synchronisms and correlations of spring phenology between apical and lateral meristems in two ...

37 Pages·2016·0.85 MB·English

1 SYNCHRONISMS AND CORRELATIONS OF SPRING PHENOLOGY BETWEEN
2 APICAL AND LATERAL MERISTEMS IN TWO BOREAL CONIFERS
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4 SERENA ANTONUCCI1,2*, SERGIO ROSSI2,3, ANNIE DESLAURIERS2, FABIO
5 LOMBARDI1, MARCO MARCHETTI1, ROBERTO TOGNETTI1,4
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7 1 Dipartimento di Bioscienze e Territorio, Università degli Studi del Molise, Pesche (IS), 86090, Italia
8 2 Département des Sciences Fondamentales, Université du Québec à Chicoutimi, Chicoutimi (QC), G7HSB1,
9 Canada
10 3 Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Provincial Key
11 Laboratory of Applied Botany South China Botanical Garden, Chinese Academy of Sciences, Guangzhou,
12 China
13 4 The EFI project centre on mountain forests (MOUNTFOR), Edmund Mach Foundation, San Michele
14 all’Adige, 38010, Italy
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16 *Corresponding author: tel [phone] Fax [phone]
17 e-mail: [email protected]
18 Keywords: Balsam fir, black spruce, bud break, cambium, temperature, xylogenesis
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20
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21 Abstract
22 Phenological synchronisms between apical and lateral meristems could clarify some aspects
23 related to the physiological relationships among the different organs of trees. This study
24 correlated the phenological phases of bud development and xylem differentiation during
25 spring 2010-2014 in balsam fir (Abies balsamea Mill.) and black spruce [(Picea mariana
26 Mill. (BSP)] of the Monts-Valin National Park (QC, Canada) by testing the hypothesis that
27 bud development occurs after the reactivation of xylem growth. From May to September,
28 we conducted weekly monitoring of xylem differentiation using microcores and bud
29 development with direct observations on terminal branches. Synchronism between the
30 beginning of bud development and xylem differentiation was found in both species with
31 significant correlations between the phases of bud and xylem phenology. Degree-day sum
32 was more appropriate in assessing the date of bud growth resumption, while thermal
33 thresholds were more suitable for cambium phenology. Our results provide new knowledge
34 on the dynamics of spring phenology and novel information on the synchronisms between
35 two meristems in coniferous trees. The study demonstrates the importance of precisely
36 defining the phases of bud development in order to correctly analyse the relationships with
37 xylem phenology.
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39 Introduction
40 Phenology is one of the most responsive plant traits driven by climate (Badeck et al. 2004),
41 making trees reliable indicators of climate change (Donnelly et al. 2004). Phenological
42 observations record earlier events in terms of bud break, shooting, leafing and flowering,
43 which may be related to the recent spring warming and extended growing seasons (Chuine
44 and Beaubien 2001, Menzel et al. 2006). In spring, bud development is mainly regulated by
45 temperature (Sarvas 1972), although other interacting factors such as chilling and
46 photoperiod can play an important role (Heide 1993, Häkkinen et al. 1998, Partanen et al.
47 2001, Rossi et al. 2007). Air temperature is a good predictor of bud development and shoot
48 elongation, once ontogenetic development begins (Schwalm and Ek 2001).
49 Bud swelling is a phenological event of growth resumption composed of sequential events
50 during which the embryonic shoots and leaves rapidly proliferate and emerge from the bud
51 scales. Generally, studies on apical meristem have identified bud swelling with the phase of
52 leaf unfolding (Takahashi and Koike 2014, Cuny et al. 2012, Moser et al., 2009), while few
53 studies have taken into account all bud development phases (Dhont et al. 2010, Huang et al.
54 2014, Rossi and Bousquet 2014).
55 As with bud development, stem growth also occurs through different phases. Secondary
56 xylem production is a complex process that derives from cell periclinal and cataclinal
57 division in the vascular cambium. The differentiation of annual rings in conifers involves
58 the formation of tracheids that pass through several phases before reaching their final form
59 (Deslauriers et al. 2003). Several studies have revealed an early onset of cambium activity
60 and xylem cell production as well as a longer duration of xylem formation and this has
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61 often been related to the influences of the on-going climate change (Huang et al. 2011,
62 Boulouf-Lugo et al. 2012). The activity of meristems involves a number of biochemical
63 processes resulting in a sequence of phases of development and maturation. These phases
64 are identified according to the morphological or anatomical changes and can last from a
65 few days, in primary meristems, to several weeks, in the case of secondary meristems
66 (Rossi et al. 2013, Rossi 2015).
67 Aloni (2015) showed how leaf development (related to the primary meristems) and biomass
68 accumulation (related to the secondary, or lateral, meristems) are regulated by a number of
69 environmental factors. These factors influence the wood production and its biochemical
70 traits, but also the amount and differentiation of vascular cells. Moreover, physiological
71 factors, such as carbohydrate availability and hormone distribution, could determine the
72 dynamics of wood formation (Uggla 1998, 2001, Rossi et al. 2008). Studies on bud and
73 cambium phenology have revealed different patterns and links between primary and
74 secondary meristems. In conifers, the last phase of bud development, often identified as bud
75 break, has been found to occur either before or after the onset of xylem cell production
76 (Ladefoged 1952, O’ Reilly and Owens 1989, Rensing and Owens 1994, Rossi et al. 2009,
77 Cuny et al. 2012), while in diffuse porous species, cambial reactivation takes place
78 immediately after bud break (Schmitt et al. 2000, Čufar et al. 2008). Thus, the relationships
79 between apical and lateral meristems are still unclear, at least in conifer species.
80 Understanding these relationships is crucial for identifying the timings of C-uptake and
81 seasonal exchanges of water, nutrients and gases between the land surface and atmosphere
82 (Chen and Xu 2012). In order to elucidate the role of meristems as indicators of the effects
83 of climate change, in particular warming temperature, the relationships between primary
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84 and secondary growth need to be defined for a mechanistic understanding of the growth
85 dynamics in all tree organs (Huang et al. 2014).
86 There are still few ecophysiological studies aiming to explain the relationships between bud
87 and cambial phenology and their synchronism over the growing season (Deslauriers et al.
88 2009, Huang et al. 2014). Characterisation of the relationships between environmental
89 factors, particularly temperature, and meristem activity is crucial to understand the fate and
90 differentiation of meristem cells and their coordination under environmental stresses. Heat
91 accumulation above a threshold temperature, commonly expressed in degree-days, has
92 often been used to predict or verify the effect of temperature on biological processes
93 (Baskerville and Emin 1969). In this study, having monitored and compared the timings of
94 bud and xylem development in two conifer species of the boreal forest, we identified the
95 spring temperature thresholds and cumulated degree-days required for triggering the first
96 phases of bud and cambium development. The study was conducted during five growing
97 seasons, investigating in detail all the phenological phases for growth resumption of bud
98 and cambium and their climatic drivers, to improve understanding of seasonal patterns and
99 responses to inter-annual and long-term variation in climate. The hypothesis that the
100 beginning of bud development occurs after the reactivation of xylem growth was tested.
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102 Materials and Methods
103 Study site and tree selection
104 This study was conducted in the Monts-Valin National Park (QC, Canada). Two sites at
105 different altitudes were selected in the black spruce-feather moss bioclimatic domain,
106 Gaspard (abbreviated as GP) and Lagacé (LA), located at 227 and 900 m a.s.l., respectively
107 (Table 1). The climate in the region is dominated by dry and moderately polar air masses in
108 winter and moist maritime air masses in summer (Sheridan 2002). Absolute minimum
109 temperature reaches -41 °C, and mean air temperature ranges between -1.4 and 2.5 °C, with
110 annual precipitation higher than 1000 mm. The sites experience between 204 and 256 frost
111 days year-1.
112 Primary and secondary growth was monitored weekly from May to September, during the
113 2010-2014 growing seasons, in balsam fir [Abies balsamea (L.) Mill.] and black spruce
114 [Picea mariana (Mill.) BPS]. Three trees per species were randomly selected in each site,
115 choosing healthy dominant or co-dominant individuals.
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117 Bud phenology
118 Two north-facing and two south-facing branches per tree were selected in the bottom part
119 of the canopy. On each branch, the phases of bud development were recorded on terminal
120 buds according to Rossi and Bousquet (2014): (1) open bud, with the scales starting to
121 separate and a pale spot visible at the tip; (2) elongated bud, with lengthening scales; (3)
122 swollen bud, with smooth and pale coloured scales but no visible needle; (4) translucent
123 bud, with needles visible through the scales; (5) split bud, with open scales but needles still
124 clustered; and (6) exposed shoot, with needles completely emerged from the surrounding
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125 scales and spreading outwards. All data were computed in days of the year (DOY), by
126 averaging the data observed in the field.
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128 Cambial phenology
129 Wood microcores (2 mm in diameter) were extracted from the stem at breast height, using a
130 Trephor tool (Rossi et al. 2006a). The samples contained the preceding 4-5 tree rings and
131 the developing annual layer with the cambial zone and adjacent phloematic tissue. The
132 microcores were placed in Eppendorf microtubes with an ethanol solution (50% water) and
133 stored at 5 °C to avoid tissue deterioration. Microcores were dehydrated with successive
134 immersions in ethanol and D-limonene and then embedded in paraffin according to Rossi et
135 al. (2006a). In order to obtain thin slices, the samples were cut (7 µm in thickness) using a
136 rotary microtome. The sections were stained with cresyl violet acetate (0.16% in water) and
137 examined within 10-15 min under visible and polarized light at 400-500× magnification to
138 differentiate the developing and mature xylem cells.
139 The radial number of cells was counted along three radial rows, according to the criteria
140 described in Rossi et al. (2006b). Observations under polarized light discriminated the
141 zones of enlarging and cell wall thickening of tracheids. The progress of cell wall
142 lignification was detected with cresyl violet acetate reacting with lignin (Antonova and
143 Shebeko 1981). Lignification was shown by a colour change from violet to blue. The colour
144 change over the whole cell wall revealed the end of lignification and the tracheid reaching
145 maturity (Gričar et al. 2005). The dormant cambium consisted of 3-4 cells. In spring, when
146 the diameter of the new xylem cells was at least twice that of a cambial cell, the onset of
147 cell division was considered to occur. Xylogenesis was considered to have started when at
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148 least one radial file of enlarging cells was observed (Rossi et al. 2008) (Fig.1). In late
149 summer, when no further cell was observed in wall thickening and lignification, xylem
150 formation was considered complete.
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152 Weather stations
153 In both sites, a standard weather station was installed in a forest gap to measure temperature
154 and precipitation. Data were collected every 15 min during the five years of monitoring and
155 recorded as hourly averages by means of CR10X data-loggers (Campbell Scientific
156 Corporation, Logan, UT, USA). Daily mean values were calculated with the time series
157 obtained from the 24 measurements per day.
158
159 Data analysis
160 Timings of bud phenology were compared between species, DOY and years with analysis
161 of covariance (ANCOVA). Phases of bud and xylem phenology were compared using
162 analysis of variance (ANOVA), while the Tukey test was applied for multiple comparisons.
163 The correlations between phenological phases were tested through Pearson correlations.
164 Degree-days sum (DDS) at the onset of bud and cambium phenology were calculated as a
165 sum of the positive differences between mean daily air temperature and the threshold value
166 0 °C (Man and Lu 2010). This threshold was used for calculating DDS according to Man
167 and Lu (2010), who suggested 0 °C as an optimal base temperature for black spruce.
168 Thermal thresholds (THR) for bud and xylem phenology were assessed using logistic
169 regressions to calculate the probability of active meristem growth at a given measured daily
170 temperature. Binary responses were coded as non-active (value 0) or active (value 1)
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171 growth, and temperature thresholds were calculated when the probability of active
172 meristem growth was 0.5 (Rossi et al. 2008). Coefficient of variation was used for
173 comparing DDS and THR, given that these measurements take non-negative values. All
174 statistical analyses were conducted using JMP (Version 11, SAS Institute Inc., Cary, NC,
175 USA).
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177 Results
178 Bud phenology
179 The phases of bud phenology occurred earlier in fir than in spruce at both altitudes, though
180 showing different trends in the investigated years. The period of bud development was
181 shorter in fir than in spruce, and longer in 2010 and 2013 than in other years. In 2014, the
182 trend of the two species was similar at lower elevation, and almost identical at higher
183 elevation. Occasionally, some intermediate phases were not recorded, especially at low
184 elevation, because they occurred within the same sampling week. The first bud
185 development phase occurred in the second half of May. The two species showed a week of
186 difference for phase 1, occurring on average on DOY 138 and 145 in fir and spruce,
187 respectively. Phase 6 occurred approximately 4 and 5 weeks after phase 1 in fir and spruce,
188 respectively (Figure 2). In LA (900 m a.s.l.), bud phenology started 2 weeks later than in
189 GP (227 m a.s.l.). ANCOVA model was highly significant (p<0.001) for both altitudes,
190 with F=435.77 and 431.51 for 227 m and 900 m a.s.l., respectively (Table 2). As expected,
191 the bud development phases occurred on different DOY. The two species showed different
192 phenological development behaviour and the years considered differed from each other. All
193 the factors considered were statistically significant with p<0.001.
194
195 Xylem phenology
196 In the analysed cross section, cambial cells were characterized by thin cell walls and small
197 radial diameters. At the onset of cambial activity, the cambial zone began to widen rapidly
198 as the number of cells increased, revealing that cell division had started. During cell
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