Faster root growth speed nature of Afghan wheat landraces under dried soil condition

Chiho Motokawa¹, Nao Sato¹, Emdadul Haque²*, Farid Niazi², Hiroyuki Tsuji², Noriaki Kojima¹, Tomohiro Ban²

¹ Yokohama Science Frontier High School, Japan

² Kihara Institute for Biological Research, Yokohama City University, Japan

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

Abstract

The rapid root growth into the deep soil could significantly improve the capture of the underground water in the rain-fed wheat cropping system in Afghanistan. It generally contributes for vegetative growth of effective tillers and grain filling when crops are vulnerable to midterm and terminal drought. Wheat landraces adapted to the Afghan environments were hypothesized to be faster and deeper root growing capacity as they are used to grow in the dried soil condition. An efficient system for root growth speed study was developed using 20, 30, 40, and 60 cm lengths of PVC pipes with two regimes of water conditions to investigate the growth speed nature of Afghan wheat landraces as collected by Dr. Kihara et al. in the Kyoto University Scientific Expedition (KUSE) to the Karakoram and Hindukush 1955. Two spring type landraces for ‘Long root’ (LR-759; KU7469) having an enormous capacity for elongation of roots over 200 cm and for ‘Short root’ possessing ca. 80 cm root (SR-815; KU7533), respectively, and one spring type modern semi-dwarf cultivar Lalmi-2 as a check were grown and recorded the emerged numbers of root from bottom of the PVC pipes with several time points. Lalmi-2 was found to grow and extend its root within 20 to 40 cm and can’t extend root growth longer and faster at the dried soil. Afghan landraces extended their roots until 60 cm within one month and interestingly, they were able to change their growth speed depend on the magnitude of the drought condition.

Keywords: root growth speed, wheat landraces, soil dryness

Introduction

7.3 billion people of the world's population is in a state of food crisis and double production of crops will be demanded in coming half century. There is an urgent need for the discovery and development of possible varieties with higher yield in the vast rain-fed area that is not currently being carried out grain production. Here we focused on root growth character to improve water harvesting in the rain-fed wheat cultivation in Afghanistan.

Most wheat in the Afghanistan is produced in the rain-fed cultivation. The climate of this region is cold in winter, hot and dry in summer. Rainfall is scanty, nowhere more than 381 mm annually with much of the precipitation as a winter snow and occasional spring rains (FAO 1972; Beekma and Fiddes 2011). Since the soil of Afghanistan is calcareous in nature (FAO 1972) and high in sand content (ICARDA 2002), it is common for water to be stored and going down into the deep as the growing season progressed. Therefore, rapid root growth, deep into the soil during the early season may increase capture of water and nitrogen and generally coincides with tillering and grain development when crops are vulnerable to midterm and terminal drought in rain-fed cropping regions (Wassion et al. 2012). The value of targeting the capture of deeper soil moisture with selected root traits in a breeding programme is 2-fold (Wassion et al. 2012). Once stored beyond the evaporation zone, it becomes a known source of crop water, while the in-season rainfall is unpredictable at the time of wheat growing season. Many wheat production systems could benefit from improvement of the storage of soil moisture through management (Hunt and Kirkegaard 2012) and soil moisture exploitation through root genetics. The Afghan wheat landraces (AWLR), local/native wheat varieties of Afghanistan, are important primary gene pool resources and were hypothesized to be faster and deeper root growing capacity as they are used to grow in dry environment.

Several kinds of AWLR 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 made the practical cultivation conditions of Afghan rain-fed areas using a various length of PVC pipes where a few centimeters top soil layer were controlled by the day-to-day watering that does not allowed the water to pass the top layer. When grown under these conditions, it was considered that the root might be passed immediately the dried zone below top wet layer within a shortest time if varieties that fits the purpose.

Materials and Methods

Based on our preliminary root and growth habits study (Unpublished) two spring type AWLR, one with long root LR-759 (KU7469) and one short-root SR-815 (KU7533) were selected in this study. Lalmi-2 was used as a check. Seeds were germinated on moist filter paper in petri dishes in dark conditions at 23^oC. Three healthy germinated seeds were sown at a 2-cm depth in each of soil-filled PVC pipes (Soils: Kobayashi Sangyo Co. LTD., Japan). Since the extension of the middle of the root inside the long pipe can’t be observed, pipes having four different lengths such as 20 cm, 30 cm, 40 cm and 60 cm with about 5.5 cm diameter were used.  The bottom of the pipes were fixed with plastic nets so that the roots can pass through it. The upper 5 cm layer of each pipes were modified with 2% water-holding gel (San Tech, Kochi, Japan) so that it can keep the surface moisture for longer period (Fig. 1). Before seed sowing, all the pipes were moistened once with water until drainage through the bottom. The pipes for wet conditions were then placed on cup so that they can work as moisture conditions as normal. All the pipes for wet conditions were watered with definite amount for each pipe lengths so that it can saturate the soils inside the pipe in every second day. In case of dry conditions, only the upper surface (5-20 cm based on pipe lengths) of the pipes were saturated with measured volume of water. The experiments were conducted in a greenhouse maintained at a temperature of approximately 20^oC day/night, with natural light (14h day/ 10h night) at the Kihara Institute for Biological Research, Yokohama City University in 2014-2015.

The emergence of roots were monitored every day and statistically calculated on 4 time points; 16, 20, 27 and 30 days after sowing. Finally plants were removed from the pipes at 30 days of sowing, photographed and other morphological parameters were checked when required.

Results and Discussion

Our primary attempt was to establish drought treatment in the long pipe soil that would provide reproducible effects on wheat growth. Firstly, in order to observe the difference in the growth of the aerial parts, tiller number and shoot length was determined (data not shown). Moreover, in order to observe the effects of stress due to drying was measured leaf shape, number of stomata (SUMP method) to find any difference between the wet and dry soil conditions within the same genotype. The results showed that shoot growth including shoot length, leaf shape, and number of stomata was reduced under dry condition and it was apparent in Lalmi-2 plants, particularly for 60 cm pipe (data not shown). It showed a good correlation between treatment intensity: i.e., moisture magnitude and pipe length, and the results: i.e., changes in the shoot growth and the genotypes. The effects of the depth of dryness to affect the seedling growth reduction is general consequence under drought condition. Selection programs for high shoot vigor have resulted in lines with more vigorous early root growth (Wasson et al. 2012) and may be associated with a deeper root system (Richards et al. 2007). Thus, the system we used in this study worked as expected and implying phenotypic variation among genotypes.   

Faster root growth and Prospects of AWLR

Lalmi-2 was found to grow and extend its root within 20 cm to 40 cm and can’t extend root longer and faster at dry condition. As expected the Afghan landraces extended their roots until 60 cm within one month and interestingly, they were able to change their growth speed depend on the magnitude of soil dryness with different pipe lengths (Fig. 2 and Fig. 3). The timing of root emergence was earlier in LR-759 than SR-815 and Lalmi-2 in accordance with the severity of soil dryness inside pipes until 60 cm. To our knowledge, this is the first report to describe the timing of root growth speed in the seminal roots of AWLR.

Reason behind faster root growth speed and relation to flowering

Two features behind the root growth speed are; faster rate of root system elongation and/or a narrow angle of root system (Wassion et al. 2012). While the faster rate of root system elongation depends on processes within the root apex (that determine cell division and expansion), shoot vigor and less number of shoot tiller, the narrow root angle system depends on soil gravitropism. Root tips were shown to drive gravitropic response in an auxin-dependent mechanisms that alters cell elongation. A strong gravitropic response is thought to be a marker of the underlying hormonal control of root depth. Therefore genotypes with any of these features would be interesting to select in the screening program for rain-fed area. Although at present we don’t have precise data, LR-759 would be great to study for further confirmation with this regard. Furthermore, the duration of root elongation in wheat is approximately related to the duration from sowing to flowering as downward growth ceases around the time of flowering and onset of grain development (Wasson et al. 2012). Therefore, another effective approach to increase root depth has been to increase the time to reach flowering. In our condition, while Lalmi-2 and SR-815 started flowering at the 30 days, LR-759 was still without flowering (Fig. 3). The delay in flowering might be one of the reason that allowing long root plant to continue growing into deep to catch stored water. We are now studying this point in the laboratory.

 Faster root growth AWLR and its prospects to rain-fed area of Afghanistan

Today most rain-fed wheat in Afghanistan is in the Northern and Eastern regions, and accounts for 60% of total production with little access to irrigation. The trend in Afghan agriculture is towards less water for irrigation, and storing and use of deep water, either summer rains or irrigation, will become increasingly important. Our hypothesis on “the advantages on the faster root landraces over stored water downing” is given in Fig. 4. Briefly, Afghan landraces with the high growth speed and long root system, like LR-759, will be beneficial compared to SR-815 (and of course to Lalmi-2) since their root is predicted to grow more than 200 cm within the early period of wheat growing season.

References

Beekma J and Fiddes J (2011) “Floods and droughts: The Afghan water paradox”, Center for Policy and Human Development (CPHD).

FAO (1972) Soil bulletin 21-Calcareous soil. Country reports, Afghanistan.

Hunt JR, Kirkegaard JA (2012) Re-evaluating the contribution of summer fallow rain to wheat yield in southern Australia. Crop and Past Sci 62: 915-929.

ICARDA reports (2002) Needs assessment on soil and water in Afghanistan, Future harvest consortium to rebuild agriculture in Afghanistan. Aleppo, Syria.

Richards RA, Watt M, Rebetzke GJ (2007) Physiological traits and cereal germplasm for sustainable agricultural systems. Euphytica 154: 409-425.

Wasson AP, Richards RA, Chatrath R,  Misra SC,  Prasad SV, Rebetzke GJ, Kirkegaard JA, Christopher J and Watt M (2012) Traits and selection strategies to improve root systems and water uptake in water-limited wheat crops. J Exp Bot 63: 3485-3498.

Yamashita K (1965) Cultivated plants and their relatives. Results of the Kyoto University Scientific Expedition to the Karakoram and Hindukush Vol. I. Japan: Koei Printing.