EFFECT OF ROOT TEMPERATURE ON BUDBREAK, SHOOT GROWTH, AND FRUIT-SET OF "CABERNET SAUVIGNON" GRAPEVINES

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1 EFFECT OF ROOT TEMPERATURE ON BUDBREAK, SHOOT GROWTH, AND FRUIT-SET OF "CABERNET SAUVIGNON" GRAPEVINES W. M. Kliewer Biochemist, Department of Viticulture and Enology, University of California, Davis Presented at the Annual Meeting of the American Society of Enologists, June 22, 1973, Monterey, California. The financial support by the California Wine Advisory Board is gratefully acknowledged. Accepted for publication April 1, ABSTRACT Three-year-old dormant 'Cabernet Sauvignon' vines, growing in 5-gallon containers in a greenhouse, were pruned to two 10-node canes and then grown for 9 weeks in water baths With root temperatures kept at 11, 15, 20, 25, 30, and 35 C. Air temperatures were the same for all treatments, fluctuating between a minimum of 20 C at night and a maximum of 32 C in the day. Budbreak and bloom occurred 3 to 8 days earlier at C than at 11 C. The number of buds that broke per vine increased with temperature, and was 2 to 3 times as great at C as at C. Total shoot growth per vine, measured as length or dry weight, was maximal at 30 C root temperature, as was also the total number of leaves and leaf area per vine. Average shoot length, dry weight per unit length of stem, leaf area, and leaf and cluster dry weights were significantly less at 35 C than at lower root temperatures. With an increase in temperature between 15 and 35 C, there was a decrease in percent dry matter in stems but an increase in leaves. The number of cluster per vine was proportional to the number of buds that broke. The number of berries set per vine did not differ significantly with temperature. However, the number of berries per cluster was significantly greater at 11 C than at root temperatures of 20 C or higher, with berry set approximately proportional to leaf area per cluster. Considerable effort has been devoted to studying the influence of air temperature on the growth and composition of grapevines and their fruits (1,2,5,6, 8,9,10,11,12,26). Information is scarce, however, on the influence of root temperature (as distinguished from shoot or foliar temperature) on fruit and vine development. An exception is work of Woodham and Alexander (33), who investigated the effect of root temperatures from 11 to 30 C on the development of 'Sultana' ('Thompson Seedless') vines grown in Hoagland's nutrient solution during an 8-week period beginning with budbreak. The growth of shoots, roots, and inflorescences, as well as fruit-set, increased with increasing root temperature. Growth of shoots and roots and fruit-set were scant at 11 C root temperature. Higher temperatures favored greater amounts of top growth relative to root growth. Skene and Kerridge (23) found that 'Thompson Seedless' vines grown in nutrient solution 82 at 30 C root temperature had greater shoot elongation, increased dry-matter accumulation in both shoots and roots, and thinner and longer roots than did vines grown at 20 C. Cytokinin content was greater in roots grown at 30 C than in roots grown at 20 C. However, roots grown at 20 C had two regions of cytokinin activity on paper chromatograms, as opposed to a single region of activity in roots grown at 30 C. A problem of considerable significance in some areas in California is the failure of many buds to break on canes. In some instances, buds may break or burst but then make little further growth. General field observations have often indicated that this "blank" or "blind" bud phenomenon is a greater problem in the coolest grape growingregions, especially in poorly drained wet soils, an indication that root temperature may be an important factor in budbreak and shoot growth in grapevines.

2 ROOT-TEMPERATURE EFFECTS ON GRAPEVINESm83 Average daily soil temperatures at depths of 6 to 36 inches at time of budbreak for most grape cultivars at Davis (March 15 to April 1) generally range from 12 to 15 C in well-drained sandy loam soils (24,25). Finer-textured clay loam soils average 2 to 4 C colder. Well-drained soils at Davis warm rapidly during April and May, by bloom (May 15 to June 1) generally ranging between 20 and 25 C at 6 to 36 inches (24,25). The present investigation was made to determine the effects of a wide range of root temperatures on time of budbreak and bloom, amount of budbreak, shoot and cluster growth, and fruit-set of 'Cabernet Sauvignon' grapevines MATERIALS AND METHODS In the spring of 1971, 100 one-year-old dormant rootings of certified 'Cabernet Sauvignon' vines were planted in 5-gal. metal cans containing a mixture of soil, sand, and peat (2:2:1 by volume) and maintained in a field lathhouse as described previously (8). In both 1971 and 1972, the vines were pruned to two shoots, which were trained vertically on separate 6-ft. stakes. On January 6, 1973, 42 vines selected for uniformity were each pruned to two 10-node canes. The vines were randomly divided into six groups of seven vines each, and each group was transferred on January 12 into a separate water bath 90 X 90 X 60 cm deep (Fig. 1). All water baths were equipped with heating and cooling facilities, and each was maintained at one of the following temperatures: 11 C (52 F), 15 C (59 F), 20 C (68 F), 25 C (77 F), 30 C (86 F), and 35 C (95 F). The water baths were located in a greenhouse and, by continuous rapid circulation of water with a centrifugal pump, were maintained within _1 C of the desired temperatures. Water level in the tanks was maintained at approximately the same level as the rooting media in the metal cans. The soil temperature in a pot in each of the 6 water baths was continuously monitored at a depth of 10 to 15 cm with a Foxboro temperature recorder and did not differ by more than ±2 C from the temperature of the water bath. Immediately prior to putting the vines in the water baths, two 6-mil polyethylene bags were placed around each container to prevent water from entering through the drainage holes of the cans. The soil surface of each container was covered with aluminum foil to prevent solar radiation from heating the soil (Fig. 1). The aerial portions of all vines were at the ambient temperature of the greenhouse, maintained between limits of 20 C (night) and 32 C (day). A large fan was used to circulate the air in the greenhouse and help maintain uniform air temperatures. Fig. 2 is a thermographic chart showing typical diurnal fluctuation of air temperatures for a 1-week period. During the first week of the experiment bud temperatures at various node positions along the canes were measured with thermistors (Yellow Springs Instrument Co.) to determine if Fig. 1. Two of the six water baths used for controlling root temperatures are shown, each containing 7 vines. The water bath in the foreground shows vines immediately after shoots were harvested from the 10-node canes. The pump at the bottom of figure on the left was used to circulate water continuously and help maintain uniform water bath and soil temperature, and the aluminum foil at the top of the pots was used to reflect incoming solar radiation and also aid in maintaining uniform soil temperature. the water bath temperatures influenced the temperatures of the buds along the canes differently. Temperatures of buds at nodes 1 to 10 on a cane usually did not differ by more than 1 C and never differed by more than 2 C within and between treatments. Light intensity was notcontrolled, and was dependent on incoming solar radiation. To keep the light intensity as high as possible, however, no whitewash was used on the greenhouse. The plants were kept well watered, and were supplied with 2 liters of Hoagland's No. 1 nutrient solution at 3-week intervals. The time of budbreak for each node position on each cane was recorded every second day, as were also the times of initial bloom (first visible opening of flowers) and final bloom ( % of flowers in bloom). A bud was considered to have broken

3 84--ROOT-TEMPERATURE EFFECTS ON GRAPEVINES Fig. 2. Thermograph chart showing typical diurnal fluctuations in air temperature for a one-week period. when the first green leaf was visible through the bud scales. Except for vines with root temperature held at 35 C, all shoots were harvested March 16, days after bloom. A malfunction in the relay switch on the 35 C water bath caused the root temperature to reach 55 C three weeks after the experiment began, so the vines in this treatment were replaced with a new set of dormant vines on February 3. To keep treatment times the same, the shoots from vines in this treatment were harvested 22 days later than those in the other treatments. The mean ambient day temperature during the period from March 16 to April 6 (35 C treatment vines only) averaged 2.2 C higher than during the preceding 6 week period. In all treatments, fruit-set had been completed by the time the shoots were harvested. The lengths of primary shoots and inflorescences on each vine were measured at harvest. The numbers of leaves and clusters per shoot and per vine were recorded, as was also the number of berries set per cluster. However, the number of flowers which abscissed during the bloom and fruit-set periods was not obtained. The oven-dry weights of leaf blades, petioles, stems, and fruit clusters were determined for each shoot and vine. To estimate total leaf area per vine, all leaf blades on a typical shoot from each treatment were removed, and their total area was determined with a planimeter. The blades were then dried at 70 C, and the ratio of cm 2 of leaf area per gram dry weight was determined. This ratio and the total leaf blade dry weight per shoot was used to estimate total leaf area per shoot and per vine. Nitrate in petioles was determined by the procedure of Paul and Carlson (18). Am. J. Enoh Viticult., Voh 26, No. 2, 1975 RESULTS For comparison of the number of days to budbreak and to bloom, the two terminal nodes on each 10-node cane were used, since these two nodes broke in nearly all plants. Starting from the time the vines Table 1. Effect of root temperature on time of budbreak and bloom of 'Cabernet-Sauvignon' vines (air temperature maintained between 20 and 32 C was common to all treatments). No. of days No. of days to No. of days between budbreak and to budbreak initial bloomy bloom Root temp. Node Node Node Node Node Node C (OF) 9 z 10 z (52) 29a 24a 55a 53a 26a 29ab 15 (59) 27ab 25a 53ab 5lab 26a 26ab 20 (68) 25bc 21b 50ab 47bc 25a 26ab 25 (77) 23c 20b 48b 45c 25a 25a 30 (86) 23c 21b 49b 46c 26a 25a 35 (95) 22c 20b 52ab 50ab 30b 30b xdata are the average of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. ynumber of days to budbreak and initial bloom were counted from the time the vines were placed in the temperature-controlled water baths. "Nodes 9 and 10 refer to the two terminal buds on each 10-node cane.

4 ROOT-TEMPERATURE EFFECTS ON GRAPEVINES--85 Fig. 3. General appearance of 'Cabernet Sauvignon' vines grown at five different root temperatures immediately prior to harvesting the shoots. Note the lack of shoots ("blank', or "blind" buds) on basal parts of canes from vines grown at root temperatures of 11 C (52 F) and 15 C (59 F) root temperatures. Root temperatures 68 F, 77 F, and 86 F correspond to 20 C, 25 C, and 30 C respectively. were placed in the water baths, the number of days to budbreak and initial bloom was 4-8 more at 11 and 15 C than at C (Table 1). However, the number of days between budbreak and bloom did not differ significantly at temperatures from 11 through 30 C. But at 35 C, the period between budbreak and Table 2. Effect of root temperature on budbreak of "Cabernet-Sauvignon' vines (air temperature maintained between 20 and 32 C was common to all treatments), x 45.0 Jf Root No. of buds temp. that grew Percent C ( F) per viney budbreak...,. 7=-, (52) 3.7a (59) 4.6ab (68) 6.0bc (77) 6.8c 34.3 ~ J/ 30 (86) 9.7d (95) 10.8d 54.3 xdata are the average of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. yeach vine was pruned to two 10-node canes [ i I i 1! C ROOT TEMPERATURE ( C) Fig. 4. Influence of root temperature on total shoot dry weight of 'Cabernet Sauvignon' vines. Shoots were harvested 14 to 20 days after bloom. The vertical bar indicates the difference between treatment means required for significance at the 5% level.

5 86--ROOT-TEMPERATURE EFFECTS ON GRAPEVINES Table 3. Influence of root temperature on dry weights (g) of stems, leaf blades, petioles, and fruit clusters per vine (air temperature maintained between 20 and 32 C was common to all treatments). Root temp. Fruit Total C ( F) Stems Leaf blades Petioles clusters shoot Table 5. Influence of root temperature on total number of leaves and leaf area per vine, and average area and dry weight of leaf blades (air temperature maintained between 20 and 32 C was common to all treatments). Root Total no. Av. dry wt temp. Total leaf area of leaves Av. area per per leaf C ( F) per vine (cm 2) per vine blade (cm 2) blade (g) 11 (52) 10.29a 11.11a 1.00a 1.72a 24.12a 15 (59) 16.55abc 13.68ab 1.53ab 1.98a 33.74ab 20 (68) 19.56bc 16.06bc 1.83b 2.82b 40.27ab 25 (77) 22.14c 18.02c 1.92b 3.64c 45.72b 30 (86) 21.07c 21.47d 2.50c 4.06c 49.10b 35 (95) 13.65ab 17.34c 1.81b 1.99a 34.79ab xdata are the average of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. bloom was 4 to 5 days longer (p < 0.05) than for vines grown at 25 or 30 C root temperatures. Root temperature had a striking effect on the number of buds that grew per vine (Fig. 3, Table 2). The average increased from 3.7 at 11 C to 10.8 at 35 C, out of a total of 20 nodes retained per vine. Total shoot dry weight, and the dry weights of leaf blades, petioles, stems, and fruit clusters, were highest at C, significantly greater (p < 0.05) that at 11 or 35 C (Table 3, Fig. 4). However, the dry weights of shoots, as well as of their component parts, did not differ significantly at root temperatures from 15 to 30 C. Table 4. Influence of root temperature on shoot and cluster length, stem dry weight per unit length, and total shoot length per vine (air temperature maintained between 20 and 32 C was common to all treatments), x Root Av. length Stem dry wt Av. cluster temp. Total shoot of shoots per cm length length C ( F) length (cm) (cm) (mg) (cm) 11 (52) 191.0a 51.4a 51.6ab 9.78a 15 (59) 270.0b 59.0a 60.0b 10.10a 20 (68) 340.6c 56.8a 55.2ab 10.57a 25 (77) 369.8cd 53.9a 57.9b 10.33a 30 (86) 427.6d 44.0a 49.2ab 9.27a 35 (95) 307.0bc 28.3b 44.0a 8.11a xdata are the average of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. Treatments were terminated 14 to 20 days after bloom. 11 (52) 3220a 50.0a 64.4a 0.222ab 15 (59) 3963ab 60.0ab 65.4a 0.226b 20 (68) 4655ab 67.6abc 68.8a 0.237b 25 (77) 5223bc 82.0cd 63.7a 0.220ab 30 (86) 6264c 103.8e 60.3a 0.207ab 35 (95) 4602ab 98.3de 46.8b 0.177a xdata are the mean of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. Treatments were terminated 14 to 20 days after bloom. Total shoot length per vine increased with increasing root temperature up to a maximum at 30 C. This was followed by a sharp decline at 35 C (Table 4). However, average length per shoot did not differ significantly at root temperatures from 11 to 30 C but was significantly (p < 0.05) less at 35 C (Table 4). Stem dry weight per cm length was significantly less (p< 0.05) at 35 C than at 15 or 25 Co However, weight per unit stem length did not differ significantly among the other temperature treatments. Average cluster length per vine was not significantly different among treatments. The trend, however, was toward an increase in cluster length with an increase in root temperature up to C, followed by a decrease at higher temperatures (Table 4). Numbers of leaves and total leaf area per vine increased with increasing root temperatures between 11 and 30 C, followed by sharp decreases at 35 C (Table 5). Total leaf area was significantly greater (p < 0.05) at 30 C than at 11, 15, 20, or 35 C. Average leaf area and dry weight per leaf blade were significantly less (p < 0.05) at 35 C than at the other temperatures (Table 5). However, neither area nor dry weight per leaf blade was significantly different at temperatures from 11 to 30 C. Number of clusters per vine increased with increasing root temperature, and was proportional to the number of buds that grew per vine (Tables 2, 6). There was no significant difference in number of clusters per shoot among treatments. The number of clusters per vine, however, was significantly greater (p < 0.05) at 30 and 35 C than at 11 and 15 C. But at 20 C or higher, the number of clusters per vine did not differ significantly among treatments (Table 6). The number of berries set per cluster and per square decimeter of leaf area was significantly greater (p < 0.05) at11c than at 20 Corhigher (Fig. 5). This was true whether the number of berries set per cluster was compared by node position or

6 ROOT-TEMPERATURE EFFECTS ON GRAPEVINES--87 Table 6. Influence of root temperatures on number of clusters, fruit-set, and cluster dry weight per vine (air temperature maintained between 20 and 32 C was common to all treatments), x Root No. of No. of Total no. of No. of berries Leaf area Av. cluster temp. clusters berries set berries set set per dm 2 per cluster dry wt per C ( F) per vine per cluster per vine leaf area (cm 2) vine (g) 11 (52) 5.0a 80.7a 404.7a 12.5a a 15 (59) 6.lab 55.2ab 339.1a 8.5ab a 20 (68) 9.8bc 30.6b 301.3a 6.5b a 25 (77) 8.6bc 37.4b 320.7a 6.1b a 30 (86) 11.6c 39.5b 460.4a 7.3b a 35 (95) 11.3c 32.1b 362.7a 7.9b b Data are the mean of 7 replicates. Within a column, means followed by the same letter are not significantly different at the 5% level. Treatments were terminated 14 to 20 days after bloom. by using the average of all clusters on a vine. On an entire vine basis, however, the number of berries set did not differ significantly among treatments (Table 6). Average cluster dry weight was significantly less (p < 0.05) at 35 C than at 30 C or lower (Table 6), but not significantly different among temperatures from 11 to 30 C. DISCUSSION The most striking finding in this investigation was the highly significant increase in number of buds that broke per vine with increase in root temperatures from 11 to 35 C, even though aerial parts of the vines were in the optimum temperature range for growth (20-32 C). The greater number of shoots per vine as a result of the increased budbreak largely accounted for the increases in total shoot length, number of leaves, total leaf area, and number of clusters per vine at root temperatures up to 30 C. Both the greater number of buds that broke per vine, and the shorter time interval required for budbreak to occur at root temperatures from 25 to 35 C than at 10 and 15 C may be explained, at least in part, by temperature influence on hormonal activities in the roots, especially those of cytokinins. Plant roots are a primary source of cytokinins (7,23, 31), and bleeding sap or xylem exudate from grapevines is rich in endogenous cytokinins (15,17, 22,23). Cytokinins have been shown to hasten budbreak (29), regulate inflorescence growth and sub- Fig. 5. Influence of root temperature imposed for 9 weeks (beginning approximately 3 weeks before budbreak) on fruit cluster development of 'Cabernet Sauvignon' vines. The photograph was taken on the day shoots were harvested, 14 to 20 days after bloom. Am. J. Enol. Viticuit., Vol. 26, No. 2, 1975

7 88--ROOT-TEMPERATURE EFFECTS ON GRAPEVINES stitute for roots in maintaining the growth of young inflorescences (14,15), increase fruit-set (27,28), increase berry growth of some grape cultivars (28), and mobilize organic nutrients in grapevines (20,30). Skene and Kerridge (23) found that the cytokinin content in root exudate from 'Thompson Seedless' was greater in vines grown at 30 C than in vines grown at 20 C. They also noted a difference in the types of cytokinins present at the two temperatures. Tables 3-5 and Fig. 4 indicated that total shoot and cluster growth per vine was greater at root temperatures of C, in agreement with findings of Woodham and Alexander (33). This is also the optimum air temperature range for photosynthesis (11,13), shoot growth (1,11), net assimilation rate (11), and fruit-set (5,26) in grapevines. On the other hand, the number of berries set per cluster and per unit leaf area was significantly greater at 11 C root temperature than at 20 C or higher (Table 6), whereas on a per-vine basis the number of berries set did not differ significantly with temperature. This result is in direct contrast to findings of Woodham and Alexander (33), who reported that both fruit-set and cluster length were markedly less at 11 C root temperature than at 20 C, and both those in turn, were less than at 30 C. The discrepancy between our results and those of Woodham and Alexander was most likely due to differences in experimental conditions, resulting in widely different ratios of leaf area to fruit cluster. In the present investigation, total leaf area per vine was approximately half as much at 11 C as at 30 C. However, the number of clusters per vine was less than half as much at 11 C as at 30 C, resulting in about 30 % greater leaf area per fruit cluster (Table 6). Woodham and Alexander (33), in contrast, found nearly 12-fold less leaf area per vine and per cluster at 11 C than at 30 C root temperature, which probably accounts for the very poor fruit-set under their conditions. Winkler (32), Coombs (3), and Skene (21) demonstrated that the number of berries set per cluster depends mainly on the supply of organic nutrients to the clusters. Therefore, one would expect fruit-set to increase with an increase in the ratio of leaf area to fruit cluster. In addition to the greater amount of leaf area per cluster at 11 C than at higher root temperatures, the rates of root elongation during the ~bloom and fruit-set periods were probably less at the low temperatures (11-15 C) than at the higher temperatures, thus reducing competition for photosynthate between fruit clusters and shoot- and root-growing points. Average shoot length, stem dry weight per unit length, area and dry weight per leaf blade, and cluster dry weight were significantly less (p < 0.05) at 35 C than at lower root temperatures (Tables 4-6). Table 3 also indicates that, with increase in root temperature, leaves contributed progressively more and stems progressively less of the total shoot dry weight. Buttrose (1) also found proportionally less dry matter accumulated in stems and more in leaves with an increase in air temperature from 20 to 30 C. He also showed that area per leaf blade was less at 30 C air temperature than at 25 or 20 C. The reason for the reduced growth of the grape shoots, leaves, and fruit clusters at 35 C root temperature than at lower temperatures is not definitely known. However, several phenomena in other fruit crops are known to be affected by above-optimum root temperatures- increased respiratory rate, resulting in reduced carbohydrate and organic acid content of roots (4,16); accumulation in roots of products of anaerobic respiration, especially ethanol and acetaldehyde (4); reduction in water uptake and transportation when roots are exposed to supraoptimal temperatures over prolonged periods (4); reduction of cytokinin levels in both roots and leaves (4,23) ; reduced chlorophyll content of leaves (16) ; and reduced potassium and zinc levels in leaves (19). Any one or combination of these could account for the reduced vine growth at 35 C root temperature than in the optimum root temperature range of C (Tables 3-6, Fig. 3). Leaves from vines grown at 11 C were slightly paler green than leaves from vines grown at higher temperatures, an indication that nitrogen uptake by roots may have been reduced by the relatively cold soil conditions. Nitrate analysis of petiolar tissues at harvest revealed that plants grown at 11 C averaged 0.10% NO:~, compared with 0.12% NO3 for vines grown at root temperatures from 15 to 30 C, thus tending to substantiate visual observations. No other symptoms of nutrient stress were observed. Shoots on vines grown at 11 C also had some indications of water stress when air temperatures approached 30 C, although in no case did permanent wilting occur. Reduced ability of roots to take Ul~ water at low soil temperatures is well known. Th~ fact that average shoot length did not differ sig nificantly among vines grown at 11,30 C root tern peratures indicates that water stress was not ex cessive at 11 C. The findings in this investigation dramaticall indicate the importance of root temperature in co~ trolling the budbreak and growth of shoots and fru clusters in grapevines. Ways of modifying soil te~ peratures through various cultural practices a presently under investigation, as is also the infl ence of various rootstocks on the performance grapevines grown under various soil temperature LITERATURE CITED 1. Buttrose, M. S. Some effects of light intensity and perature on dry weight and shoot growth of grape-vine. Bot. 32: (1968). 2. Buttrose, M. S., C. R. Hale, and W. M. Kliewer. Effe temperature on the composition of 'Cabernet Sauvignon' be Am. J. Enol. Vitic. 22:71-5 (1971). 3. Coombe, B. C. Fruit set in grape vines: the mech of CCC effect. J. Hort. Sci. 45: (1970).

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