Water Balance of Cut and Intact «Sonia» Rose Plants

Transcription

1 Centre for Agrobiological Research, Wageningen, The Netherlands Water Balance of Cut and Intact «Sonia» Rose Plants H. C. M. DE STIGTER With 4 figures Received January 18, Accepted February 20, 1980 Summary For validly interpreting certain aspects of cut-flower physiology it is a prerequisite to compare the behaviour of cut flowers with that of intact ones, side by side, under the same environmental conditions. Fresh weight, water loss and water uptake of hydroponically grown, single-stem roses with or without a root system were followed throughout one blooming period, by twice-daily weighings. In the intact plants shoot plus flower weight could be estimated reliably with a water-displacement technique eliminating root weight. Cut roses standing in water very soon started to develop a characteristic pattern of weight fluctuations, in accordance with the light-dark alternation, losing ca weight in the light and regaining it in darkness. The pattern persisted after petal shedding, but on a roughly 50 % reduced scale; this means that both petals and foliage contributed to the fluctuations observed. In the intact roses actual weight losses only occurred shortly before petal shedding, in the light, and did not persist thereafter; thus here it was only the senescing petals that lost some of their water-retaining capacity. Total daily water loss and uptake of the cut roses were identical to those of the intact ones in the first 24 h, but rapidly declined subsequently, levelling off to some In the same lapse of time overall water loss and uptake of the intact roses slowly declined as well, by some 30 0/0. In the dark periods water uptake of the cut roses did not decline, as it did in the light, but remained constant and equal to that of the intact roses for most of the time. This is taken to mean that the phenomenon of rapidly declining overall water uptake of cut roses is not caused by a massive, solid plugging of the xylem vessels, as almost generally inferred in the literature. An alternative hypothesis is put forward. Key words: Rose, cut flower, water balance, vascular occlusion. Introduction The cutting operation is an event of paramount importance for a cut flower's further life. However, scant attention appears to have been given to a direct, experimental study of the cutting effect per se, by way of side-by-side comparisons of cut and intact flowers, under the same environmental conditions. Complications of technical experimental nature may well have been one reason for this apparent neglect. Either one would have to study the cut flower's behaviour in the glasshouse where the mother plants are being grown, or the intact plants should be moved into Z. Pflanzenphysiol. Bd. 99. S

2 132 H. C. M. DE STIGTER the cut flower's climate room. Both of these approaches have certain practical drawbacks, but it would by far be preferable to study cut flowers and intact plants under controlled conditions. A workable solution would be to grow potted plants in a glasshouse and to move them into a climate room for the experiment proper. Recently, NICHOLS and Ho (1979) used this method to compare aspects of the energy balance in cut and intact rose flowers. Quite a different attempt was that of DURKIN and Kuc (1966): by approach-grafting they provided rose stems with an auxiliary root system. Thus comparisons with non-grafted controls could be made under identical conditions. By growing single-stem rose plants from rooted cuttings, in a climate room in hydroponic culture, in batches at 2-3 weeks intervals, we acquired a year-round source of uniform experimental material that was excellently suited for the mentioned purpose of comparing cut and intact plants. This paper and subsequent ones will report some results of experiments related to aspects of the water management of roses, cut and intact. Materials and Methods The Hybrid Tea-Rose «Sonia» (= «Sweet Promise») used in the experiments was grown from cuttings. The method of making the cuttings was essentially that of MOE (1971). Shoots bearing a coloured flower bud were cut up into one-node segments, each with one compound leaf in top most position. The bases were dipped in a 750 ppm solution of potassium indolylbutyrate, 1 em deep, for 5 sec; the adhering drop was allowed ca. 5 min to be sucked in. After dipping the leaf in 0.1 % «Benlate» + wetting agent, and the base in 10 0 /0 «Captan» powder, the cuttings were stuck in a well-moistened rooting medium consisting of 3 parts of perlite and 1 part of sifted peat. Rooting took place under polythene cover, in a climate room at 25 C and a light intensity of 12 W m- 2 (14 h day-i). Under these conditions rooting as a rule was sufficient after 2'/2 weeks. Hydroponic culture After harvesting and washing, the rooted cuttings were transferred to hydroponic culture. The method described by STEINER (1965) was so modified that a 60 X 120 em phytotron cart would accommodate 24 plants. These were mounted in such a fashion that they could be handled individually or rearranged at any time, each having available 15 X 16 em of space. The nutrient solution was the «universal» one of STEINER (1972): KH2P mg; MgS0 4 '7H mg; K 2 S mg; Ca(N0 3 h. 4H mg; KN mg per litre water. Total ionic strength 30 mg ion 1-1; osmotic value 0.72 atm at 20 C, ph ca. 6 (weekly checked). This solution was supplemented with iron chelate and micro elements. Climatic conditions Temperature 20 C ± 0.2; relative humidity ca. 70 Ofo; horizontal air displacement ca. 0.3 m S-I; irradiance 75 W m- 2 at plant base with a photoperiod of 14 h; light quality: high pressure mercury lamps, Philips HPLIN 400 W, with some admixture from incandescent bulbs. Under these conditions it takes about 6 weeks for a «Sonia» cutting with «dormant» axillary bud to develop into a single-stem plant with a coloured flower bud «<cut-ripe stage»), an average stem length of ca. 55 cm, and a shoot weight of ca. 20 g. More details, on other cultivars and observations of plant behaviour outside the scope of this paper, will be reported and commented upon elsewhere. Z. Pflanzenphysiol. Bd.99. S

3 Water balance of rose plants 133 Weight determinations Roses were cut under water and placed individually in 100 ml beakers, after defoliation of the basal 8 cm of stem. To minimize evaporation the stems were inserted through two overlapping pieces of «paraplast» film. Fresh weight and amounts of water taken up and transpired were determined by periodic double weighings: total beaker + plant and beaker without plant, respectively. For making the same determinations with intact roses, comparable plants were transferred to one-litre cylinders, taking care that cut and intact plants had the same geometry with respect to relevant environmental factors, such as distance to lamps and wind exposure. Cylinder weight with and without plant yielded total plant weight: shoot-plus-roots. Root weight was subtracted by placing the plant on a fork mounted on the weighing platform of the balance, with the roots just completely immersed in water: Figure 1. This technique rests on the assumption that the specific weight of the roots is sufficiently close to that of water. It gave reproducible results provided that firstly, the root systems were always immersed to exactly the same depth and that secondly, care was taken to lift the plants from the cylinders with consistent slowness, so that no excessive and varying quantities of water were lost by dripping off the roots. As a check severed root systems of different sizes were weighed after mild centrifugation, for 1 minute, in a small household centrifuge. Quite consistently the weight of centrifuged root systems was ca. 65 Ofo of that of roots plus adhering water as determined by the water-displacement technique. Aeration of the plants, while growing and being observed on the cylinders proved not to be necessary provided that the nutrient solution was renewed daily. Altogether, for a complete weight and water-balance account the intact plants required 5-fold weighings: 1, total cylinder + plant; 2, cylinder without plant; 3, cylinder with plant on fork and roots in water; 4, ditto, but old nutrient solution replaced by fresh; 5, new total of cylinder + plant. Summing up, the method is somewhat laborious and time consuming (some 5 min per plant), but as a recompense one has a completely non-destructive method for partitioning intact-plant weight into shoot and root weight. As a result overall growth, shoot! root ratios and water management of individual plants can be followed through time. Fig. 1: Weighing system for intact plants. Z. P/lanzenphysiol. Bd.99. S

4 134 H. C. M. DE STIGTER Times of weighing. Initially for reasons of economizing on energy costs, the 14 h photoperiod was set at to h. At the same time it turned out that this produced the possibility of adopting a schedule of two weighing periods per day, without too much personal inconvenience: the first and most critical one at the end of the light period, shortly before h, and the second around h, i.e. after 6 h of darkness, more than half the dark period. Unless otherwise specified, the terms «rose» and «flower» are meant to denote the whole of a leafy shoot plus flower(bud). Results Figures 2 and 3 give the fresh weights, and water loss and uptake of intact rose plants and of cut roses standing in water, in percentages of the initial values. The 100 % initial levels in Fig. 2 representing absolute water losses of 65.6 and 63.0 g day-l for the cut and intact roses, respectively, it may be stated that during the first 24 h both water loss and water uptake were practically equal in the two treatments. To verify this finding a similar experiment was made with plants that were about one week younger. All plants were left intact during that one week, after which half of the plants were cut and placed in water; the other half remained intact all the time. Table 1 summarizes some of the results. Not only in the intact plants, but also in the intact -+ cut plants, water losses during the 24 h before and after the cutting date were exactly the same. By the 9th day after the cutting date, however, water losses had dropped to 67.4 % and 18.4 Ofo of the initial values, resp. (or 69.1 % and 17.1 % on the basis of comparison of Fig. 2). As Fig. 3 shows, the fresh weight of the cut roses soon, i.e. within 2 days after cutting, started to strongly fluctuate in accordance with the light-dark alternations, weight losses during the light and recovery during the dark often being in the order of 10 % of the initial weight. The intact plants did not show this fluctuation until ~ : : i djjs Fig. 2: Water loss and uptake of cut and intact roses, per 24 h, as percentages of water loss during the first 24 h: 65.6 and 63.0 g of water, resp. Water loss of cut roses:... Water loss of intact roses: - Continuous thin lines: corresponding water uptake. Z. Pflanzenphysiol. Bd.99. S

5 Water balance of rose plants 135 Table 1: Fresh weight and water loss of rose plants initially intact and in green-bud stage (cf. Ho and NICHOLS, 1977; stage G: calyx split, corolla just visible). After 6 days one half severed from root systems and placed in water. Weight and water loss expressed as percentage of initial value: 17.9 g and 15.7 g plant weight; 92.9 g (5.80 g cm- 2 h- 1 ) and 73.4 g (6.10 g cm- 2 h- 1 ) water loss per 24 h, for intact and cut plants, resp. day before (-) fresh weight, g water loss, g per day or after (+) cutting date intact cut intact cut (74.9) 100 (73.1) 100 (102.5) 100 (93.0) (95.8) (93.5) 97.1 (99.5) (100.0) (100) (100) (108.4) (106.6) 97.6 (100) (100) (155.3) (125.6) 67.4 (69.4) 18.4 (17.1) In parentheses: percentages calculated on basis of values on day 0 (fresh weight) or day + 1 (water loss) taken as /0, for correspondence with Figures 2 and t. : ~ c.. = ~ ;: 100 = M ~ ~... ~. f" " ".;... / d.ys Fig. 3: Fresh weight of cut (...) and intact roses (-), in latter case total weight minus root weight, at end of light period, and towards end of dark period, as percentage of initial weight: 20.6 and 21.6 g, resp. On day 8 and 10, resp., petals showed symptoms of advanced senescence, and were removed for weight determination: 7.9 and 14.4 g, with 18.8 and 12.1 % dry matter content, resp. z. P/lanzenphysiol. Bd.99. S

6 136 H. C. M. DE STIGTER much later and only to a much smaller extent, in the days preceding petal removal. After this date actual weight losses no longer occurred in the intact plants, while in the cut roses weight fluctuation was still present, though much reduced. Splitting up total water loss and uptake per 24 h (Fig. 2) into water loss and uptake per light and dark period results in Fig. 4. The water uptake surpluses and deficits of this Figure exactly reflect the fresh weight gains and losses of Fig. 3, resp. Furthermore it should be noted that, whereas in the light water uptake of the cut roses decreased rapidly with time, uptake in the dark remained equal both to the initial value and to that of the intact plants; only after petal shedding it fell to somewhat lower values. ~5 A... ~5 ~4 ;3 E2 B Fig. 4: Water loss and uptake of intact (A) and cut (B) roses in light and dark, expressed in g h- 1 Water loss: columns end with -; water uptake: columns end with... Surpluses are vertically hatched, deficits are solid black. Column widths correspond to duration of light and dark periods: 14 and 10 h, resp. Arrows: moments of petal removal, d. Fig. 3. Discussion Choice of material and methods MAYAK et al. (1974) rightly stated that the water balance is recognized as a major factor determining the quality and longevity of cut flowers. However, there is a Z. PJlanzenphysiol. Bd. 99. S

7 Water balance of rose plants 137 lack of experimental data directly comparing the water balance in cut and intact flowers under identical conditions. The present study involving the use of singlestem, own-rooted roses aims at contributing to fill this gap. The experimental material has the following advantages and capabilities: The use of single-stem cuttings allows the raising of relatively large numbers of uniform plants in the limited space of a climate room. - Simply removing the root system turns an intact rose plant into an otherwise identical cut flower. - There is no lateral branching, with shoots in different developmental stages as in glasshouse rose bushes, which would make it impossible to single out exact data on the bloom to be studied. - The hydroponic system allows accurate measurements to be made of the amounts of water taken up, and lost by transpiration; especially the former would be very difficult - if at all possible - to come by using conventionally grown material, even including potted plants (d. Ho and NICHOLS, 1977; NICHOLS and Ho, 1979). The non-destructive weighing technique described makes it possible to reliably distinguish between shoot and root weight of intact plants; the shoot part of total weight thus obtained is directly comparable to the weight of the cut roses. - «Air-embolism» (whatever its importance may be, d. ROGERS, 1973) can be completely prevented by cutting under water. As a contrast, the efficacy of recutting under water often referred to as a standard routine for preparing conventional glasshouse roses may be questioned, this most probably depending on the extent of air-filling of the xylem vessels. By using intact-plant behaviour as a reference that shows what the rose would have done if it had remained intact all the time, the very effects of the cutting operation per se will emerge more conclusively. Two aspects should be considered: Short-term effects: the immediate effects of replacing water uptake through the roots by that through the opened-up xylem vessels, involving the elimination of root resistance and the substitution of water (with lower osmotic potential) for the original tracheal sap. On account of this, enhanced water uptake might be expected as a reaction to cutting. - Long-run effects. A «cut-ripe» rose shoot is a relatively young plant part: it may reach this stage in a matter of six weeks from axillary bud break. During the ensuing flowering period of about two weeks the various developmental and physiological processes are still going on, but are they running parallel, or not, in the cut and ill the intact roses? Experimental results Fig. 2 and Table 1 give answers to the twofold consideration mentioned. Transpiration and water uptake of the cut roses during the first 24 h after cutting were identical or nearly so to those during the 24 h before cutting (Table 1), and to Z. Pjlanzenphysiol. Bd.99. S

8 138 H. C. M. DE STIGTER those of the plants that had remained intact (Fig. 2). Enhanced uptake was not detectable. During the following days water uptake and transpiration of the cut roses fell sharply, but then levelled off to a final ca. 10 %. In similar experiments not reported here, uptake of cut roses which showed symptoms of petal wilting and «bent neck» dropped to values under 10 %: %; average 8.5 %, n = 5). In roses which did not show such symptoms this value stayed above 10 %: %; average 18.6 %, n = 6). So the phenomenon of «bent neck» and wilting in cut roses is related to the degree of impairment in water uptake. In the mean time water turnover of the intact plants (Fig. 2), after a slight increase, gradually decreased to a final ca. 70 % of the initial value. In similar experiments this effect was found consistently: %; average 66.9 %, n = 6. These results show that water uptake of the cut roses would have dropped by some 30 % anyhow: apparently leaves transpire less actively upon getting older. With sunflower BLACK (1979) found the youngest fully expanded leaf to be most aotive in this respect. The fresh-weight curves of Fig. 3 show the capabilities of the weighing system for intact plants and the considerable advantage of weighing twice a day. The rates of weight increase of the intact plants are clearly different during light and dark periods; with single weighings these different rates as well as the striking pattern of light-dark fluctuations of the cut roses would go completely unnoticed. From the degrees of weight fluctuation before and after petal removal it may be concluded that in the cut roses both petals and foliage contributed to the phenomenon, but that in the intact plants only the senescing petals were responsible for actual losses of plant weight in the light. By removal of flower or leaves, or both, CARPENTER and RASMUSSEN (1974) found the respective contributions to water uptake by «Forever Yours» roses to be 20.4 % and /0. In view of the dynamic nature of flower unfolding with concomitant water absorption, we feel that percentages like these should be regarded as momentary recordings depending on the particular stage of flower development. With this restriction, in the present study petals and foliage contributed about equally to the weight changes. The cut roses did not show any symptoms of petal wilting or pedicel bending (<<bent neck»). Accordingly, the general fresh-weight trend was upwards for most of the time, mainly because of flower expansion. Nonetheless, flower expansion was much better in the intact roses, as evident both from the higher final plant weight (some 160 % of initial weight) and from the petal weight at the time they were removed: 14.4 g with 12.1 % dry matter content (= 1.74 g), versus 7.9 g and 18.8 % dry matter content (= 1.49 g) in the cut roses. As a result, the latter had lost much of their «display value». Fig. 4 reveals that although water uptake of the cut roses in the light declined rapidly, uptake in darkness remained practically constant and equal to that of the intact plants. In the literature overall decreases in water uptake by cut roses are Z. PJlanzenphysiol. Bd.99. S

9 Water balance of rose plants 139 generally reported and interpreted as being due to an increasing degree of vascular occlusion, either of a «microbial» type at the cut stem base or a «physiological plugging» higher up the stem (AARTS, 1957; LINEBERGER and STEPONKUS, 1976). However, because of extent, distribution and moment of appearance, RASMUSSEN and CARPENTER (1974) considered the occurrence of blocking materials as a secondary phenomenon, an effect rather than a cause, the decreasing ability of the cut rose to take up water rather being a physiological phenomenon assuming the form of a «decreasing harmonic curve». In an earlier paper (CARPENTER and RASMUSSEN, 1973) they already argued against a «natural physical block» by stating that «the rate of water uptake could be turned off and on by alternating dark and light periods coupled with a gradual decrease in water uptake during the light period». Although actual water losses were not recorded in their experiments, they also held reduced uptake rather than excessive loss responsible for the observed loss in petal and pedicel turgidity. The present results conclusively show the vascular-occlusion hypothesis to be untenable in its present form since solidly plugged rose stems would not conduct normal amounts of water in darkness, under conditions of much reduced stress. That water-uptake deficits were always followed by normal uptake in the dark indicates the existence of a reversible type of uptake restriction. The following hypothesis is proposed, on the basis of xylem vessels consisting of overlapping series of elements more or less frequently terminated by pitted membranes (ZIMMERMANN, 1978). Upon cutting a rose stem and placing it in water, tiny bodies or slimy particles are formed in the opened-up vessels, at first freely swimming in the tracheal fluid. As more of them are formed, more will get trapped against the pores in the lateral and apical pit membranes and stay adhered by transpirational pull. Longitudinal and lateral water passage will gradually become impeded so that, in the light, with initially open stomates, water uptake will lag behind transpirationalloss. This results in decreasing water potentials and once these become sufficiently low, the situation may worsen by vessel cavitation. In darkness the reduction in transpirational pull will allow occluding particles to be more or less completely loosened from the pit pores, so that enough water may pass to fill up any cavitations, if present. Thus a situation of relatively unhampered water uptake will be restored. All the events involved, stomate opening and closing, acceleration and deceleration of transpiration, build-up of more negative water potentials and vessel cavitation, need time to develop: they do not simply turn on and off with the light switch. This may readily explain Fig. 3 of CARPENTER and RASMUSSEN (1973). The pore-occluding particles suggested are by no means to be identified with the solid plugs reported by others. These plugs which may even take the form of coherent strings (LINEBERGER and STEPONKUS, 1976) are to be regarded as secondary phenomena (RASMUSSEN and CARPENTER, 1974). The presently proposed bodies probably would escape light-microscopic observation. As the triggering process likely has some connection with the wounding of the stem base, the site of events should Z. P/lanzenphysiol. Ed. 99. S

10 140 H. C. M. DE STIGTER be in the lowermost open segment delimited at the upper end by the first transverse membrane in the individual vessels. Acknowledgements Thanks are due to Mr. A. G. M. BROEKHUYSEN for able technical assistance, and to Ms. H. COUPERUS for preparing the Figures. Note Finishing revision of the manuscript, I came across D. ]. DURKIN: Effect of millipore filtration, citric acid, and sucrose on peduncle water potential of cut Rose flower. ]. Amer. Soc. Hort. Sci. 104(6), , 1979, stating «. it is conceivable that obstruction of flow through pit membranes could be accomplished by obstructing particles at undetectably low levels». This statement is very much in line with the hypothesis proposed above. References AARTS, J. F. TH.: Over de houdbaarheid van snijbloemen. With summary: On the keepability of cut flowers. Meded. Landbouwhogesch. Wageningen 57(9), 1-62 (1957). BLACK, C. R.: The relationship between transpiration rate, water potential, and resistances to water movement in sunflower (Helianthus annuus L.). ]. expo Bot. 30, (1979). CARPENTER, W. J. and H. P. RASMUSSEN: Water uptake rates by cut roses (Rosa hybrida) in light and dark. J. Amer. Soc. Hort. Sci. 98, (1973). - - The role of flower and leaves in cut flower water uptake. Scientia Hort. 2, (1974). DURKIN, D. J. and R. Kuc: Vascular blockage and senescence of the cut rose flower. Proc. Amer. Soc. Hort. Sci. 89, (1966). Ho, L. C. and R. NICHOLS: Translocation of 14C-sucrose in relation to changes in carbohydrate content in rose corollas cut at different stages of development. Ann. Bot. 41, (1977). LINEBERGER, R. D. and P. L. STEI'ONKUS: Identification and localization of vascular occlusions in cut roses. ]. Amer. Soc. Hort. Sci. 101, (1976). MAYAK, 5., A. H. HALEVY, S. SAGlE, A. BAR-YOSEPH, and B. BRAVDO: The water balance of cut rose flowers. Physio!. Plant. 31, (1974). MOE, R.: Kultur von Topfrosen nach neuen Erkenntnissen. Gartenwelt 71, (1971). NICHOLS, R. and L. C. Ho: Respiration, carbon balance and translocation of dry matter in the corolla of rose flowers. Ann. Bot. 44, (1979). RASMUSSEN, H. P. and W. J. CARPENTER: Changes in the vascular morphology of cut rose stems: a scanning electron microscope study. ]. Amer. Soc. Hart. Sci. 99, (1974). ROGERS, M. N.: An historical and critical review of postharvest physiology research on cut flowers. HortSci. 8, (1973). STEINER, A. A.: A new method for growing plants in water culture. Act. Bot. Neer!' 14, (1965). STEINER, A. A.: Plantenteelt zander aarde. Landbouwk. Tijds. 84, (1972). ZIMMERMANN, M. H.: Vessel ends and the disruption of water flow in plants. Phytopath. 68, (1978). Dr. H. C. M. DE STIGTER, Centre for Agrobiological Research, P.O. Box 14, 6700 AA Wageningen, The Netherlands. z. Pf/anzenp'hysiol. Ed. 99. S