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Water relations in two cultivars of Napier grass under variable water supply and temperature conditions

Solomon Mwendia1, Isa Yunusa1, Brian Sindel1, Wal Whalley1 and Innocent Kariuki2

1 University of New England, Armidale, NSW 2351, Australia. Email smwendia@une.edu.au
2
Kenya Agricultural Research Institute, Muguga South, Kenya

Abstract

Napier grass (Pennisetum purpureum Schumach.) is the main fodder for the dairy industry in eastern and southern Africa as in many other tropical/subtropical regions of the world. Plant water relations were assessed for two cultivars of Napier grass; Local and Bana, the latter being an infertile hybrid between P. purpureum and P. glaucum. Plants were grown in a controlled environment at diurnal maximum temperatures of either 25 or 35C to assess their tissue water relations in response to variable water supply (25, 50 or 100% field capacity) to mimic current and possible future climatic conditions in east Africa. At 25% watering and 25C Local had higher midday relative water content (RWC) than Bana, but not at 50 or 100% field capacity, at which RWC was higher for Bana than for the Local. At the cooler 25C, Bana attained a leaf water potential (LWP) minimum of -2.88 MPa at 25% watering, while Local had -2.27 MPa. At 100% watering and 35C, Bana had higher RWC and LWP than Local at 25%. In a hotter environment, Bana appears to maintain higher water status than Local at 25% when soil is at field capacity while other watering responses appear similar for both cultivars.

Key words

Dairy, East Africa, fodder, heat stress, ratooning, water stress

Introduction

Napier (elephant) grass (Pennisetum purpureum Schumach.) is a perennial C4 grass that is important for dairy fodder crops in Latin and South America (Martha et al. 2004) and East Africa (Orodho 2006). Its high yields, persistence and regeneration capability (Nyambati et al. 2011) make it desirable for fodder. Napier grass is moderately drought tolerant (Agromisa 2005), but its regeneration and growth can be highly constrained by prolonged and/or transient water stress (Yanxian et al. 2008). High temperatures exacerbate water stress that will further reduce growth and therefore biomass production (Barnes et al. 2007). High temperature and water shortage are two factors constraining plant productivity in many regions of the world (Shah and Paulsen 2003), and this situation may get worse with the predicted increases in global temperatures of between 1.7 and 4.9C by the year 2100 (Wigley and Raper 2001). Drought tolerance in pasture grasses is defined as the ability to use the least amount of water to continue transpiring for the longest time (Zhou et al. 2009) while maintaining leaf hydration to be able to recover on rewatering (Norris and Thomas 1982). There is limited physiological understanding of these attributes in Napier grass. Analysis of water relations could help in the identification of Napier grass traits that could be used for selecting cultivars that are drought tolerant. The objective of this study was to identify whether water relations traits differ in two related cultivars of Napier grass found in Australia.

Materials and methods

Study design

The experiment was a complete randomised design with two factors; Napier grass at two levels and water supply at three levels in two temperature environments and replicated four times. The experimental unit was a plastic bin 39 cm in diameter and 46.5 cm deep. The study was conducted at the University of New England, New South Wales, Australia.

Planting and watering

Each of the 48 bins was filled with 50 kg of sandy Alfisol (Klingebiel and Montgomery 1961) soil collected from Trevenna University Farm. Diammonium phosphate fertiliser (18:20:0 of N,P,K) was applied at a rate of 13 g/bin, equivalent to 26 kg P/ha. One Napier grass cane (with two nodes) was planted per bin with one node below the soil on 12 May 2011. One kilogram of alcathene beans per bin was spread on the soil surface to control water evaporation. Bins were labeled and their placement randomised. The canes were initially raised over 28 days. Watering and temperature treatments were initiated on 10 June, 2011. Subsequent watering was undertaken when the tillers wilted. After first harvest, all the bins were watered to field capacity. Subsequent watering (ratoon 1 and 2) reverted to the water treatments.

Treatments

Bana and a Local cultivar of Napier grass were collected at Atherton in Queensland, Australia. Bana in Australia is an infertile interspecific hybrid between P. purpureum Schumach. x P. glaucum (L.) R. Br. used as windbreaks between crops in tropical and sub-tropical areas in Australia. The Local cultivar (P. purpureum) was introduced into Atherton some years ago but its origin is uncertain. The experiment was carried out in two glasshouses. Temperature in the two glasshouses was automatically set at 25C and 35C respectively during the day (10 am-6 pm) and dropped by 10C nightly over 3 hours (7-10 pm) to minimums of 15C and 25C respectively. Temperatures rose gradually in the morning (7-10 am) to the pre-set maximums. Water supply was set at 25, 50 or 100% of field capacity. To determine the amount of water required to achieve these levels, eight “recharge” bins were set up (four in each glasshouse). A recharge bin had 50 kg soil with four holes at the base and placed on a pallet to allow excess water to drain through overnight into a collecting basin. The basin had a polythene sheet fastened over the top with an inlet at the centre to allow water to enter while curtailing evaporation. There were two replicates of recharge bins for each Napier grass cultivar at each temperature. The difference between amount of water applied and the filtrate constituted the amount of water to recharge the soil to field capacity; this amount was used to rewater the 100% treatments and the other water supply treatments were scaled accordingly.

Data collection

Leaf water potential (LWP) was measured by a pressure chamber (Scholander et al. 1965) while relative water content (RWC) was measured according to the procedure given by Barrs (1968). For LWP, samples were excised from the youngest fully developed leaf and immediately pressurized in the chamber. A pressure reading was recorded once water appeared on the excised edge. This was done around midday at weekly intervals, and at predawn (5 am) and around noon (12 pm) on the day of watering which occurred at 6 pm. Following watering, further LWP measurements were made during the following day at 5 am, 7 am, 9 am, midday, 4 pm and 7 pm. For RWC, Napier grass leaf segments were excised as for LWP at the same time and preserved in a cooler box. These were weighed fresh (F) and then floated on deionised water for 4 h at room temperature before being mopped with a filter-paper to obtain the ‘fully turgid’ weight (T). They were then dried for 24 h in an oven at 80C to give the dry weight (D). The RWC was calculated as: ([F–D]/[T-D]) x 100. Data were collected for two consecutive ratoonings.

Data analyses

Data were tested for normality by probability plots in Minitab statistical software before analysis of variance. Means were separated by least significant difference and standard errors calculated in Excel.

Results

Leaf water potential and relative leaf water content

When all measurements taken at 5 am and 12 pm before watering were pooled together at 35C, there was an interaction of cultivar by watering level for both RWC and LWP. The Local cultivar at 25% watering had the lowest RWC means (P<0.05) at both predawn and midday and was significantly less than Bana at 50 and 100% watering (Table 1). LWP was lowest for Local at 25% watering and Bana at 25% watering at 5 am at noon respectively.

Table 1. Effect of cultivar-watering interaction on RWC and LWP at 35C.

 Cultivar and

RWC (%)

LWP (MPa)

Water treatment

5 am

12 pm

5 am

12 pm

Bana 25%

75.4

61.4

-2.17

-2.85

Bana 50%

76.9

63.5

-1.97

-2.27

Bana 100%

81.9

65.2

-1.28

-1.82

Local 25%

64.0

55.1

-2.77

-2.44

Local 50%

72.8

59.3

-2.31

-2.68

Local 100%

75.5

58.3

-1.67

-2.21

LSD (P=0.05)

11.5

8.0

0.79

0.52

At 25C there was a significant interaction (P<0.05) between cultivar and watering level for both RWC and LWP. Local at 50% watering had the lowest pre-dawn RWC and was similar to all treatments except Bana at 50% and 100% watering. At noon, Local at 50% watering again had the lowest RWC. Bana 100% watering had the highest RWC and was not significantly different to Bana at 50% and Local at 25% watering. LWP of Bana 25% watering was lowest at both readings, while Bana 100% watering had the highest LWP. There was no effect of watering treatment on LWP on Local (Table 2). The cultivar effect was further demonstrated by diurnal RWC and LWP at 35C (Figure 1).

Table 2. Effect of cultivar-watering interaction on RWC and LWP at 25C.fc

Cultivar and

RWC (%)

LWP (MPa)

Water treatment

5 am

12 pm

5 am

12 pm

Bana 25%

73.5

63.4

-2.50

-2.88

Bana 50%

86.6

77.8

-1.25

-2.16

Bana 100%

90.6

81.1

-0.74

-1.33

Local 25%

84.5

74.8

-1.73

-2.27

Local 50%

73.6

63.2

-1.95

-2.66

Local 100%

79.9

67.7

-1.39

-2.03

LSD (P=0.05)

12.4

10.9

0.91

0.87

Diurnal RWC differed (P< 0.05) between Bana and Local at 35C before watering. LWP differed at 5 am before watering and at midday on the day after watering (P<0.05). The Local cultivar was more water stressed before watering but had better hydration by midday the day after watering. This was the trend for both RWC and LWP (Figure 1).

Figure 1. Diurnal changes in (a) relative water content (RWC) and (b) leaf water potential (LWP) for Napier grass cultivars Bana and Local in the glasshouse at 35C.

Discussion

The Napier grass cultivars responded to some extent differently to water availability and temperature. These responses can be attributed to differences in genetic makeup of the cultivars as they were under the same controlled environmental conditions. Colom and Vazzana (2001) reported differences in LWP and RWC for three cultivars of the C4 grass Eragrostis curvula under water stress conditions. They reported that the cultivars Tanganyika and Ermelo had lower water potentials (-3.4 and -3.0 MPa respectively) than Consol (-2.6 MPa). The LWP in this study fell as low as -2.8 MPa (Table 2). This was almost twice as low as the nominal permanent wilting point (-1.5 MPa) for agricultural crops (Lambers et al. 2008). However, the low value we observed was consistent with those obtained with other crops that have high tolerance of drought, such as sorghum in which a LWP of -2.6 MPa was reported (Turner et al. 1978).

Both cultivars were more stressed at 35C (Table 1 versus 2) as a result of lower vapour pressure deficit at 25C throughout the trial period (data not shown). It is known that LWP, as an indicator of water stress, reflects a combination of many factors, including vapour pressure deficit, stomatal regulation, internal plant hydraulic conductivity and soil-water availability (Sousa et al. 2006). The cultivars could also have different capacities for osmotic and elastic adjustments, which were not determined in this study. Wilson and Ludlow (1983) observed a range of osmotic adjustments (0.16-0.31 MPa/day) amongst three tropical grasses due to water stress.

Conclusion

Napier grass cultivars are likely to have different inherent adaptation to water and heat stress as shown in this study, suggesting that a selection program for such traits may prove beneficial. At higher temperature, Bana appears to maintain higher hydration when soil moisture is not limiting than Local especially when Local is under limited soil moisture.

Acknowledgements

The lead author is funded by an Australian Government Development Scholarship (Ausaid). The authors are grateful to the technical staff at the School of Environmental and Rural Science, especially Mr Michael Faint, for logistical assistance.

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