Greater growth efficiency of Kihara Afghan wheat landraces (KAWLR) in response to high CO₂ under CaCO₃ soil

Nao Sato², Emdadul Haque^1*, Chiho Motokawa², Noriaki Kojima², Hiroyuki Tsuji¹, Tetsu Kinoshita¹, Tomohiro Ban¹

¹ Kihara Institute for Biological Research, Yokohama City University, Japan

² Yokohama Science Frontier School, Japan

^* Corresponding author: Emdadul Haque (haque@yokohama-cu.ac.jp)

Abstract

Increase in CO₂ concentration increase atmospheric O₂ and soil Ca²⁺ through the production of plant biomass and decomposition of CaCO₃, respectively. To learn the effect of CO₂ on wheat under calcareous soil, a typical problem in Afghanistan, a biotron wheat culture system was developed in combination of CaCO₃ containing two-chambered cup and high CO₂ (800-1000ppm). Two spring type Kihara Afghan wheat landraces (KAWLR) collected by Dr. Kihara et al., one with long root type (LR-595; KU11274) and another with short root type (SR-815; KU7533), and one spring type modern cultivar Lalmi-2 as a check were grown. Overall, total growth and growth efficiency of wheat under CaCO₃ at high CO₂ concentration was better, with the highest in KAWLR. Wheat grown on control and CaCO₃ at ambient CO₂ (400-500ppm) had positive correlation on leaf length and wide, meaning the longer lengths the wider leaf, while the correlation become reverse under high CO₂, meaning the longer lengths the narrower leaf, particularly in KAWLR. Moreover, the high CO₂ and CaCO₃-grown leaf were much thicker compared to ambient CO₂ condition, with the chlorophyll content remained unchanged. This altered leaf form, so called “mature leaf type”, is usually get favor by environmental stresses. Water adsorption was higher at high CO₂ (Lalmi-2<SR-815<LR-595), but the number of opened stomata in LR-595 was same in both CaCO₃ and control conditions, indicating its higher leaf water potential and/or water use efficiency. Taken together the effect of future elevated CO₂ will be minor in wheat, particularly in KAWLR under calcareous Earth.

Keywords: Afghanistan, Wheat, Landraces, CaCO₃, Plant nutrient

Introduction

It is generally saying that elevation in CO₂ and thereby high temperature is not good for plants. However, studies on responses of crops to elevated CO₂ often report to increase growth, use efficiency of nitrogen and water when nutrient availability is optimal or near optimal (Nord et al. 2015). Various C₃ plant species shown that elevated CO₂ stimulates leaf photosynthesis and photosynthetic carbon gain (Tsutsumi et al. 2014). Thus, the elevated CO₂ increasing O₂ via C₃ plants. Though the majority of studies consider elevated CO₂ in isolation, plant responses to elevated CO₂ may be affected by other environmental factors, including soil properties (Nord et al. 2015). With this regards, a second advantage of CO₂ is to decompose CaCO₃ to increase Ca²⁺, which is very essential plant nutrient (Giel and Bojarczuk 2011). A classification of our earth is (i) calcifuge or low Ca²⁺ and acidic with wet soil, and (ii) Calcareous or high Ca²⁺ and alkaline with dry soil. While the former one is widely used for rice, the later one is for wheat cultivation. The presence of excessive CaCO₃ in the calcareous Earth including Afghanistan (15-40%)(FAO 1972) is a typical phenomenon in rain-fed region. Interestingly nitrification rate is generally greater (Kishchuk 2000) and water deficiencies (by evaporation or absorption) is higher (Sibbett, 1995) on calcareous soil. In our study (Unpublished), under normal atmospheric CO₂ (approximately 400 ppm), wheat particularly landraces showed higher nitrogen absorption efficiency and less or no reduction in leaf greening traits under saturated CaCO₃ conditions with optimal fertilizers. Here, we would like to know how modern wheat and landraces react to elevated CO₂ when grown on high CaCO3 condition. There is a concern about the performance of both cultivated and wild plants in future climates characterized by elevated CO₂ (IPCC 2007a). Since Afghan wheat landraces is used to grow in this types of marginal area of the Earth, we hypothesized that wheat could efficiently manage their growth and development in response to high CO₂ under CaCO₃ soils same as or better than the normal condition.

Kihara Afghan wheat landraces (KAWLR) with distinct root system differences were prepared from the collection of Dr. Kihara et al. in KUSE 1955 (Yamashita et al. 1965) and then we grow them in two-chambered semi-hydroponic cultivation condition having CaCO₃ in combination with biotron breeding condition with the double supply of CO₂ concentration.

Materials and methods

Based on our previous studies on root system (Osmani et al. 2015) and growth habits (Stanikzai al. 2015) two spring type KAWLR, one with long root LR-595 (KU11274) and one short-root SR-815 (KU7533) were selected in this study. One spring type semi-dwarf modern wheat Lalmi-2 was used as a check. The methodology followed here are according to Haque et al. (2015) with the combination of biotron culture (Fig. 1). In briefly, after 2 d of imbibition followed by 4 ºC treatment, seeds were sowed at a 2.5-cm depth in soil-filled (Volcanic soil; Kaneko Seeds, Japan) upper cup of two-chambered cups. The bottom cup contained water and different concentration of CaCO₃ solutions. The bottom of the upper cup contained four holes (about 4 mm diameter) so that plant growing in the upper cup can absorb the gradient water and nutrients through bottom cup. In this study, we used two treatment solutions: (i) Tap water (control) and (ii) Saturated CaCO₃ (T). Liquid fertilizer (HYPONeX, Japan) was added into the treatment solutions at initial solution followed by once per 9 days. The solution of the bottom cup were changed in every 3 days. The experiments were conducted in a biotron chamber (SANYO MLR-350, Japan) with (800-1000 ppm) or without (400-500ppm) the supply of CO₂ through cylinder connecting with CO₂ data recorder (MCH-383SD, Taiwan). We maintained the culture chamber at a temperature of approximately 20-25 ºC day/night, with constant supply of white light (200 μmol m^-2S^-1) at the Kihara Institute for Biological Research, Yokohama City University in 2014-2015. The total chlorophyll contents (SPAD value) were measured in the last leaf. Plants were then harvested from individual cup at 5 weeks after treatment initiation and root and shoot traits were measured. We measured leaf shape and number of stomata (SUMP method).

Results and Discussions

Growth of modern wheat and landraces to high CO₂ under CaCO₃

Overall, shoot lengths in all the 3 cultivars were increased in response to high CO₂ under CaCO₃ condition by 1-3 cm (Fig. 2). For roots, when the LR-595 was better in water, SR-815 was growing well at CaCO₃ saturated aqueous solution, the Lalmi-2 difference was variable depending on CO₂ concentration. Shoot (Fig. 2D) and root (Fig. 2E) drymass responses to high CO₂ under CaCO₃ condition was almost in similar fashion of shoot and root length data. Wheat grown on H₂O and CaCO₃ in ambient CO₂ concentration have positive correlation on leaf length and wide correlation, meaning that the longer lengths the wider leaf (Fig. 3A). However, when the CO₂ concentration is higher, the correlation become reverse, meaning the longer lengths the narrower leaf (Fig. 3B). The later tendency was much obvious in both KAWLRs. Moreover, the leaf was thicker in high CO₂ than the ambient CO₂ condition (400-500ppm) without much changing the chlorophyll content as measured until 5 weeks after treatment initiation (data not shown) This scenario is in agreement with former report (Nie et al. 1995) where spring wheat does not affected in their chlorophyll content until flag leaf in response to high CO₂. On the other hand, while the water adsorption rate was higher at high CO₂ biotron chamber (L-2<SR-815<LR-595) under both control and CaCO₃ with the highest in CaCO₃ condition (data not shown), the number of opened stomata in LR-595 was almost same in CaCO₃ and H₂O conditions (Table 1). This results was supported by the higher water use efficiency hypothesis of plants at high CO₂ concentration (Hao et al. 2011).

Learning the reason of higher growth response of wheat to high CO₂

Alteration in leaf form widely very according to plants species (Tsutsumi et al. 2014) and the results which is most agreement with our findings is reported by Thomas and Bazzaz (1996). The narrower but thicken leaf growth under CaCO₃ condition at elevated CO₂ is a kind of acceleration of “mature” or “high light” type leaves (Thomas and Bazzaz 1996) as found particularly in KAWLR (Fig. 2 and Fig. 3). This observation recalls management/minimization of environmental effects by genotypic potentials. For example, production of mature leaf forms is often favored by high light levels and root removal or restriction, while shading, nutrient additions, and removal of actively photosynthesizing tissue favor production of young/immature leaves (Thomas and Bazzaz 1996). Although we did not assayed leaf carbohydrate content, the first hypothesis on the alteration of leaf form is that CO₂ enhance leaf carbohydrate levels, and that one or another carbohydrate (perhaps sucrose) are directly involved in a signal transduction pathway that ultimately results in altered patterns of cell proliferation and expansion (Thomas and Bazzaz 1996). Two other alternative physiological explanations are direct hormonal interaction or increased leaf water potential under elevated CO₂ (see more detail on Thomas and Bazzaz 1996). The higher water absorption capacity by KAWLR under high CO₂ without changing the opened stomata (Table 1) might increase the leaf water potential. Thus, these narrower and thicker leaf type with higher water absorption/use-efficiency and unchanged chlorophyll content in CaCO₃ soil under higher concentration of CO₂ might be one of the reason of overall greater total growth and growth efficiency of KAWLR (Fig. 4). Furthermore, the photosynthesis rate might be higher in KAWLR, even the chlorophyll content was not altered (data not shown). For the efficient photosynthesis, it is not necessary to increase the chlorophyll contents in CO₂. However, there might be some of the mechanism in the soil condition management. One possibility is CO₂ might regulating pathways beneficial to soil condition by regulating necessary elements and/or soil buffering via the secretion of mugineic acid by plant root. We are now analyzing these factors, particularly, the status of mugineic acid in response to high CO₂, as reported to be influenced by other environmental stimuli such as light (Mino 2007).

In overall conclusion, better performance of KAWLR compared to Lalmi-2 against high CO₂ is suggesting that the effect of future elevated CO₂ concentrations will be minor in wheat, particularly in KAWLR, in calcareous and dry soil, since drought does not affected plant growth and production significantly under elevated CO₂ (Kimball 2001).

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