The interactive responses of fertigation levels under buried straw layer on growth, physiological traits and fruit yield in tomato plant

ABSTRACT The experiments were conducted on tomato plants to study the interactive responses of water levels (W70%: 70% of water consumption and W90%: 90% water consumption) and nitrogen rates (N100%: 100% of recommended and N80%:80% of N1.0) under two straw mulching conditions (NS; no straw introduced and WS: with buried straw layer) on growth and physiological parameters for two fruit growing years to assess the interactive responses of fertigation under buried straw layer on the changes in plant fresh and dry biomass, roots biomass, photosynthesis rate (PN), stomatal conductance (GS) and chlorophyll fluorescence of tomato plants. Buried straw layer was proved to improve plant biomass, photosynthesis rate and other physiological traits such as chlorophyll contents (Chl), maximum electron transport rate (ETRmax), maximum quantum yield (FV /FM ) and GS under the lower fertilizer (N80%) and irrigation levels (W70%). However, increasing fertilizer and irrigation level decreased these parameters significantly (p < .05 to p < .001) under buried straw layer. Conversely, increased fertilizer (N100%) and irrigation (W90%) levels increased these parameters significantly (p < .05 to p < .001) under no straw condition. The overall findings revealed that buried straw layer could relieve stress developed by limited irrigation water and fertilizer and benefit plant growth, physiological parameters and fruit yield of tomato plant.


Introduction
Tomato is a widely grown plant due to the high nutritional value of its fruit and great economic importance (Savić et al. 2008). China is one of the leading tomato producing countries with a production of 50,552,200.0 metric tons, and it is 6.82% of the world's grown tomatoes (FAO 2016). Tomato growth parameters are strongly influenced by two significant cultural manageable factors such as nutrients and irrigation water (Ozbahce and Tari 2010;Chen et al. 2013). Inefficient and overuse of nitrogen fertilizer is a severe issue in China with an adverse economic and environmental impacts . According to the investigation by the Chinese Ministry of Agriculture, the amount of fertilizer applied is higher and beyond the safe level (Shuqin and Fang 2018).
Conventional methods of high input of nutrient and water management are still being used by most of the farmer's community to achieve higher yield. However, the extreme usage of nutrients and water have also led to decrease fertilization efficiency by increasing leaching of nutrients and resulted in severe degradation of soil environment especially in the Southern China, which produces most of the food (Delang 2017) as well as water pollution . Therefore an appropriate management is a crucial measure to improve crop yield, quality and fertilizer use efficiency (Kennedy et al. 2013;Luo and Li 2018).
Fertigation is a method by which irrigation and fertilization can be applied precisely. However, fertigation requires sufficient skills and knowledge of soil and vegetables that farmers are lacking in many vegetable production areas. Over fertilization and irrigation are still common under fertigation. A higher application of nitrogen fertilizer may cause leaching of NO 3 -N correspondingly (Ullah et al. 2017) and ultimately disturb the nitrate contents in fruits of tomato (Wang et al. 2015). At the same time, higher application of water will inevitably promote leaching and waste of fertilizer (Ullah et al. 2017;Li et al. 2018). To fulfill the high demand of food, there is conflict to recommendations to lessen the use of fertilizer in China as it is considered as a risk of production decrease, despite the extensive evidence. Too much fertilization results in low fertilizer use efficiency, increases costs of production, and needlessly larger losses of nutrients triggering severe ecological problems such as groundwater nitrate contamination and greenhouse gas emissions. In the current situation, additional measures are immensely needed to minimize fertilizer wastage and mitigate the soil degradation (Delang 2017).
Photosynthesis is the key physiological process of plants and can provide 90% of plant biomass (Wang et al. 2018). Leaves are the main contributors to the crop productivity (Ashraf and Bashir 2003), and their photosynthetic activities are crucial significantly for harvestable fruit yield. Leaf gas-exchange such as photosynthesis rate and stomatal conductance or chlorophyll fluorescence parameters such as light curve, maximum electron transport rate and maximum quantum yield based on the operating quantum efficiency of electron transport can be used to estimate leaf photosynthetic capacities.
Water deficit negatively affects water potential of plants which results in, dehydration, stomatal conductance reduction and changes in fluorescence (Pessarakli 2005). The decline in photosynthesis can be credited to a decrease in intercellular carbon dioxide concentration partially to metabolic elements (Lawlor and Cornic 2002) and partially because of closing of stomata, which subsequently leads to photo-inhibitory damage of PSII reaction centers (Souza et al. 2004). The deficiency of nutrients can directly disturb photosynthetic actions and restrict partitioning of assimilates from the leaves to the fruits (Kanai et al. 2011). On the other hand, applied nutrients can be used to synthesize the components of the photosynthetic apparatus (Sugiharto et al. 1990), including rubisco, chlorophyll and carotenoid containing membrane proteins (Bungard et al. 1997).
The crop yield of plant is mainly based on its photosynthetic capacity (Pessarakli 2005). Leaf photosynthesis is the main reason for the majority of the variation in biomass and crop production (Takai et al. 2010). However, three differing correlations between leaf photosynthesis and crop yield have been reported by different researchers: no correlation (Chongo and McVetty 2001), positive (Ashraf 2003;Hubbart et al. 2007) and negative (Long et al. 2006). Lawlor (1995) described that these clashing findings could be clarified by the fact that limiting plant and environmental dynamics interact strongly for regulating photosynthesis, which subsequently affects crop yield. Therefore, it is important to improve understanding of the approaches underlying biomass production under different conditions (Jaimez et al. 2008).
The response of either fertilizer or irrigation on photosynthetic effects of tomato plants have been well studied, however, the interactive effects of fertigation under an isolated straw layer on these factors have not been studied. Furthermore, how could leaf photosynthesis affects other physiological traits under these conditions? Therefore, the objectives of this study were to assess the interactive responses of different fertilizer and irrigation levels under a buried straw layer on growth, photosynthesis rate, stomatal conductance and Chlorophyll fluorescence parameters for two growing season.

Site description and cropping conditions
The experiments were carried out in April-July 2017 and 2018 in a greenhouse without temperature control under nature light environments, situated at State Key Laboratory of Efficient Irrigation and Drainage of Hohai University, Nanjing, China at 31°95´N, 118°83´E and 15 m above the sea level. Microclimatic parameters inside the greenhouse are shown in Table 1. The soil physical and chemical properties are presented in Table 2. Six-weekold uniform seedlings of tomato (Solanum lycopersicum L.) were transplanted in beds, and each bed consisted of three tomato plants having a distance of 50 cm between two plants. Plant density was maintained as 3.33 plants.m −2 . A nylon cord was used for plants vertical support. Pruning was also done to uphold the appropriate growth by following the well managed local agronomic practices.

Experimental design
Three factors randomized complete block design with three replications was established to assess the effects of irrigation and nitrogen regimes under buried straw layer on growth and physiological traits of greenhouse-grown tomato. Two water levels (W 70% : 70% of water consumption; and W 90% : 90% water consumption) and two nitrogen rates (N 100% : 100% of recommended and N 80% :80% of N 100% ) under two straw mulching conditions (NS; no straw introduced and WS: with buried straw layer) were applied (Table 3). A 5 cm thick straw layer was buried at a 20 cm depth in the soil. Each block had eight rows and 24 plants. Before transplanting, the top soil was rototilled and the beds were manually raised. For N 100% , 225 kg.ha −1 and for N 80% , 180 kg.ha −1 of pure nitrogen was applied in the form of CO(NH 2 ) 2 , 112.5 kg.ha −1 of P 2 O 5 was applied to all the treatments in the form of KH 2 PO 4 and 135 kg.ha −1 of K 2 O was applied to all the treatments in the form of KH 2 PO 4 and K 2 SO 4 .

Irrigation requirements
The same level of irrigation water was applied for the first 21 days to ensure the proper growth of the plants and treatments were imposed on day 22. Volumetric soil moisture contents θ v were monitored by a Time Domain Reflectometer with 4-days interval during both growing seasons. The irrigation frequency was kept at every two days from commencement of irrigation regimes until the harvesting of the crop. Irrigation was done by the gravity drip irrigation bags having a capacity of 1.5 L. The storage bags were hanged at a height of 1.5 m to store irrigation water to irrigate a single plant. Every bag had two drippers, which were buried at a depth of 2 cm on either side of a plant at a distance of 10 cm from the stem of the plant. Fertigation was done in four equal splits along the season.
Soil water deficit (W SD ) was computed by using Equation (1).
where soil water deficit (mm), θ t1 and θ t2 are the volumetric water contents (%) in root zone at time t1 and t2, respectively and Z rt is the depth of root zone (cm)  Amounts of irrigation water for treatments were computed by using Equation (2).
where W I is irrigation water (mm), W SD is water deficit before irrigation (mm) and K n is the rate of water deficit (0.9 for W 90% and 0.7 for W 70% ). Crop consumptive use (ET) was estimated using the following water balance method: where ET (mm) is the crop consumptive use, I W (mm) is irrigation water, ΔS (mm) is a change in soil water storage, D and R, are losses due to deep percolation and run-off (mm), respectively. Deep percolation and Run-off can be ignored for drip irrigation system (Yuan et al. 2001;Chen et al. 2015). Thus, Equation (3) can be simplified:

Plant growth parameters
The plant material was collected at the end of the experiments for both seasons and washed by tap water. After recording fresh weight, all the material was dried at 70°until the sample weight was held constant. Then the dry weight of biomass and roots were recorded.

Photosynthesis rate and stomatal conductance
Photosynthesis rate (P N ) and stomatal conductance (G S ) of two mature leaflets in the upper canopy were measured during both years on 38, 64 and 86 DAT after transplanting for flowering, fruiting and harvesting stages respectively using a portable photosynthesis system (Li-6400) from 9:00 to 11:00 in the morning under natural conditions. The measurements were conducted at temperature natural conditions and CO 2 at 400 µmol. mol −1 .

Chlorophyll fluorescence
Light response curves and F V /F M were measured using Mini-PAM II (Walz, Germany) on 36, 62 and 85 days after transplanting during both years. Light curve measurements were done at 12 PAR levels from 0 to 1500 µmol.m −2 .s −1 . ETRmax was determined using the following method developed by (Silsbe and Kromkamp 2012).
Using the above expression, ETRmax can be determined as: where ETR; electron transfer rate, F V /F M ; maximum quantum yield PAR sat ; saturation light intensity (µmol.m −2 .s −1 ) and α, β and I are leaf absorptance coefficient, distribution of light energy between PSI and PSII and light intensity (µmol.m −2 .s −1 ) respectively and were derived from the light response curves.

Chlorophyll content
A non-destructive chlorophyll meter SPAD-502 (Japan) was used to record the relative leaf greenness using the same leaves used for other physiological parameters.

Statistical analysis
Three-way analysis of variance was achieved using Statistix 8.1 statistical software and LSD was applied for means comparisons at p < .05 significance level. Linear regression was performed on Microsoft excel 2016 to test the correlation of maximum electron transport rate, chlorophyll contents and stomatal conductance with photosynthesis rate for both seasons.

Water consumption
The amount of water consumed under different treatments during different growth stages is shown in Table 4. During the seedling stage, the same quantity of irrigation was applied to all the treatments for plant optimum establishment. The lower fertilizer and irrigation levels under buried straw layer utilized lowest amount of water for both years. On the other hand, higher fertilizer and high irrigation levels with no straw layer used highest amount of water during both years.

Plant and roots fresh and dry biomass
The aboveground plant biomass, root biomass and root length at harvesting time are shown in Table 5. In general, plant fresh  weight (PFW) and plant dry weight (PDW) were both significantly increased under buried straw layer in both growing seasons (p < .001). The mean values of FPW increase with increasing fertilizer significantly (p < .05 in 2017 and p < .001 in 2018) and no significant increase was observed in PDW with increasing fertilizer in 2017 but a significant increase was noticed in 2018 (p < .05). Similarly, increasing irrigation water also increased both FPW and DPW in both experimental seasons (p < .001). The interactions of the buried straw layer with fertilizer, fertilizer with irrigation and irrigation with buried straw layer increased both FPW and DPW significantly (p < .001) in both growing seasons. The buried straw layer significantly increased roots fresh weight (RFW) (p < .001) and root dry weight (RDW) (p < .01) for both experimental season. Increasing fertilizer did not increase RFW significantly for both seasons but RDW was significantly increased in 2017 experimental season (p < .01). Irrigation also had significant effect on both RFW and RDW and increased RFW for both seasons (p < .001) but different significance was observed in RDW (p < .05 in 2017 and p < .001 in 2018). The interactions of the buried straw layer with fertilizer, fertilizer with irrigation and irrigation with buried straw layer significantly increased RFW and RDW (p < .01 to p < .001) in both growing seasons. The T 4 (WSN 80% W 70% ) treatment resulted in the highest RFW values (34.18 and 35.44 g) followed by T 5 (NSN 100% W 90% ) (33.59 and 37.35 g) in 2017 and 2018, respectively. The lowest values of RFW (18.04 and 21.03 g) were recorded in T 3 (NSN 80%-W 70% ) in 2017 and 2018 respectively. The highest RDW was achieved in T 5 treatment (7.76 and 8.17 g) followed by T 4 treatment (7.35 and 7.76 g) in consecutive seasons. The lowest RDW values (5.01 and 5.39 g) were recorded in T 1 (NSN 80% W 90% ) and T 7 (NSN 100% W 70% ) in 2017 and 2018 respectively. Root length (RL) was significantly decreased under buried straw layer in 2017 (p < .05) but no significant effect was observed in 2018. RL was significantly increased by increasing fertilizer and irrigation water (p < .001). The interaction of buried straw layer and fertilizer had no significant effect on RL. A significant increase in RL was noticed when fertilizer interacted with irrigation in 2017 but no significant effect was observed in 2018. Similarly, the interaction of irrigation and buried straw mulching had no significant effect on RL in 2017 but RL increased significantly in 2018. The highest (53.48 and 53.95 cm) and lowest (39.30 and 36.00 cm) RL was observed in T 5 and T 4 .

Fluorescence parameters
The response of the buried straw layer, fertilizer levels, irrigation regimes and their interactions on chlorophyll contents (Chl) and maximum electron transport rate (ETRmax) are represented in Table 7. A polynomial relationship between light intensity and electron transport rate was fitted (Figure 1) in all three growth stages (p < .001). During the flowering stage, irrigation affected the Chl significantly whereas irrigation and the interactive effect of fertilizer and irrigation had significant effects on ETRmax (p < .05) for both growing seasons. Comparing with T 5 (NSN 100% W 90% ), highest decrease in Chl was noticed in T 3 (NSN 80% W 70% ) (7.18%) and T 7 (NSN 100% W 70% ) (6.24%) during 2017 and 2018 respectively, however, T 8 (WSN 100% W 70% ) had highest decrease in ETRmax for both years during flowering stage. Fertilizer, irrigation, interactive effects of buried straw layer and fertilizer, and the interactions of straw layer and irrigation significantly affected Chl but fertilizer, interactive effect of fertilizer and buried straw layer, and the interaction between irrigation and buried straw layer significantly affected ETRmax during both years (p < .05 to p < .001) at the fruiting stage. Comparing with T 5 , the highest decrease in Chl and ETRmax was noted in T 6 (WSN 100% W 90% ) (8.98%) in 2017 whereas in 2018, highest decrease was observed in T 4 (4.39%) and

Correlation of G S , ETRmax and Chl with P N
The linear regression relationships of G S , ETRmax, Chl and G S with P N during 2017 and 2018 is shown in Figure 2 which shows that correlations obtained for ETRmax, Chl and G S with P N during both years were highly significant (p < .001). The linear regression coefficients (R 2 ) between P N and G S were 0.963 for 2017 and 0.9173 for 2018. Although, the correlation between ETRmax and P N was significant but its values were 0.3819 and 0.4847 in 2017 and 2018, respectively. The correlation between Chl and P N was also significant and the R 2 of these relations were 0.6677 for 2017 and 0.3441 for 2018.

Fruit yield
During both years, the highest decrease in yield was observed in T 7 (M N F C I 70 ) when compared to T 5 (M N F C I 90 ) . The highest fruit yield (8.73 and 10.58 kgm −2 ) was achieved by T 5 (NSN 100% W 90% ) followed by T 4 (WSN 80% W 70% ) (6.57 and 9.19 kgm −2 ) whereas lowest fruit yield (4.47 and 6.76 kgm −2 ) were achieved inT 7 (NSN 100% W 70% ) in 2017 and 2018 respectively (Table 8). T 5 resulted with the highest yield, but it increased the input cost of irrigation water and fertilizer. On the other hand, T 4 has no significant decrease in the fruit yield but it decreased the input costs of the water and fertilizer respectively.

Discussions
Irrigation is a substantial factor of plant growth therefore increased irrigation deficiency can lead to decrease plant vegetative measurements by decreasing soil water constants and consequently reduce water availability to the plant growth. On the other hand, the buried straw layer can hold moisture and decreases the chances of plant stress. In our findings, buried straw mulching significantly affected the ns ns *** * *** * ** ns *** *** *** *** N*W ns ns ns ns ns ns * * ns *** *** *** S*W ns ns *** * *** * *** ns *** *** *** *** S*N*W ns ns ns ns ns ns ns ns ns ns ns ns Note: S: Straw, N: nitrogen, W: Irrigation water. Values within the same columns followed with different letters are significantly different at p < .05 according to LSD test. ns; not significant, *; significant at p ≤ .05, **; significant at p ≤ .01 and ***; significant at p ≤ .001 between the treatments.
soil water contents when compared with the treatments having no straw layer. Similar trends have been reported by Mingze et al. 2016). The treatments T 4 and T 5 (NSN 100% W 90% ) recorded lowest and highest water consumption but no significant difference for most vegetative measurements was noticed between these two treatments except the plant root attributes. Similar trends for roots have been reported by Man et al. (2016) and ) who stated that keeping soil water contents at a moderate level enhances the root distribution in the topsoil layers. Li et al. (2010) also noted that deficit irrigation levels increase root density in the topsoil layers. These results match with our findings which reveal that optimum levels of moisture in the root zone can improve roots density compared to deficit or full irrigation. It was concluded by Harris (1992) that root length is possibly a better measure of the absorbing capability of roots, which is also proved by our results that under W 90% treatments, roots had vertical whereas roots had a horizontal pattern under W 70% treatments due to the difference in wetting pattern of irrigation under different irrigation levels. There is a close correlation between plant nutrient absorption and water uptake. During the process of water absorption by plant roots, the nutrients move to the roots surface. Limiting the plant water uptake also restricts the delivery of nutrients to the plant roots. The processes of diffusion and mass flow, responsible for bathing the plant roots with nutrients slow down as the soil dries. The tomato plant requires a balanced management of fertilizer for optimum growth because it is a heavy remover of nutrients and responds well to the fertigation (Hebbar et al. 2004). In addition, deficit irrigation and fertilizer levels may lead to adversely affect the plant growth. On the other hand, buried straw layer can decrease the impact of water and fertilizer deficiency by holding water and nutrients in the topsoil layer (Zhao et al. 2014). As in our findings, application of higher water and fertilizer with no straw layer and lower water and fertilizer with buried straw layer had no significant difference in most vegetative growth parameters. Similar trends have also been reported by (Chen et al. 2016;Chen et al. 2019). It is found in our results that plant growth parameters are more sensitive to mulching and irrigation than fertilizer. It is also reported by (Xiukang and Yingying 2016) that irrigation has more effects on plant growth than fertilizer.
The interactive effect of fertilizer and irrigation significantly affected P N and G S at the flowering stage during both years (p < .001) but no significant difference was notice because all the plants were treated with same practices for the day 1 to day 21 and treatments were established on 22nd day. At fruiting and harvestings stages, combination of higher levels of fertilizer and irrigation improved P N and G S under no buried straw conditions whereas lower fertilizer (N 80% ) and irrigation (W 70% ) levels had a positive effect on P N and G S under buried straw layer. The reason of improved P N and G S by the lower irrigation and fertilizer under buried straw layer treatment (T 4 ) could be the effect of better plant growth under T 4 , which was improved due to the horizontal distribution of roots at the topsoil layer, which increased plant nutrients and water uptake. Under increased fertilizer and lower irrigation, both P N and G S were decreased at fruiting and harvesting stages. Liu et al. (2012) advised that nitrogen and water are closely associated, and drought tolerance cab be enhanced by applying nitrogen at a lower rate which subsequently can improve water uptake and photosynthetic capacity. Consequently, an optimal low nitrogen supply would be adopted under the deficit conditions. It is also reported by Garcia et al. (2007) that the water-stressed tomato plants showed a decrease in stomatal conductance due to an increase in leaf osmotic potential and decrease in leaf water potential. The optimal addition of nitrogen can contribute to recover the adversative drought effect by a process of elastic and osmotic adjustment. In our findings, a direct relation was observed between P N and G S , as P N was significantly increased with the rise in G S (Figure 2) . Hebbar et al. (2004) stated that chlorophyll contents of tomato were significantly affected by the interaction between fertilization and irrigation. However, no significant effect of the interaction between fertilization and irrigation was noticed during our study but the interactive effects of buried straw layer with fertilizer as well as with irrigation affected Chl significantly. Photosynthesis was significantly influenced by chlorophyll contents (Figure 2) which is also stated by Hebbar et al. (2004) and Xiukang and Yingying (2016), who described that photosynthesis is the main function of leaves however, fertilization and irrigation are the main factors which progress leaves growth and subsequently improves plant growth, crop production and crop quality. Chlorophyll fluorescence analysis allows near-instantaneous, non-invasive measurement of main aspects of electron transport and photosynthetic light capture (Campbell et al. 1998) and responds to quantum yield, state of energy distribution in the thylakoid membrane and the extent of photoinhibition (Wang et al. 2007). ETRmax and P N were found to have a significant correlation (Figure 2, p < .001). The possible reason due to which T 4 (WSN 80% W 70% ) could get higher yield and T 6 (WSN 100% W 90% ) decreased the fruit yield is horizontal distribution of plant roots in T 4 , which supports higher nutrients availability. While, under T 6, most part of the roots was in the lower side of straw layer, which reduced the nutrients uptake ability of the plants as most of the nutrients, sustained by straw layer. Also, the straw layer could reduce leaching loss of nitrogen and water due to the characteristic of straw-lack of capillary (Zhao et al. 2014). If the soil moisture of the upper-layer is decreased due to evapotranspiration loss then straw use could overcome the adversative effects of water stress by increasing plant water status, improving plant growth and water use efficiencies (Abd El-Mageed et al. 2018). The increment in nitrogen fertilization reduced tomato yield under no straw and W 70% treatment (T 7 ). This result indicated that increasing nitrogen rate under water deficit will be relatively inefficient, could be because of an adverse effects of excessive nitrogen on yields (

Conclusions
It was revealed in the present study that growth and physiological parameters were all affected by fertilizer, irrigation and buried straw layer. Buried straw layer improved both growth and physiological parameters under lower fertilizer (N 80% ) and irrigation levels (W 70% ) but decreased under higher fertilizer (N 100% ) and irrigation levels (W 90% ) during both years. Plant and roots fresh and dry biomass, photosynthesis, stomatal conductance, ETRmax, F V /F M and chlorophyll contents were significantly improved in T 5 (NSN 100% W 90% ) followed by T 5 (WSN 80% W 70% ). P N was noticed to be correlated with G S , ETRmax, and Chl significantly suggesting that osmotic effect was the key factor influencing the photosynthetic process. It was observed that under limited water conditions, buried straw layer could be applied to relieve the adverse effect of deficit irrigation and fertilizer levels for improving plant growth and physiology and fruit yield.

Disclosure statement
No potential conflict of interest was reported by the authors.