Stomatal aperture rather than nitrogen nutrition determined water use efficiency of tomato plants under nitrogen fertigation

Highlights

• Intensified ABA signaling and decreased plant water status caused partial stomatal closure, resulting in improved WUE_i.

• The increase in leaf δ¹³C under the reduced water regimes was caused primarily by reductions in g_s.

• Stomatal aperture rather than N nutrition predominated regulation of plant WUE under fertigation.

• Under sufficient N supply, WUE can be improved by moderate watering without affecting fruit yield significantly.

Abstract

Fertigation can improve water use efficiency (WUE) compared with conventional separate supply of water and fertilizers to plants. Yet the mechanisms underlying the improved WUE under fertigation remain largely elusive. Therefore, the impact of water and nitrogen (N) on leaf gas exchange, plant water relations, ABA signaling and WUE as well as leaf δ¹³C and δ¹⁸O were investigated in order to unravel how water and N modulate plant WUE. Results showed that reduced soil water regimes under N fertigation caused partial closure of stomata via decreased plant water status and intensified root-to-shoot ABA signaling, resulting in improved intrinsic WUE (WUE_i). Decreased soil water regimes increased plant WUE (WUE_p) and leaf δ¹³C, and the increased leaf δ¹³C was due to reduced g_s and/or higher specific leaf N content enhanced photosynthetic capacity. Leaf δ¹⁸O and δ¹³C further indicated that the significant increase in leaf δ¹³C under the reduced water regimes was caused primarily by reductions in g_s compared with N nutrition. Therefore, g_s rather than N nutrition predominated regulation of plant WUE under fertigation. Moderate soil water regimes with sufficient N supply are recommended for fertigation in terms of achieving high fresh fruit yield, WUE and nutrient uptake.

Introduction

Water and nitrogen (N) are the two most limiting factors for plant production globally. In order to increase yields, large amounts of fertilizers, especially N fertilizer, are applied to the soil each year. However, this leads to a dramatic reduction in N use efficiency (Tilman et al., 2002). Meanwhile, the arable land areas under irrigation are continuously increasing. This deteriorates the environment due to loss of N into water resources (Sebilo et al., 2013). To cope with these challenges, efficient management strategies are crucial for increasing water and N use efficiency towards agricultural sustainability (Quemada and Gabriel, 2016).

Fertigation has been widely used in crop production in many regions of the world. Compared with the supply of water and nutrients separately during conventional irrigation and fertilizer application, fertigation delivers water and nutrients simultaneously through irrigation systems to the vicinity of active roots, and this can facilitate water uptake and enhance the bioavailability of soil nutrients. Moreover, fertigation enables multiple applications of nutrients with required dosages during specific growing seasons in coordination with plant demand for more precise water and nutrient management (Bar-Yosef, 1999; Hebbar et al., 2004; Alva et al., 2008; Qin et al., 2016; Zhou et al., 2017). Thus, plant growth, yield, water and nutrient use efficiency under fertigation can be improved compared with conventional fertilizer application (Locascio et al., 1997; Singandhupe et al., 2003; Mahajan and Singh, 2006; Bhat et al., 2007; Badr et al., 2010). The advantages of fertigation over traditional fertilization methods also include uniformity of nutrient application, less nutrient loss through seepage or runoff, a reduction of soil compaction and mechanical damages to growing plants (Rapadopoulos, 1988; Asadi et al., 2002; Bryla and Machado, 2011).

The N effect under fertigation has received considerable attention. Klein et al. (1989) found that N concentration in the apple leaves was decreased significantly under low N fertigation at 50 kg ha⁻¹ compared to other increased N treatments. Asadi et al. (2002) reported that N treatments at 150 and 200 kg ha⁻¹ resulted in high yield of corn compared to low N at 100 kg ha⁻¹ or no N treatment under N fertigation. Castellanos et al. (2013) observed highest yield and water use efficiency (WUE) of melon plants under N fertigation at 160 kg ha⁻¹ compared with other varied N rates. Zhang et al. (2017) noted that the low N treatment during N fertigation at 110 kg ha⁻¹ reduced the yield and WUE of winter wheat significantly, and they speculated that the yield differences were due to the significant reduction of photosynthetic rate (A_n) and stomatal conductance (g_s), however, the medium N treatment at 190 kg N ha⁻¹ had similar yield and WUE as the high N treatment at 290 kg N ha⁻¹. Sinha et al. (2017); Mali et al. (2017) and Jayakumar et al. (2017) showed that high amount of fertilizers during fertigation increased the yield and WUE. However, the concomitant impact of water and N on WUE under fertigation in the aforementioned studies was not further investigated.

Plants exert some control over water loss from leaves via the narrowing of stomatal apertures, which is commonly observed in plants grown in drying soils. Such stress response is initiated and regulated by chemical and hydraulic signals. Root–derived abscisic acid (ABA), which is transported through the transpiration stream to the shoots, acts as early signals of soil drying (Blackman and Davies, 1985; Zhang and Davis, 1990; Tardieu et al., 1996; Sauter et al., 2001; Bahrun et al., 2002; Dodd et al., 1996; Liu et al., 2005; Dodd, 2005; Schachtman and Goodger, 2008). Hydraulic signals are produced when shoot water status decreases as a consequence of limited water uptake by roots (Chazen and Neumann, 1994; Comstock and Mencuccini, 1998). Water deficit induced root-to-shoot ABA signaling and decreased plant water status cause reductions in leaf expansion growth and stomatal opening (Gowing et al., 1990; Zhang and Davis, 1990; Davies and Zhang, 1991; Davies et al., 1994, 2002; Hartung et al., 2002; Wilkinson and Davies, 2002; Comstock, 2002), thereby WUE of plants is improved. In addition, previous studies showed that ABA may correlate with nitrogen in several plant species (Wang et al., 2010; Kiba et al., 2011 and literature cited therein).

Carbon isotopic composition (δ¹³C) provides a time-integrated measurement of plant WUE (Farquhar and Richards, 1984; Farquhar et al., 1989). There is a strong positive correlation between δ¹³C and WUE in many crops including tomatoes (Martin and Thorstenson, 1988; Ellsworth et al., 2017). It is well established that the higher WUE and δ¹³C are associated with a lower ratio between cell (C_i) and atmospheric (C_a) CO₂ concentrations (C_i/C_a). The decrease of C_i/C_a is due to the decrease of g_s or the enhancement of photosynthesis (A_n), or both (Condon et al., 2004). Oxygen isotopic composition (δ¹⁸O) reflects transpiration rates, as there is no further discrimination for the element during photosynthesis. After photosynthetic CO₂ assimilation, the δ¹⁸O signal of water is transferred to plant tissues (Farquhar et al., 1998; Yakir, 1998; Barbour et al., 2000; Barbour, 2007). Increasing g_s results in less enrichment at the sites of evaporation within leaves caused by increased transpiration (Barbour, 2007). Therefore, δ¹⁸O can aid our understanding of plant responses to water stress because the relative effect of g_s and photosynthetic capacity on changes in WUE can be separated (Farquhar et al., 1998; Barbour et al., 2000; Chaves et al., 2003).

Although the impact of N on plant growth, yield and N utilization under fertigation have been well documented, the mechanisms underlying the improved WUE and its modulation by water and N are still poorly understood. Therefore, the purpose of this study was to investigate the physiological responses of tomato plants subjected to different combinations of water and N treatment under fertigation and to explore the mechanisms regulating plant WUE. Specifically, we examined plant water relations, leaf gas exchange, ABA signaling, leaf δ¹³C and δ¹⁸O, to determine the relations of these factors with plant WUE.