Cambial activity and annual rhythm of xylem production of Pinus kesiya Royle ex. Gordon (Pinaceae) in relation to phenology and climatic factors growing in sub-tropical wet forest of North East India

Flora 206 (2011) 198–204

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Cambial activity and annual rhythm of xylem production of Pinus kesiya Royle ex. Gordon (Pinaceae) in relation to phenology and climatic factors growing in sub-tropical wet forest of North East India N. Dhirendra Singh, N. Venugopal ∗ Plant Anatomy and Reproductive Biology Laboratory, Centre for Advanced Studies in Botany, School of Life Sciences, North Eastern Hill University, Shillong 793022, Meghalaya, India

a r t i c l e

i n f o

Article history: Received 9 December 2009 Accepted 3 April 2010 Keywords: Annual xylem production Sub-tropical wet forest Regression analysis Phenology Vessel differentiation Seasonality

a b s t r a c t The interrelationship between phenological events, climatic factors, periodicity of cambial activity and seasonal production of xylem was examined in Pinus kesiya Royle ex. Gordon growing in sub-tropical wet forest of Meghalaya state, India. Reactivation of dormant cambium occurs after sprouting of new needles during the middle of February. Since the formation of reproductive cones takes place simultaneously with vegetative bud break and needle formation, cone formation could also lead to the enhancement of cambial activity. The activity of cambium and xylem production decline gradually towards November and cease from end of December to end of January. There was no correlation between needle fall and cambial activity. Due to the production of three flushes of new needles and branches in a year the tree never becomes completely leafless. It was evident from correlation and regression analysis that the annual course of average temperature plays an important role for the reactivation of vascular cambium after dormancy. The differentiation of xylem elements correlated with mean temperature in the first place and secondly with precipitation. Increase in length of fusiform initials and their derivatives could be correlated with relative humidity, precipitation and mean maximum temperature. Dormancy was imposed by low temperature and less precipitation. The data are discussed in the light of cambial activity, xylem production and phenological events. © 2010 Elsevier GmbH. All rights reserved.

Introduction The activity of vascular cambium is not uniform throughout the year and determined by the interaction of internal and external factors (Grotta et al., 2005; Iqbal, 1994; Iqbal et al., 2010; Larson, 1994; Philipson et al., 1971; Reinders-Gouwentak, 1965). The majority of the past studies on cambial activity pertain to plants growing in temperate regions, with definite seasonal climates (Antonova, 1996; Antonova and Stasova, 1997; Bailey, 1920; Bannan, 1955, 1962; Rensing and Samuel, 2004). Also trees growing in the tropics and in arid and semi-arid regions in semi-deciduous forests show seasonal variation in cambial activity and annual rhythms of xylem and phloem differentiations (Amobi, 1974; Borchert, 1999; Fahn et al., 1968; Larson, 1994; Lisi et al., 2008; Rao and Rajput, 2001a; Venugopal and Krishnamurthy, 1987). The effect of genetic and environmental factors on shoot growth and xylem formation has been studied in the West African tropical tree Terminalia superba Engl. and Diels (Combretaceae) (Longman et al., 1979). Periodicity

∗ Corresponding author. E-mail addresses: nvgopal [email protected], [email protected], [email protected] (N. Venugopal). 0367-2530/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.flora.2010.04.021

of wood formation in twigs of 11 tropical trees was studied in different vegetation types, such as lowland forest, savanna and mangrove swamps in Nigeria (Amobi, 1974). Wood production has been estimated in a natural forest stand in Cameroon using tree ring analysis (Worbes et al., 2003). Cambial activity and annual rhythm of xylem production in trees and shrubs have been studied in Israel (Fahn, 1958). There was a correlation in cambial growth and rainfall in the dipterocarpous forest of Peninsular Malaysia (Killman and Thong, 1995). The activity of cambium and differentiation of its derivatives are influenced by environmental factors (Antonova and Shebeko, 1986; Denne, 1976; Denne and Smith, 1971; Ford et al., 1978; Iqbal et al., 2010; Wodzicki, 1971). Vessel chronologies, tracheid length, maximum density, latewood cell wall proportion, latewood density, resin duct density, ray height and microfibrils were used as variables for dendroclimatic studies (Pumijumnong and Park, 1999; Wemmer and Grabner, 2003). The effect of flood on tree growth in the Amazon forest in relation to phenology and analysis of rings has been studied in detail (Dezzeo et al., 2003; Parolin et al., 2006; Schöngart et al., 2002; Worbes, 1997). Periodicity of wood production, tree ring formation in relation to phenology and climatic factors was studied in 24 tree species from semi-deciduous forest in Brazil (Lisi et al., 2008). Seasonal activity of vascular cambium and production of xylem in relation to different climatic factors in

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Fig. 1. Annual course of climatic parameters in the study area of P. kesiya; monthly means for the period of 2 years from March 2001 to February 2003 (data from Central Seismological and Meteorological Observatory, Government of India, Shillong station; arrow indicates separation of 2 years).

sub-tropical wet forest trees were reported only in Dillenia indica Linn. (Dilleniaceae) (Venugopal and Liangkuwang, 2007). However, this type of study has not yet been done with gymnosperms growing in sub-tropical wet forest regions. The present study on annual rhythm of cambial activity and xylem differentiation in relation to phenology as well as climatic factors has been undertaken in Pinus kesiya Royle ex Gordon, a dominant species in sub-tropical wet pine forest of Northeast India.

Materials and methods Study area and general climate The present study was conducted at the Laitkor Protected Forest of Meghalaya, in and around Shillong (25◦ 34 N; 91◦ 53 E; 1500 m a.s.l.). The elevation above sea level is characterized as a mountain or wet hill forest climate, with low temperature and relatively high precipitation. With increase in altitude, various climatic parameters such as pressure, temperature and precipitation undergo remarkable changes (Lal, 2002). The soil is a loam, reddish brown in color and lateritic in origin. The pH ranges from 5.7 to 06.4 (Mishra et al., 2003; Porwal et al., 2000; Singh, 1996). This study area belongs to the sub-tropical wet climatic regions (Champion and Seth, 1968). On the basis of variation in temperature, rainfall and wind, a year in this region may be divided into four distinct seasons: (1) winter (December–February), (2) pre-monsoon or summer (March–May), (3) monsoon (June–September), (4) retreating monsoon (October–November). The region receives abundant southwest monsoon during monsoon season (June–September) and during winter (December–February) and it receives occasional showers from northeast monsoon. Highest rainfall during the study period was in July (Fig. 1). In winter mean temperature ranges from 10.0 to 14.5 ◦ C and during summer mean temperature ranges from 17 to 20 ◦ C (Fig. 1).

Field observations Twig samples of 1.5–2 cm in diameter were collected from 10 Pinus kesiya trees at the end of every month. Collected samples were still in the field, immediately after sampling, fixed in FAA (formalinaceto-alcohol) and Glutaraldehyde in phosphate buffer of pH 7.2. The timing of the different phenophases such as formation of new needles, formation of male and female cones and their maturation, seed dispersal and needle fall was recorded from March 2001 to February 2003 from the forest stand. The sample materials were processed through customary methods of dehydration in alcohol series and propanol. The material was embedded in paraffin and glycol methacrylate (Technovit 7100® ). Sections were taken at a thickness of 5–10 ␮m by using Leica RM 2125 KT microtome and were stained either with Safranin and fast green FCR (Johansen, 1940) or toluidine blue O (Feder and O’Brien, 1968). For micro-measurements recently formed xylem tissue was carefully teased out and macerated according to Jeffrey’s method (Berlyn and Miksche, 1976). The numbers of cell layers in the cambial zone and widths of the cambial zone and the differentiating xylem zone were measured. The cambial zone included the cambial initials and their derivative mother and daughter cells (Waisel et al., 1966). The mean length values of fusiform initials and xylem tracheids were measured from slides of 100 randomly chosen fusiform initials and tracheids. All measurements were carried out with an ocular micrometer. Photographs were taken with Olympus BX 41 microscope. Data analysis The mean value and standard deviation were calculated for all measurements. Statistical analysis of relationship between climatic factors (monthly mean, minimum and maximum temperature, rainfall and relative humidity) and anatomical variables such as number of cambial layers, cambial zone width, differentiating xylem zone width, mean length of fusiform initials and mean length of tracheids were calculated by using Pearson’s correlation coeffi-

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Table 1 Phenology of Pinus kesiya growing in North East India. Different phenophases

Time period

Formation of new needles, three flushes in a year

First flush: middle of February Second flush: June second week Third flush: middle of October 1. Needles which flushed in February fall from October of the current year to March of the second year 2. Needles which flushed in second week of June in the first year fall during January to June of the second year 3. Needles which flushed in the middle of October in the first year fall during March to October of the second year Second week of March Middle of March to the end of April

Needle fall

Male cone formation Maturation of male cone and pollen dispersal Female cone formation Seed dispersal

During the first week of March in alternate year Next year April to May

cient. Multiple (partial) regression analysis was done and (t) values were calculated to determine the influence of a particular climatic factor on cambial activity and xylem production (Schweingruber, 1988; Zar, 1974). Number of layers in the radial file of the cambial zone and cambial zone width were used to indicate the cambial activity, and width of differentiating xylem zone and mean length of fusiform initials and tracheids were used to indicate the annual production of xylem. Monthly mean, maximum and minimum temperature, rainfall and relative humidity for 24 months (March 2001 to February 2003) were used as climatic variables.

width of the cambial zone increased considerably. The fusiform initials show a binucleate condition with 3–4 nucleoli in each nucleus (Fig. 3). Average data pertaining to them are shown in Table 2 for different months of the years 2001–2003. Cambial cells began to enlarge radially in the end of February, underwent periclinal division in the first week of March and reached peak in April with 8–10

Results Phenology of trees In Pinus kesiya the annual growth in terms of elongation of the branch apices commonly consists of three growth flushes with new apical buds being formed between two flushes. Thus three whorls of shoots were formed in 1 year, 1st in the middle of February, 2nd in the second week of June and 3rd in the middle of October. Therefore, P. kesiya exhibits recurrent flushing behavior of needles and branches thrice in a year, which is followed by three steps of needle fall. The needles appeared in the end of February drop from October of the current year to March next year; the needles which appeared in 2nd week of June are lost in the following January to June and those appeared in the middle of October are lost in March to October of next year. The cambial reactivation occurs 2 weeks after sprouting of the first flush of needle formation. The formation of male cones occurred simultaneously along with the first flush of needle formation in the second week of March. Pollen dispersal takes place from the middle of March to the end of April. Female cones develop in P. kesiya during March in alternate years. Maturation of female cones and dispersal of seeds take place from April to May in the subsequent years (Table 1). Cambial activity and differentiation of xylem In P. kesiya the vascular cambium is non-storied, with axially elongated fusiform initials and more or less isodiametric ray initials. Cambial rays are predominantly uni- to biseriate, but unicellular and occasionally multiseriate rays were also noticed. During the active period the cambial zone is wider and cell walls of both fusiform and ray initials are thin and surrounded by differentiating xylem and phloem elements (Fig. 2). In contrast during the dormant period, the cambial zone is narrow with relatively thick radial walls and surrounded by mature xylem and phloem (Fig. 4). The beaded nature of cell walls of fusiform initials becomes prominent in tangential longitudinal sections (Fig. 5). Initiation of cambial activity was marked by radial swelling of fusiform initials and active vacuolation and followed by periclinal divisions into fusiform initials. Consequently the number of cells in the cambial zone and the

Figs. 2–5. 2. Transverse section of stem showing the active cambium zone (CZ) consisting of 8–10 layers in the radial file and differentiating xylem zone (Dx). Vertical arrow indicates the width of differentiating xylem; phloem region (Ph). Bar = 110 ␮m. 3. Tangential longitudinal section of stem showing the fusiform initials (Fi) and ray initials (Ri) during active period. Note the binucleate condition of fusiform initials and cell walls of fusiform initials are very thin. Bar = 180 ␮m. 4. Transverse section of dormant vascular cambium consisting of only three layers in the radial file. The fusiform initials (Fi) contain starch grains (S) after iodine and potassium iodide test. Note the tracheids (T) are radially compressed and thick walled with narrow lumen. Bar = 30 ␮m. 5. Tangential longitudinal section of dormant vascular cambium consisting of fusiform initials (Fi) and ray initials (Ri). Note the beaded nature of thick cell walls. Bar = 150 ␮m.

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Table 2 Seasonal changes in the parameters of vascular cambium and its derivatives in P. kesiya. Months

Number of cambial cell layers

Average number of cambial cell layers

March April May June July August September October November December January February

8–10 8–10 7–8 7–8 8–9 8–9 7–8 6–7 5–6 3–4 3–4 4–5

9.0 9.0 7.5 7.5 8.5 8.5 7.5 6.5 5.5 3.5 3.5 4.5

± ± ± ± ± ± ± ± ± ± ± ±

0.52 0.81 0.67 0.78 0.78 0.67 0.84 0.70 0.67 0.52 0.51 0.51

Average of cambial zone width (␮m) 135.0 135.0 112.5 112.5 127.5 127.5 112.5 97.5 82.5 52.5 52.5 52.5

± ± ± ± ± ± ± ± ± ± ± ±

5.41 5.43 3.35 3.32 3.81 3.33 4.54 2.93 3.31 1.52 1.52 1.51

Average of differentiating xylem zone width (␮m)

Average length of fusiform initials (␮m)

300.2 ± 296.3 ± 250.6 ± 215.7 ± 196.1 ± 160.5 ± 110.2 ± 80.3 ± 60.6 ± – – –

2089.1 2690.2 2688.1 2744.6 2994.0 2905.6 2807.6 2526.8 1985.2 1562.4 1562.4 2086.6

15.12 11.8 8.03 8.61 5.81 6.42 3.31 2.43 2.42

± ± ± ± ± ± ± ± ± ± ± ±

146.21 174.82 188.12 233.23 209.51 203.32 224.61 101.12 119.13 110.81 110.42 166.91

Average length of tracheids (␮m) 2298.6 3093.7 3225.8 3293.5 3592.8 3341.4 3228.7 2779.5 2143.8 1710.7 1710.7 2190.9

± ± ± ± ± ± ± ± ± ± ± ±

160.81 247.51 241.92 230.54 287.00 200.42 242.12 166.76 150.00 119.71 114.42 131.43

–, no xylem production. ±, standard deviation.

cells in each radial file (Fig. 2). The cambial activity was then slowed down during May and June and remained more or less constant continuously from July to the end of November (Table 2). Cessation of xylem production and cambial dormancy were observed from the end of December to the first week of February. Crystals, starch and protein bodies were observed during the dormancy of the cambium (Fig. 4). Differentiation of xylem tissue The secondary xylem of P. kesiya consists of tracheids and xylem rays which are homogenously composed of only procumbent cells. The bordered pits are dispersed uniseriately across the radial surface aspect, with distinct bars of Sanio in between the bordered pits. Resin canals are mostly restricted to the latewood. Axial and ray parenchyma cells are present. Xylem production was noticed for a total period of about 9 months in P. kesiya (Table 2). It started with the formation of tracheids in the middle of March, nearly 20 days after the sprouting of new needles and branches. This tissue differentiation continued up to the last week of November. Tracheids produced during March were thin walled (8–10 ␮m) and the number of bordered pits per unit area (180/mm2 ) was more than found in the late wood elements (120/mm2 ). Resin canals were generally formed in the beginning of the differentiation of latewood elements and they continued to develop till the end of November. Tracheids abutting with xylem rays had half bordered pits. The latewood elements were radially more compressed and thick walled than those produced during other months (15–25 ␮m). The tangential lumen diameter of tracheids was large during their initial production in March and April (70–80 ␮m). During the onset of cambial activity ergastic substances were reduced in their amount and finally totally lost. Relationship between climatic factors, cambial activity and xylem production As the reactivation of cambium started by the second week of February and reached its peak activity during March and April, monthly mean temperature showed a better relationship with the number of cambial cell layers and cambial zone width, respectively (r = +0.81, t ≥ +2.99) than any other meteorological factor (Table 3). Therefore, the temperature seems to be the most important parameter influencing the initiation of cambial activity after dormancy. After cambium reactivation, there was an increase both in the cambial zone width and in the number of cell layers in the radial file. The cambial zone width showed good correlation with monthly mean temperature (r = +0.89, t ≥ +4.8). A negative corre-

lation was given with monthly mean maximum temperature and monthly mean minimum temperature. A positive but not significant correlation was given with rainfall data (r = +0.33). Thus the increase in the width of the cambial zone was most influenced by the changing temperatures. Correlation with relative humidity also gave a non-significant negative correlation (r = −0.33, t ≥ −0.87). However, relationships were slightly different between data of differentiating xylem zone width and climatic factors. Between monthly mean temperature and width of the differentiating xylem zone a high significant correlation existed (r = +0.90, t ≥ 5.23). But rainfall also correlated significantly with the differentiation of the xylem elements (r = +0.71, t ≥ +2.5). Other factors, such as mean minimum temperature and relative humidity, had negative correlations with the differentiation of xylem tissue. Mean maximum temperature, rainfall and relative humidity had a significant relationship with mean length of fusiform initials and mean length of tracheids (Table 3). Thus, changes in the length of fusiform initials and tracheids were probably influenced by several meteorological parameters, viz. maximum temperature, rainfall and relative humidity. It results that the cambial activity is mainly correlated with monthly mean temperatures whereas the differentiation of xylem elements could be correlated with several meteorological parameters, viz. mean temperature, rainfall, and relative humidity.

Discussion and conclusion Cambial reactivation and xylem differentiation of Pinus kesiya started 2 weeks after the onset of bud break. Such a pattern was observed also, e.g., by Fahn and Werker (1990) in sub-tropical and temperate plants, like Quercus boissieri Reut. (Fagaceae), Pistacia atlantica Desf. (Anacardiaceae), in Pinus strobus L. (Pinaceae) (Murmanis, 1971) or Pyrus communis L. and Pyrus malus L. (Rosaceae) (Evert, 1961, 1963). The same phenomenon was observed in the tropical species Tectona grandis L. f. (Verbenaceae) (Rao and Dave, 1981; Rao and Rajput, 2001a,b; Venugopal and Krishnamurthy, 1987). In P. kesiya new needles are produced thrice in a year as shown in Table 1 (see also Das and Ramakrishnan, 1986). Trees growing under different local climatic conditions show reactivation of cambium in different months (Zimmermann and Brown, 1971). But comparative studies of the timing of cambial activity in co-existing trees from sub-tropical wet forests are still lacking. Concerning within-genus comparisons, Pinus species growing in the Western Himalayas show similar phenological characters as that of P. kesiya (Dogra and Sahai, 1984; Sahai, 1987). Though there is shedding of needles intermittently, the tree never becomes leafless in any season of a year due to production of new needles in

+2.6* +2.6* +0.96* +0.96* +0.72* +0.73* +2.5* +3.5* +0.71* +0.71* +0.72* +0.82* −0.83 −1.38 +0.99 +0.99 −0.32 −0.27 +2.80* +2.78* +0.99* +0.99* +0.75* +0.75* Significant at p < 0.05. *

+0.93 +0.93 −0.20 +0.37

−0.50 +0.97

+0.96 +0.96 +0.96 −0.29 −0.33 −0.08 +0.40 +0.87 +2.5* +0.71 +0.71 +0.71* +0.18 +0.33 +0.71* +0.99 +0.99 +0.99 +0.93 +0.93* +0.93* +0.81 +0.89* +0.90*

Average number of cambial cell layers Average cambial zone width (␮m) Average width of the differentiating xylem zone (␮m) Average length of fusiform initials (␮m) Average length of tracheids (␮m)

t

+2.99 +4.8* +5.23*

+0.09 −0.02 +0.46

+0.99 +0.99 +0.99

+0.06 −0.05 +1.28

−0.11 −0.13 −0.57

−0.6 −0.33 −1.73

R2 r R2

*

R2 r

*

*

r

R2

t

r

t

r

R2

t

Monthly mean relative humidity (%) Monthly mean precipitation (mm) Monthly mean minimum temperature (◦ C) Monthly mean maximum temperature (◦ C) Monthly mean temperatures (◦ C) Climatic factors

Anatomical variables

Table 3 Correlation coefficient (r), coefficient of multiple determination (R2 ) and (t) values of cambium parameters and those of its derivatives versus different climatic factor.

+0.20 −0.87 −0.2

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t

202

three flushes, and this is paralleled by the extended period in production of xylem elements. In the Himalaya area of occurrence of P. kesiya its cambial activity is continuous throughout the year except during the winter season starting from December to the first week of February. The formation of male cones occurs simultaneously along with the sprouting of needles. Therefore, it is difficult to find out whether the cambial reactivation is more linked to leafing or to flowering or to both equally (Fahn and Werker, 1990; Reinders-Gouwentak, 1965; Venugopal and Krishnamurthy, 1987). Dormancy of the vascular cambium is imposed strongly by the climatic factors low temperature and precipitation. During winter no needle fall occurred, in contrast to most reports from tropical or Mediterranean deciduous trees (Dave and Rao, 1982; Fahn, 1982; Lipschitz and Lev-Yadun, 1986; Philipson et al., 1971; Venugopal and Krishnamurthy, 1987). Pinus kesiya has a distinct annual rhythm in cambial activity which results in the formation of discernible annual growth rings (Carlquist, 1980; Chowdhury, 1964; Fahn et al., 1986). Lipschitz et al. (1981) reported two flushes of xylem production in Cupressus sempervirens L. (Cupressaceae) growing in the Mediterranean climate and a similar phenomenon was reported in Tamarix aphylla L. (Tamaricaceae) (Fahn, 1958). Many tropical trees show multiple ring formation in correspondence with the number of bud breaks within a growth period (Amobi, 1974). The continuous production of xylem from middle of March to end of November in P. kesiya produces one xylem ring per year despite the three flushes of needles. Based on this annual periodicity, a corresponding tree ring pattern is formed in P. kesiya stems. Probably starch and crystals of calcium serve as source materials for new cell wall synthesis (e.g., carbohydrates and calcium pectate) when the cambial derivatives are rapidly produced. Riding and Little (1984) observed maximal and minimal starch content in the xylem of Abies balsamea (L). Mill (Abietaceae) associated respectively with the period of cambial dormancy and reactivation (see also Amobi, 1973; Essiamah and Eschrich, 1985; Parker, 1960; Pomeroy and Siminovitch, 1971; Sauter, 1966; Tsuda and Shimaji, 1971). When there are two flushes of cambial activity and dormancy, the accumulation and depletion of starch and calcium take place twice in a year (Venugopal, 1986; Venugopal and Krishnamurthy, 1987). Parolin et al. (2006) also reported that the stored carbohydrate is reutilized when there is demand in Amazon forest floodplain trees. The timing of reactivation of cambium, peak activity, xylem production and dormancy in P. kesiya was related here to variations in climatic factors such as temperature, precipitation and relative humidity. As the temperature rises from February onwards, reactivation of cambium occurs and reaches its peak activity leading to intensive xylem production during March–April in the present study. Pumijumnong and Wanyaphet (2006) studied the cambial activity in the same species Pinus kesiya growing on podzolic soil of low water holding capacity in Northern Thailand at an altitude of 1070 m a.s.l. They reported that cambial activity and xylem production showed significant relations rather to rainfall than to temperature. By contrast, P. kesiya growing in the sub-tropical wet forest of northeast India showed a significant relationship only with temperature for the reactivation of cambium and xylem production. These differences may be due to differences in the altitudes of the plant occurrences, and in particular to soil type and its moisture content. On the one hand, from our measurements correlations became obvious between amount of rainfall, relative humidity, and mean maximum temperature and the increase in length of fusiform initials and tracheids. Studies on Dillenia indica from lower altitudes in the Himalayas showed a positive relation between mean minimum temperature and cambial activity (Venugopal and Liangkuwang, 2007). This was not the case in P. kesiya, probably due to the altitude-related dif-

N. Dhirendra Singh, N. Venugopal / Flora 206 (2011) 198–204

ferences in local climate. Higher temperature was reported also to be conducive to cambial reactivation and xylem production in the boreal tree Picea glauca Moench (Pinaceae) (Gregory and Wilson, 1968). Kramer and Kozlowski (1979) summarized, that temperature can be a significant factor for bud break following cambial reactivation and subsequent shoot growth. On the other hand, a raise and fall in temperature under a Mediterraneantype climate was reported to have no effect on cambial activity in Cupressus sempervirens L. (Lipschitz et al., 1981) and in Eucalyptus camaldulensis Dehn. (Waisel et al., 1966). It appears that the temperature factor does not act independently from other influencing parameters, and the law of limiting factor may be in operation (Coile, 1936; Keen, 1937). The physiological mechanism by which higher temperature promotes cambial reactivation in many trees is not very clear. Wort (1962), on the basis of in vitro experiments, suggested that the increase in temperature is responsible for release of auxin reserves from tissues adjacent to cambium, and that in turn will activate the cambium cells. Catesson (1962), on the other hand, reported that the influence of increased temperature promotes vacuolation of fusiform initials. Relative humidity does not show clear relationship with cambial behavior. In the past, the role of rainfall under drought conditions on cambial behavior and xylem production was studied much more intensively than that of other factors. Higher rainfall was reported to be conducive to cambial reactivation in several plants growing especially in the tropics and under a semi-arid climate (Dave and Rao, 1982; Glock, 1955; Reinders-Gouwentak, 1965; Roger, 1981). In the present study the mean rainfall data correlated less with cambial activity than temperature. Rainfall probably is an important factor only in regions where soil moisture content varies greatly depending on rainfall (Rao and Rajput, 2000, 2001a,b). In P. kesiya cambial reactivation, peak activity of the meristem, and xylem production were not mainly limited by rainfall because in the sub-tropical wet forest of North East India usually enough soil moisture is available throughout the year (Amobi, 1974; Borchert, 1998, 1999; Fahn, 1959). Only during December–February the monthly mean precipitation is below 50 mm, but the soil type is oxisols where field capacity (30–40%) is high (Brady and Well, 2002; Pandey, 2004; Porwal et al., 2000; Triparthi, 2002). Cambial reactivation after dormancy rather was induced here by a slight increase in temperature i.e. from 15 to 18 ◦ C at the end of February. However, rainfall, relative humidity and mean maximum temperature values correlated with an increase in length of fusiform initials and tracheids in P. kesiya. For Pinus kesiya growing in sub-tropical wet forests of northeast India temperature played the most significant role in triggering cambial activity and xylem production. The reactivation of vascular cambium after dormancy is correlated with flushing of new needles and buds. Increased mean temperature and rainfall in spring probably have a synergistic effect on the peak activity of vascular cambium and xylem production and differentiation. Only the increase in the length of fusiform initials and its derivatives is better correlated with mean maximum temperature, relative humidity and rainfall. The relationships between environmental parameters and cambium activity of P. kesiya populations differ. In northeast India, with monthly precipitation rates >300 mm for 2/3 of the year and a high water storage capacity of the soil, but annual temperature fluctuations between 12 and 20 ◦ C, it is the increase of spring-time temperature that breaks cambial dormancy. In Thailand, with an annual course of temperatures between 20 and 30 ◦ C, but high precipitation rates only during 1/3 of the year (Pumijumnong and Wanyaphet, 2006) the water factor is distinctly superior to the temperature factor governing the activity of the P. kesiya axial meristem.

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Acknowledgements This study was carried out with the aid of a grant No. 38-0941/98/EMR-II received from the Council of Scientific and Industrial Research, New Delhi. Our thanks are due to the officials of Forest Department, Government of Meghalaya, who helped in the field collection. We also thank Head of the Department of Botany, North Eastern Hill University, Shillong, for providing all facilities to conduct this study. We also thank to Mr. S.C. Sahoo, scientist in-charge, Central Seismological observatory and Meteorology, Shillong Station, for providing climatic records.

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