Protea night-life: Ecological and commercial significance for hydraulics 1. Heidi-J Hawkins A,C, Hans Hettasch B and Michael D Cramer A

ADepartment of Botany, University of Cape Town, Private Bag X1, Rondebosch, 7701, South Africa; BArnelia Farms, P.O. Box 192, Hopefield, 7355, South Africa

CCorresponding author Tel: +27 21 650 2442 Fax: +27 21 650 4041 Email: [email protected]

1 Submitted Feb 2009 in detailed format to Functional Biology as ‘Hydraulic redistribution by Protea ‘Sylvia’ (Proteaceae) facilitates soil water recharge and water acquisition by an understorey grass and shrub’. Please use the latter article to cite information .

1 Introduction During the day , if plant stomata are open, water moves along the (ψw) gradient from soil to leaf to atmosphere via the , i.e . transpiration occurs (Fig. 1a) . At night , or whenever stomata close , that gradient is removed ... However,However,However, asas longlong asas plantplant rootsroots spanspan wetwet (usually(usually deeper)deeper) and drier (usually shallow) soil areas, another water gradient exists , i.e. between wet ter and drier soil layers. In this case, w ater moves along the ψw gradient from wet soil toto thethe root,root, along the root xylem andand outout intointo drier soil , resulting in soil wetting (Fig. 1b). This passive process is called hydraulic redistribution and has been documented for more than 50 taxa includingincluding trees, shrubs and grasses , particularly in summer -drought Mediterranean areas, but also in deserts , tropical forests (Jackson et al. 2000) and crop ( Sekiya and Yano 2004 ).

Hydraulic redistribution is thought to be important forfor soil soil water water recharge and facilitating nutrient nutrient uptake in water -limite d and summer -drought , e.g. Great Basin of the western United States ; Mediterranean -type ecosystems such as the Australian kwongan, the Californian chaparral and the Cape Florist icic Region Region (CFR) of South Africa . However,However, the ecological and commercial importance of hydraulic redistribution has not been well -demonstrate d in the field.

South African Proteaceae are often the largest shrubs in the CFR (2 up to 10 m, Rebelo 2001) and havehave been shown to continue to transpire throughout thethe summersummer drought drought in the west of of the the CFR CFR while while other shallower -rooted plants cease to tr anspire (van der Heyden and Lewis, 1988). Not only do ProteaceaeProteaceae continue to transpire throughout summer,summer, but Proteaceae grow predominantly duringduring thethe hot, hot, dry summer months (Rebelo 2001). 2001). Roots are dimorphic with deep ‘sinker ’ roots and shallow lateral ro ots (Mortimer et al. 2003) si milar to Australian Proteaceae (Pate et al. 1995). Rooting depths for shrub -forms of SA Proteaceae are known to reach at least 3.6 m (Higgins et al. 1987) but may extend deeper depending on the water table . This implies that the deeper root ed Proteaceae have access to deep water reserves which sustain photosynthesis, transpiration and growth through the summer.

Fig. 1. Soil water moves along water potential OurOur preliminary data using probes gradients. In the day the gradient is usually from soil showed that soil moisture in summer near lateral toto plantplant toto atmosphere atmosphere (a), during the the nightnight or or whe whenn roots (0.2 -0.4 m d epth) of Protea ‘Sylvia’ was stomatastomata are are closed and the gradient to atmosphere i iss indeed higher than slightly deep er in the soil profile removed, wat er can move from wet to drier soil in a (e.g. 0.5 m, Fig. 2 ) while being similar to deeper process called hydraulic redistribution (b). Pictures soil layers ( 1.0 m, Fig. 2) suggesting that hydraulic modified with permission courtesy of FC Meinzer, redistributionredistribution toto dry upper soil layers was occurrioccurring.ng. USDA Forest Servic e, Forest Science Lab, Corvalis, Therefore, we hypothesize d that Proteaceae are able OR, 2003. to hydraulic ally redistribute water from deeper 2 moist soil layers to upper drier soil layers where nutrients occur. Redistributed water may aid in improving the water and nutrient status of the proteas, but it may also positively influence surrounding plants (Dawson 1993) and thus may be an important component maintaining diversity in these systems. In addition, if commercially grown Proteaceae are encouraged to have deep root systems via ‘deep’ irrigation versus frequent shallow irrigation events, these crops may be made more tolerant to drought stress.

Fig. 2 Soil moisture profiles adjacent to Protea ‘Sylvia’ plants grown on aeolian sands at Arnelia Farms, Hopefield, South Africa, before supply of heavy water label. Values are means ± standard error (n=4).

Brief materials and methods Our field experiment was accomplished by sinking PVC plumbing pipes 1.2 m into the soil profile next to farmed five-year old Protea ‘Sylvia’ plants. Plants were irrigated via these pipes to simulate deep water supply and to encourage root growth near the pipe. After ca. 2 months, plants were either supplied with 25 L labelled water overnight or not. Labelled water, or so-called heavy water was characterized by the ratio of heavy 3 hydrogens (2H) to light hydrogens (1H) as δ2H (measured in ‰). Any increase in δ2H of the upper soil overnight was interpreted as evidence of hydraulic redistribution or hydraulic lift. Likewise, an increase in δ2H of shallow-rooted plants ( Leysera gnapholodes , Asteraceae) under the protea was interpreted as evidence of water parasitism by the shrub, since the shrub had no access to water at depth. Lithium, an analogue of potassium that is not metabolised by plants and thus may be traced, was placed around plants receiving labelled water to determine whether a short-term supply of water at depth, and subsequent hydraulic redistribution, could contribute to nutrient uptake. A pot experiment with Sylvia and the shallow-rooted Fynbos grass, Cyanodon dactylon (Poaceae), was used to support field findings. In addition, an irrigation experiment tested the practical implications of hydraulic redistribution on sandy . In this experiment, Sylvia received 90 L water week -1, supplied via surface drippers (shallow irrigation), half surface drippers and half via a 1.2 m deep pipe (half shallow/half deep irrigation) or via the 1.2 m pipe only (deep irrigation). Deep irrigation was meant to simulate less frequent, high volume irrigation. Plants receiving deep irrigation were acclimatized to this irrigation over a few months to allow for root growth near the pipe.

Results and Discussion It was found that the large Sylvia plants growing in aeolian sands in the so-called Strandveld Fynbos (Arnelia Farms, Hopefield, on the west coast of the CFR, 33 °2’49’’S, 18 °20’30’’E) took up the labelled water at a depth of 1.2 m as was evident from the label found in Sylvia stems (Fig. 3a). This confirmed that Sylvias had root access to the water supplied at depth, as was hoped. More importantly, Sylvia redistributed labelled water from 1.2 m depth to the 0.2 to 0.4 m soil layer, as indicated by increased δ2H of soil water from -24.5 ± 0.7‰ to - 8.0 ± 3.0‰ (Fig. 3b) and a 52% increase in soil water from 0.48% to 0.89% moisture content. Linear mixing models attributed 0.68 ± 0.05% of the water in the upper soil layer to the label, or 8.5 ± 0.9% when using Sylvia plants as the label source. This is a rather small redistribution of water and may be linked to the short-term nature (overnight) of the experiment. The small amount of hydraulic redistribution was apparently insufficient to result in there being any difference in Li uptake between plants receiving 25 L heavy water label at depth and those receiving no additional water overnight. This suggests that hydraulic redistribution does not make a measurable contribution to ion uptake in the short-term. However, both treatments accumulated considerable amounts of Li over the three week period subsequent to watering, meaning that both treatments had access to sufficient soil moisture to allow the Li to diffuse to the roots. It is possible that plants in both treatments made use of hydraulically-lifted water from >2m depth (from trenches we know that Sylvia roots extend at least 2 m). Therefore, the amounts of additional water supplied at 1.2 m depth may have made no difference to the amount of Li in soil solution or to that taken up by the plant in this sandy soil. Snyder et al. (2008) found that hydraulically redistributed water did not increase the amount of 15 N acquired by the desert shrub Sarcobatus vermiculatus in dune sand. Both the present and the latter study were performed on sandy soils where hydraulic redistribution may be less important due to relatively less root-soil contact compared to clayey soils. Also matric forces (capillary and electrostatic) between water and sands are small due to the relatively large pore sizes of the sand. Relatively larger gravimetric forces may mean that hydraulically-lifted water quickly drains out of the upper sand layers.

Water in stems of the shallow-rooted understorey shrub, L. gnaphalodes had similar δ2H values (338 ± 157‰) to stems of Sylvia, indicating water parasitism by L. gnaphalodes . Supporting this field data was pot 3 experiments where Sylvias redistributed ca . 17% of another type of water label (radioactive H2O) supplied, equating to 34 ± 1.2 mL plant -1. Also, the shallow-rooted local grass, Cyanodon dactylon (Poaceae), growing in the same pots as Sylvia were labelled, again indicting water ‘parasitism’. In the field, the two understory plants are likely dependent on periodic rainfall events during summer and/or reliant on water redistributed by large shrubs. Whatever the competitive interactions between protea and understorey plants during recruitment, once established, the persistence of grass and small shrubs such as the L. gnaphalodes under the canopies of Proteaceae shrubs in Fynbos are likely facilitated by water lifted by proteas during summer drought. This does

4 Fig. 3. δ2H of plants (a) and soil (b) at 20 to 40 cm soil depth the morning after supplying labelled water at depth to Protea ‘Sylvia’ roots (labelled) or not (unlabelled). Values are means ± standard error (n=8). Letters indicate difference at the P<0.05 level after a one-way ANOVA and Newman Keuls multiple range tes t.

5 not imply that hydraulic redistribution always facilitates water use by shallow rooted plants, as some plants clearly redistribute water downwards after rains (e.g. Burgess et al., 1998) resulting in the removal of water resources away from shallower rooted understory plants.

In the irrigation experiment, it was found that plants grew best on shallow irrigation compared to half shallow/half deep or just deep irrigation (Fig. 4). This confirmed to an extent what was found above, i.e. that hydraulic redistribution occurs and allows plants to acquire sufficient nutrients to grow but that, on a sandy soil, this does not equate to large volumes of water. It is possible that deep irrigation would make a plant more drought tolerant but this was not tested. It should be stressed that deep irrigation may well have more benefits on clay-containing soil. Regardless of soil type, the implication of hydraulic redistribution by proteas for commercial protea growers is that, after plant establishment, irrigation scheduled to encourage deep-rooted growth would enable plants access to moist soil, or even the water table, making these plants less dependent on irrigation events during the summer drought. After roots have reached this moist or wet deep layer, the grower would have the choice of irrigating to maximize production, or if water was limiting, to reduce irrigation short- term without risking plant death.

Fig. 4. Stem length of Protea ‘Sylvia’ plants grown for 130 d with deep, shallow or half deep and half shallow irrigation. Values are means ± standard errors (n=15).

Conclusions Protea ‘Sylvia’ redistributed water from deep tap roots to surface lateral roots in both a pot and field study and this redistribution both recharged the soil water and was available to understorey plants. This is the first report of hydraulic redistribution and water parasitism in the Cape Floristic Region (CFR). We conclude that hydraulic redistribution by Proteaceae (and likely other deep-rooted plants) plays an important role in soil water recharge, water supply to shallow-rooted plants, and thus soil hydraulics and plants community structure during the summer drought of the Cape Floristic Region. Hydraulic redistribution is also an important adaptive feature that growers can consider when scheduling irrigation.

6 Acknowledgements The authors thank Molteno Brothers Pty. (Ltd.), Elgin Glen, South Africa for providing Sylvia cuttings, as well as the National Research Foundation of South Africa and the South African Protea Producers and Exporters Association for financial support during this project. The authors also thank Carin Basson for technical assistance.

References Burgess SSO, Adams MA, Turner NC, Ong CK (1998) The redistribution of soil water by tree root systems. Oecologia 115 , 306-311. Dawson T.E. (1993). Hydraulic lift and water use by plants: implications for water balance, performance and plant-plant interactions. Oecologia 95 , 565-574. Higgins KB, Lamb AJ and Van Wilgen BW (1987). Root systems of selected plant species in mesic mountain fynbos in the Jonkershoek Valley, south-western Cape Province, South Africa. South African Journal of Botany 53 , 249-258. Jackson RB, Sperry JS, Dawson TE (2000) Root water uptake and transport: using physiological processes in global predictions. Trends in Plant Science 5, 482-488. Mortimer P, Swart JC, Valentine AG, Jacobs G, Cramer MD (2003) Does irrigation influence the growth, yield and water use efficiency of the Sylvia hybrid ‘Sylvia’ ( Protea susannae x Protea eximia )? South African Journal of Botany 69 , 135-143. Pate JS, Jeschke WD, Aylward MJ (1995) Hydraulic architecture and xylem structure of the dimorphic root systems of South-West Australian species of Proteaceae. Journal of Experimental Botany 46 , 907-915. Rebelo AG (2001) Proteas. A field guide to the Proteas of Southern Africa. Fernwood Press, South Africa. ISBN 1 874950 40 7. Sekiya N, Yano K (2004) Do pigeon pea and sesbania supply to intercropped maize through hydraulic lift? – Hydrogen stable isotope investigation of xylem waters. Field Crops Research 86 , 167-173. Snyder KA, James JJ, Richards JH, Donovan LA (2008) Does hydraulic lift or nighttime transpiration facilitate nitrogen acquisition? Plant and Soil 306 , 159-166. van der Heyden F, Lewis OAM (1988) Seasonal variation in the photosynthetic capacity with respect to plant water status of five species of the Mediterranean climate region of South Africa. South African Journal of Botany 55 , 509-515.

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