www.iita.org I www.cgiar.org

Phenotypic variability of drought-avoidance shoot and root phenes and their relationships with yield under drought and low P conditions in cowpea Belko N¹*, Boukar O¹, Burridge J3, Chamarthi S¹, Togola A¹, Moukoumbi Y¹, Lynch J 3, Abberton M², Fatokun C²

1 International Institute of Tropical Agriculture (IITA), Ibadan, Oyo State, NIGERIA 2 Plant Root Biology Lab, Department of Plant Science, Penn State University (PSU), USA

Rationale and Objective

Cowpea (Vigna unguiculata (L.) Walp.) is an important food and cash crop in the semi-arid regions of sub-Saharan Africa where it is widely cultivated on marginal lands under rain-fed condition by small-scale farmers. Although the crop is relatively adapted to drought prone agro-ecologies, it could benefit from genetic improvement aimed at improving its tolerance multiple abiotic stresses (Boukar et al., 2016). Collaborative experimental research was therefore conducted by IITA and PSU with the objective to assess cowpea germplasm for its genetic variation in (i) water-saving shoot traits and (ii) root system architecture for superior adaptation to drought and low phosphorus environments. The identified lines with traits of interest will be incorporated into the cowpea breeding program for developing new high yielding cultivars with enhanced tolerance individual or combined environmental stresses.

The cowpea germplasm mini-core collection (374 lines) and 50 selected breeding lines were phenotyped in both screen-house and field conditions for their variation in drought and low P tolerance related shoot and root traits.

Whole plant transpiration was estimated gravimetrically and indirectly using infrared thermography (Photo 1: Belko et al., 2013). Plant root system architecture was evaluated using a field base phenotyping technique “Shovelomics” for adaptation to both drought and low phosphorus (Photo 2: Burridge et al., 2016).

Results and Discussion

There was large genotypic variation among the cowpea germplasm for (i) plant transpiration and canopy temperature under non-limiting water and high VPD conditions (Fig. 2a,b) and (ii) grain yield under relatively low P conditions (Fig. 2c). Root phenes i.e. adventitious and basal root numbers (ARN, BRN) and growth angles (ARGA, BRGA) and branching density (BD) also significantly discriminated between tested lines (Table 1).

IT86D-1010, IT98K-205-8, IT97-499-35, IT99K-573-2-1, TVu-9486, TVu-14788, TVu-15391, TVu-11986, and TVu-14676 with conservative water-use attributes and steep root system are potential for adaptation to drought while IT98K-506-1, IT99K-494-6, IT07K-318-33, IT99K-494-6, Tvu-2731, Tvu-5415, TVu-9797, TVu-11982 and TVu-15055 with seedling basal root number and shallow root system are potential for adaptation to low P (Table 1a,b).

Conclusion and Recommendation

The results reveal the possibility for improving the productivity of cowpea in drought-prone and poor soil environments through exploiting its genetic resources. It is important to design an integrated strategy combining plant phenomics, genomics, agronomy and modeling to maximize crop productivity in a given environment or stress scenario and to develop guidelines for farming options in the face of climate variability in sub-Saharan Africa.

References

1. Boukar et al. 2016: Front Plant Sci. 2016; 7: 757 2. Belko et al. 2013: Plant Biology 15: 304 – 316 3. Burridge et al. 2016: Field Crops Research 192: 21 – 32

Hypotheses and Methods

Edaphic resources i.e. water and P are stratified in contrasting patterns into the soil profile: Restricted leaf transpiration under high VPD (Fig.1a) and steep/profuse root system (Fig.1b) are important for adaptation to terminal drought while shallow/dense root system (Fig.1b) is beneficial for sub-soil foraging and enhanced water and phosphorus acquisition.

TCanopy = Sum ((Ti x Pxi) / Pxt) Ig = (Tdry leaf – TCanopy) / (TCanopy – Twet leaf)

Photo 1: Thermal image (a) and distribution of number of pixels over a range of plant canopy and background temperatures (b) for indirect estimation of plant canopy temperature (Tcanopy) as a proxy for whole plant transpiration rate in cowpea.

Nu

mb

er

of p

ixels

Temperatures ( C)

Range of leaves temperatures Range of backgrounds temperatures

B

(b) (a)

Photo 2: Shallow (a) and steep (b) root systems evaluated for their difference in adventitious and basal root numbers, growth angles and branching densities using a visual scoring board.

(a) (b)

Root System Architecture / Potential for

adaptation to drought and low P stress

environements

Genotype ARGA BRGA ARN BRN BD5-10cmGrain yield

(kg/ha)

IT98K-1111-1 40.00 ± 5.28 40.00 ± 2.00 12 ± 3.61 9 ± 1.20 10 ± 1.26 1207 ± 380

TVu-11982 41.11 ± 4.84 33.33 ± 3.33 11 ± 1.28 6 ± 0.56 10 ± 0.91 818 ± NA

TVu-9797 43.33 ± 3.33 27.77 ± 4.01 13 ± 5.28 10 ± 2.41 11 ± 2.02 1349 ± 422

TVu-15055 41.66 ± 4.41 35.55 ± 2.94 10 ± 1.53 5 ± 0.73 10 ± 1.60 2683 ± NA

IT07K-318-33 36.66 ± 1.92 28.88 ± 5.56 8 ± 0.51 7 ± 0.59 10 ± 0.68 3344 ± 239

IT99K-494-6 43.33 ± 6.67 36.66 ± 6.02 8 ± 1.00 4 ± 0.60 10 ± 1.76 1932 ± 627

….

….

….

….

….

….

….

IT86D-1010 57.77 ± 7.77 47.77 ± 7.77 8 ± 1.06 6 ± 1.01 10 ± 0.29 NA

TVu-11986 50.00 ± 0.00 50.00 ± 10.00 6 ± 1.45 9 ± 0.67 8 ± 0.88 1509 ± 196

TVu-6443 63.33 ± 3.33 50.00 ± 5.77 9 ± 3.21 7 ± 1.20 12 ± 2.03 1369 ± 533

IT98K-205-8 50.00 ± 0.00 46.66 ± 6.66 6 ± 0.88 5 ± 0.67 8 ± 2.20 1083 ± 48

IT97-499-35 56.66 ± 3.33 46.66 ± 6.66 10 ± 3.06 5 ± 0.58 10 ± 0.58 1391 ± 104

Tvu-14676 55.55 ± 4.44 51.11 ± 8.88 7 ± 0.33 7 ± 2.51 15 ± 3.71 2790 ± 591

Mean (33 geno) 50 40 9 7 11 1928

Shallow Root System (ARGA<45, BRGA<45,

ARN>8) potential for adaptation to low

phosphorus environment

Steep Root System (ARGA>45, BRGA>45,

BD5-10cm>8) potential for adaptation to

drought-prone environment

Genotype TPRL BRN

TVu-6443 4.0 ± 0.8 0.0 ± 0.0

IT81D-985 7.7 ± 0.9 7.3 ± 0.5

TVu-9797 13.0 ± 1.6 6.3 ± 1.2

IT89KD-288 13.0 ± 1.4 6.7 ± 1.7

TVu-15055 14.0 ± 1.6 7.0 ± 0.8

TVu-1438 13.7 ± 1.7 5.3 ± 0.5

….

….

….

IT98K-205-8 22.3 ± 1.2 12.0 ± 0.8

IT98K-506-1 22.3 ± 0.5 12.3 ± 0.5

IT96D-610 22.0 ± 0.8 12.7 ± 1.2

IT07K-318-33 21.0 ± 2.2 10.3 ± 1.2

IT99K-494-6 20.3 ± 0.5 11.7 ± 1.2

IT98K-1111-1 19.0 ± 1.4 15.0 ± 0.8

Mean (33 geno) 16.7 9.4

F value 34.22 23.44

Pr>F <.0001 <.0001

CV 9.32 13.31

LO

WH

IGH

Table 1: Classification of cowpea genotypes with contrasting adult plant morphology (a: shallow and steep root system) and seedling stage root traits (b: total primary root length (TPRL) and basal roots number (BRN)) and their hypothesized contribution to adaptation to drought and low P. Plant genotypes with TPRL and high BRN are potential for adaptation to low P and early vegetative stage drought.

(a) (b)

Water spender under high VPD

Water saver under high VPD

Fig. 1: Difference in leaf water losses in response atmospheric vapor pressure deficit (VPD) (a) and root system architecture (b) for water and P acquisition.

H2O P

5%

18%

11%

23%

10 cm

20cm

30cm

4ppm

2ppm

0.5ppm

0.25ppm

40cm

(a) (b)

Fig.2: Genotypic variation in plant TR (a) and canopy temperature depression (b) and grain yield (c) among the cowpea germplasm.

(b)

(a) (c)

Plant traits might be dynamic and interact with one another and with their environment. Agronomic crop management could influences their expression. Therefore, crop-climate-management modeling becomes an essential tool for navigating the biological complexity and testing the effects and probability of success of specific plant trait or traits combination.

Acknowledgment and Collaborators