy (SEM) of the root tips showed that minimal damage of roots was observed for pumpkin roots compared with the roots of watermelon and bottle gourd under V stress conditions. So far as we know, these results are the first evidence that grafting mitigates V stress in plants.

Vanadium (V) is the fifth most abundant transition metal, elevated levels of V are hazardous to plants. Boron (B) is an essential micronutrient for plants and can mitigate heavy metal toxicity. However, the mechanism used by B to promote tolerance to vanadium is unknown. In this study, a combination of physiological and gene expression analysis was used to explain mechanism of B (75 µM) induced V (40 mg L-1) stress tolerance in watermelon. V stress severely reduced root and shoot growth and increased the accumulation of ROS. B application improved tolerance to V by enhancing the expression of B transporter genes (ClaNIP5;1–1, ClaNIP5;1–2, ClaBOR4) that facilitated B uptake and transport while restricting V transport in plant tissues. At cellular level, the higher V retention in leaves was achieved by cell wall chelation, whereas, the higher V exclusion in vacuole of root cell was driven by elevated vacuolar H+-ATPase, H+-PPase activities, and transcript level of ClaVHP1;1, ClaPDR12–1 and ClaPDR12–2 genes facilitated by B application. Moreover, B application reduced tissue ROS cascade by enhancing antioxidant enzymatic activity and expression of superoxide dismutase (ClaCSD1–1, ClaCSD1–2, ClaCSD3, ClaMSD1) and catalase (ClaCAT2–1, ClaCAT2–2) genes that enhanced the defense mechanism of the V treated plants, improved root and shoot growth and tolerance index of watermelon. In conclusion, we demonstrate that ameliorative effect of B in tolerance to V of watermelon was based on B homeostasis and improved antioxidant defense system. These findings might help to increase watermelon production in V polluted soils.

Background Nitrogen (N) is a key macronutrient required for plant growth and development. In this study, watermelon plants were grown under hydroponic conditions at 0.2 mM N, 4.5 mM N, and 9 mM N for 14 days. Results Dry weight and photosynthetic assimilation at low N (0.2 mM) was reduced by 29 and 74% compared with high N (9 mM). The photochemical activity (Fv/Fm) was also reduced from 0.78 at high N to 0.71 at low N. The N concentration in the leaf, stem, and root of watermelon under low N conditions was reduced by 68, 104, and 108%, respectively compared with 9 mM N treatment after 14 days of N treatment. In the leaf tissues of watermelon grown under low N conditions, 9598 genes were differentially expressed, out of which 4533 genes (47.22%) were up-regulated whereas, 5065 genes (52.78%) were down-regulated compared with high N. Similarly in the root tissues, 3956 genes were differentially expressed, out of which 1605 genes were up-regulated (40.57%) and 2351 genes were down-regulated (59.43%), compared with high N. Our results suggest that leaf tissues are more sensitive to N deficiency compared with root tissues. The gene ontology (GO) analysis showed that the availability of N significantly affected 19 biological processes, 8 cell component metabolic pathways, and 3 molecular functions in the leaves; and 13 biological processes, 12 molecular functions, and 5 cell component metabolic pathways in the roots of watermelon. The low affinity nitrate transporters, high affinity nitrate transporters, ammonium transporters, genes related with nitrogen assimilation, and chlorophyll and photosynthesis were expressed differentially in response to low N. Three nitrate transporters (Cla010066, Cla009721, Cla012765) substantially responded to low nitrate supply in the root and leaf tissues. Additionally, a large number of transcription factors (1365) were involved in adaptation to low N availability. The major transcription factor families identified in this study includes MYB, AP2-EREBP, bHLH, C2H2 and NAC. Conclusion Candidate genes identified in this study for nitrate uptake and transport can be targeted and utilized for further studies in watermelon breeding and improvement programs to improve N uptake and utilization efficiency. Electronic supplementary material The online version of this article (10.1186/s12864-018-4856-x) contains supplementary material, which is available to authorized users.

Boron (B) is an essential trace element required for the physiological functioning of higher plants. B deficiency is considered as a nutritional disorder that adversely affects the metabolism and growth of plants. B is involved in the structural and functional integrity of the cell wall and membranes, ion fluxes (H+, K+, PO43−, Rb+, Ca2+) across the membranes, cell division and elongation, nitrogen and carbohydrate metabolism, sugar transport, cytoskeletal proteins, and plasmalemma-bound enzymes, nucleic acid, indoleacetic acid, polyamines, ascorbic acid, and phenol metabolism and transport. This review critically examines the functions of B in plants, deficiency symptoms, and the mechanism of B uptake and transport under limited B conditions. B deficiency can be mitigated by inorganic fertilizer supplementation, but the deleterious impact of frequent fertilizer application disrupts soil fertility and creates environmental pollution. Considering this, we have summarized the available information regarding alternative approaches, such as root structural modification, grafting, application of biostimulators (mycorrhizal fungi (MF) and rhizobacteria), and nanotechnology, that can be effectively utilized for B acquisition, leading to resource conservation. Additionally, we have discussed several new aspects, such as the combination of grafting or MF with nanotechnology, combined inoculation of arbuscular MF and rhizobacteria, melatonin application, and the use of natural and synthetic chelators, that possibly play a role in B uptake and translocation under B stress conditions.

Table S1. Summary of sequencing data quality of leaves and roots of watermelon grown under hydroponic conditions at low N (0.2 mM) and high N (9 mM); Table S2. Summary of total, multiple and uniquely mapped reads of leaves and roots of watermelon grown under hydroponic conditions at low N (0.2 mM) and high N (9 mM); Table S3. Transcript abundance of the genes that were only expressed under low N (LLN) in the leaves of watermelon seedlings grown under hydroponic conditions; Table S4. Transcript abundance of the genes that were only expressed under high N (LHN) in the leaves of watermelon seedlings grown under hydroponic conditions; Table S5. Transcript abundance of the genes that were expressed either under low N (RLN) or high N (RHN) in the roots of watermelon seedlings grown under hydroponic conditions; Table S6. The list of primer sequences used for qRT-PCR analysis; Table S7. Arabidopsis thaliana Ortholog genes to the selected candidate genes that substantially responded to low N (0.2 mM) compared with high N (9 mM) in the leaf and root of watermelon; Figure S1. Correlation between expression value of selected genes obtained by RNA-seq and qPCR in the leaf (a) and root (b) tissues of watermelon seedlings grown under hydroponic conditions exposed to different levels of N (0.2 mM and 9 mM) for 14 days. FC: fold change; r: correlation coefficient; Figure S2. Hierarchical cluster analysis map presenting differential gene expression in the leaf and root of watermelon grown under hydroponic conditions at 0.2 mM and 9 mM N. LHN: leaf high N (9 mM); LLN: leaf low N (0.2 mM); RHN: root high N (9 mM); RLN: roots low N (0.2 mM). Samples for transcriptome analysis were harvested after 14 days of N treatment; and Figure S3. The cytoscape presenting protein interaction network analysis of differentially expressed genes of leaf and root of watermelon grown under hydroponic conditions at 0.2 mM and 9 mM N. (DOCX 1230 kb)