Abstract

Plants of 16 genotypes of four Brassica species (B. rapa var. brown sarson, B. rapa var. yellow sarson, B. juncea L. Czern & Coss and B. carinata A. Br. ) were evaluated under normal, mild and severe moisture stress conditions for concurrent changes in leaf water potential (ψ_L-MPa), leaf osmotic potential (ψ_π -MPa) and turgor potential (ψ_p MPa ) at reproductive stage of development. Genetic variability for plant water relations was higher under moisture stress than mild and normal moisture environments. Among different components of water potential, the higher genetic variations were recorded for turgor potential (ψ_PMPa) followed by leaf water potential and osmotic potential. Seed yield was, however, affected due to differences in the partitioning of dry matter in different genotypes. The stress tolerance index revealed that the genotypes DHR 9401, DHR 9504, DHR 9204 and Rajat recorded higher stress tolerance and yield potential.

Introduction

Oilseed Brassicae are grown in India on light textured soil with conserved moisture from monsoon rains on a large area. Under such situations the crop at the time of reproductive development suffers due to moisture stress because of high atmospheric demand. The ability of plants to maintain leaf turgor by osmotic adjustment as leaf water potential declines is an important adaptation to moisture stress (Morgan; 1984; Singh and Singh, 1988). Hence, for higher productivity better plant water status helps in maintaining various growth and physiological processes. The study of genetic variations in plant water relations would help to identify drought tolerant genotypes. The identified genotypes can be used in hybridization programme to develop superior genotypes for drought tolerance and high seed yield. Therefore, an attempt has been made here to generate information on plant water relations of important cultivated species of oilseed Brassicae.

Material and methods

The present study was carried out during the winter season of 1997-98 at research farm of Chaudhary Charan Singh Haryana Agricultural, University, Hisar, India (29 ⁰ 10’ N, 75 ⁰ 45’E). Sixteen elite genotypes namely BSH 1 (Brassica rapa L. var. brown sarson), Jhumka (Brassica rapa L. var. yellow sarson), RH 30, BIO 902, Rajat, Varuna, Rohini, DHR 9401, DHR 9504, DHR 9204, RH 819, PCR 10 ( Brassica juncea L. Czern & Coss.), DHC 4, DHC 7, DHC 9601 and PCC 2 (Brassica carinata A. Br.) were taken for the present study. These genotypes were grown under three environments viz., Normal Irrigation (Normal ); Withholding of irrigation at 90 DAS ( Mild Stress); Withholding of irrigation at 60 DAS (Severe Stress), with three replications in randomized block design in concrete drought plots (6 x 1 x 1.5 m) filled with dunal sand. The crop was grown in a plot size of paired rows of 1 meter length keeping row to row distance of 30cm and plant were spaced 10 cm apart after timely thinning. The recommended cultural practices were adopted to raise a good crop.

The leaf water potential (ψ_L), osmotic potential (ψπ) and turgor potential (ψ_p) were measured concurrently at 12 noon on the fully expanded leaf at 100DAS. Leaf water potential was estimated by the pressure chamber method as described by Scholander et al. (1965). Osmotic potential was measured by 5100-B vapour pressure osmometer, thus, obtained in milli-osmoles per kg were converted to molarlity and finally to MPa with the help of calibration curves. The ψ_p was computed as the difference of ψπ and ψ_L. The Relative water content (RWC) was calculated as described by Weatherly (1965). The fully developed leaf of each cultivar from all the three environments were taken. The leaf water retention (LWR) was calculated as LWR (%) = 100 - Leaf water loss (LWL) during 24 hrs under shade.

Results and discussion

Plant water relations

Among the various plant water relation parameters studied, the maximum variation was observed in ψ_p followed by leaf water retention (Table 1). The variation in plant water parameters among different genotypes increased with increased moisture stress. The highest variation in ψ_L among different oilseed Brassica species was observed in B. carinata and it was minimum in B. rapa genotypes. But the variation in RWC was highest in the B. juncea genotypes followed by B. carinata and it was minimum in B. rapa. The increased moisture stress decreased the ψ_L , ψπ, ψ_P and RWC but increased the LWR_. The maximum reduction in plant water status due to moisture stress was recorded in ψ_P (133.3 %) followed by ψ_L (32.8 %) and it was minimum in RWC (13.2 %) followed by ψπ_.

Among the three oilseed Brassica species maximum reduction in ψ_P due to moisture stress was recorded in B. carinata followed by B. juncea and it was minimum in B. rapa genotypes. The ψ_L was recorded highest in PCC 2 and DHC 7 (B. carinata) and Jhumka (B. rapa) and lowest in RH 30, Varuna and PCR 10 (B. juncea) under normal irrigated conditions. However, under severe stress environment the highest ψ_L was recorded in DHC 4 (B. carinata) followed by Rohini (B. juncea). Severe moisture stress reduced the osmotic potential, probably due to more build up of solute at siliquae development stage which resulted, in lower osmotic potential and simultaneously more negative leaf water potential. Similar decrease in solute potential and leaf water potential have earlier been reported in response to soil moisture stress in oilseed Brassicae by Kumar and Singh, 1998 . The highest RWC under severe stress was observed in DHR 9401 and DHR 9504 (B. juncea). The LWR shows the sensitivity of stomata and efficacy of epicuticular layer of leaves which was recorded highest in DHC 9601 followed by DHC 7 and PCC 2 (B. carinata). It might be because of the fact, that thicker leaves of this species with higher amount of wax on leaf surface help in retaining the water loss. The lowest LWR was recorded in the genotypes of B. juncea (Rajat and Varuna). Similar variation in leaf water retention among various Brassica species were also reported by Kumar and Singh, 1998.

The highest seed yield among various oilseed Brassica species was recorded in B. juncea genotypes DHR 9401 and DHR 9504 and it was lowest in B. rapa (BSH 1). Among the three species B. juncea is most potential species from yield and economic point of view under all three environments. Although there were yield reduction in all the three oilseed Brassica species with the imposition of moisture stress but the relative reduction 11.0 % under severe stress and 8. 5 % under mild stress were observed alongwith highest yield in B. juncea. It may be because of the fact that the stress intensity was of low order. Among the B. juncea genotypes DHR 9401, DHR 9204 and Rajat yielded highest under normal irrigated environment and Rajat, DHR 9204 and DHR 9504 under severe moisture stress.

The higher yield recorded in B. juncea genotypes over other two species may be attributed to their higher RWC not only on the basis of mean but in highest yielding genotypes also i.e., DHR 9401 and DHR 9504 maintained higher turgor which may have contributed to maintenance of higher stomatal conductance and photosynthetic activity, even at low water potential because of better osmotic adjustment even under severe stress environments. Osmotic adjustment has been shown to be positively associated to seed yield in oilseed Brassica species (Singh et al. 1990). The highest LWR in RH 30 and RH 819 widely adapted varieties indicated its efficient stomatal sensitivity alongwith epicuticular layer which resulted in reasonable seed yield even under severe stress. Higher water retention reduces dehydration which will help plant leaves to survive stress and contribute to the growth of the plants (Kumar and Elstom, 1993).

Breeding oilseed Brassica for high seed yield and drought tolerance involves the identification and transfer of physiological traits responsible for drought tolerance high yielding and agronomically acceptable cultivars as drought resistance and seed yield are controlled at separate genetic loci (Blum, 1983 and Morgan, 1984). The higher seed yields in Rajat, DHR 9204, DHR 9504 and DHR 9401 (B. juncea) was because of the maintenance of their higher RWC alongwith higher LWR. The measurement of LWR is quite simple, fast and require only a simple balance therefore, it can be used successfully to screen drought tolerant genotypes (Pannu, et al. 1993). The Work of Dedio, 1975 has shown that LWR behaves as a simple inherited character. Among the different indices of plant moisture stress. LWR showed higher correlation with seed yield (Kumar and Singh, 1998).

Conclusions

Hence, on the basis of these results it may be concluded that their is not only significant variation in plant water relations. The severity of moisture stress further increased these variations in the genotypes of all the three oilseed Brassica species. Therefore, to tap the desirable variation in plant water relations, the screening should be fallowed under stressed environment. The LWR seems to be a promising simple plant water relation parameter to screen genotypes for dought tolerance. The variations available for plant water relation indices and high seed yield in the genotypes viz.; Rajat, RH 30, DHR 9204, DHR 9504 and RH 819 (B. juncea) can be exploited for future breeding genotypes tolerant to moisture stress.

References

1. Blum, A. 1983. Evidence for genetic variability in drought resistance and its implication for plant Breeding. In : Drought resistance in crops, with emphasis on rice. Los Banos, Philippines; IRRI, 53-68.

2. Dedio, W. 1975. Water relations in wheat leaves as screening tests for drought resistance. Can. J. Plant Sci.. 55 : 369-378.

3. Kumar, A. and Elston, J. 1993. Leaf expansion of Brassica species in response to water stress. Indian J. Plant Physio. 36 : 220-222.

4. Kumar, A. and Singh, D.P. 1998. Use of physiological indices as a screening technique for drought tolerance in oilseed Brassica species. Ann. Bot. 81: 413-420.

5. Morgan, J. M. 1984. Osmoregulation and water stress in higher plants. Ann. Rev. Plant Physiol. 35 : 299-319.

6. Pannu, R.K., Singh, D.P, Singh, P., Chaudhary, B.D., and Singh, V.P. 1993. Evaluation of various plant water indices for screening the genotypes of chickpea under limited water environment. Haryana J. Agron. 9 : 16-21.

7. Scholander, P.F., Hammel, H.T., Bradstreet, E.D., and Hemmingsen, E.A. 1965. Sap pressure in vascular plants. Sci. 148 : 339-346.

8. Singh, D.P., Kumar, A., Singh, P., Chaudhary, B.D. 1990. Soil water use, seed yield, plant water relation and their inheritance in oilseed Brassicas under progressive soil moisture stress. Proc. Int. Cong. Plant Physio. New Delhi, India, Feb. 15-20, 1987. 841-848.

9. Singh, R.P. and Singh, D. P. 1988. Plant water relations as a selection criteria for drought tolerance in mustard. Biologia. Plantarum. 30 : 231-235.

10. Weatherley, P.E. 1965. In : The state and movement of water in living organisum, (G.E. Fogg ed. ) Camb. Uni. Press, London and New York, 157-184..

Table 1. Leaf water potential (ψ_L-MPa), osmotic potential (ψπ -MPa), turger potential (ψ_p MPa), relative water content (RWC),
water retention (LWR) and seed yield in different genotypes of oilseed Brassicae under different environments.

S = Severe moisture stress ; M = Mild moisture stress; N = Normal irrigation