Number Of Stem Units Research Articles

Witches' brooms in Siberian stone pine as somatic mutations and initial genetic material for breeding of nut-bearing and ornamental cultivars

For the raising of the Siberian stone pine (Pinus sibirica Du Tour) nut-bearing and ornamental cultivars, the most important traits are a dense crown, slow growth and precocity. Generative mutations of this kind are eliminated by natural selection, therefore, somatic mutation rsearch is important. Among somatic mutations, the most promising one is the so-called 'witches' brooms' (WB) where crown fragments demonstrate slowed growth and intensive branching. WB occasionally occurs in native populations. According to phytopathology textbooks, WB are caused by various pathogen species (viruses, mycoplasmas, fungi). The WB of this kind are characterized by a sickly look, full suppression of reproductive functions, a short life and a nidus pattern of distribution. There are also WB of different types: with a high vitality and fertility, a long life and sporadic distribution. They occur very rarely (about 1 per 10 000 trees) across the species' range. We investigated 18 trees with WB of this type. The size of WB ranged from 0.3 to 30 m, age varied from 30 to 300 years. Male cones were absent in all WB. Female cone initiation was normal if the WB was located in the top part of a crown. Scions from WB and a normal crown (NC) of the same tree were grafted on identical rootstocks. On average, the height of 7-year-old WB grafts (WBG) was 2 times lower, and the stem diameter was 2 times higher than in the NC grafts (NCG). It was achieved due to the fundamental differences in the shoot system morphogenesis. Here are three principal differences in decreasing order of importance: (i) WBGs were characterized by the absence or near absence of apical dominance. The NCG had no more than 3 orders of branching, and the length of the 1st order axis was on average 5 times larger than the axis of the 3rd order. The WBG had 6-7 orders of branching, and the length of shoots of 5-6th orders averaged 80-90% of the length of the first orders. (ii) At an identical shoot length, the number of lateral buds in the WBG was 3-4 times more. This superiority was achieved in several ways: a greater shoot number per node, the presence of one or even two additional nodes (summer shoot) on many annual shoots, the formation of lateral long shoots outside any nodes (instead of short shoots). (iii) The number of stem units, their length, and annual shoot length in the WBG were 1.5-2 times less compared with the NCG. The first seed cones were initiated in both variants in the year of grafting. However, by the end of 5-7 year period of supervision, the WBG surpassed the NCG in the number of cones by 5-10 times. WB and NC have also been tested on seed progeny (WBP and NCP). In the WBP halfsib families, a clear tendency of splitting into two classes was observed: (1) normal seedlings, statistically indistinguishable from the homogenous NCPs and (2) seedlings with growth essentially slowed down and very intensive branching. The ratio of the two classes was about 1:1. Therefore, the studied WB are a dominant somatic mutation. The principles of using of these mutations in breeding programs are discussed.

Effects of exogenous gibberellin and auxin on shoot elongation and vegetative bud development in seedlings of Pinus sylvestris and Picea glauca.

The hormonal control of stem unit (foliar appendage and axillary structure, if present, plus subtending internode) number and length was investigated in shoots of Scots pine (Pinus sylvestris L.) and white spruce (Picea glauca (Moench) Voss). Seedlings were treated with six gibberellins (GA1, GA3, GA4, GA5, GA9 and GA20) and two auxins (indole-3-acetic acid (IAA) and naphthaleneacetic acid (NAA)) when either neoformed growth was occurring or the terminal vegetative bud was developing. Hormones were applied by drenching the shoot tip, injecting the stem or spraying the foliage. Combined results for all three application methods indicated that shoot elongation in first-year seedlings (i.e., neoformed growth) was promoted in both species by GA1, GA3, GA4 and, less obviously, by GA9. This promotion was attributable to an increase in length, rather than number, of stem units. However, the number of stem units formed during terminal bud development, as reflected in the number of needles (white spruce) or cataphylls (Scots pine) present on the shoot resulting from the terminal bud, was stimulated by GA1, GA3 and GA4 in both species and by GA9 in Scots pine. The GA-induced increase in the number of preformed stem units was associated with increased bud width in white spruce and increased bud length and resulting shoot length in Scots pine. In contrast, application of IAA or NAA either did not affect or inhibited both neoformed growth and terminal bud stem unit number, depending on the application method and concentration. We conclude that, in the Pinaceae, (1) GA stimulates the activity of both the subapical meristem during neoformed growth and the apical meristem during vegetative bud development, and (2) the early non-hydroxylation pathway, via GA9, is the major route of GA biosynthesis. The role of auxin in the control of stem unit number and length remains to be resolved.

A retrospective early test ofPinus sylvestrisseedlings grown at wide and dense spacing

Stored seeds from 30 open-pollinated families that had been field tested for 28 years provided a retrospective opportunity to assess the efficiency of early testing in Scots pine (Pinus sylvestris L.). Seedlings height at the end of first (HGP1), second (HGP2) and third growth period (HGP3), and in the third growth period, leader length (LEADER3), basal stem diameter (DIA3), number of stem units (NSU3), mean stem unit length (MSUL3), aboveground fresh weight (AGFWT3), stem dry weight (STDWT3), and shoot and needle dry weight (SHNDWT3) were studied under wide and dense spacing. Families differed at the 1 or 0.1% level of probability. Differences between spacing were significant at the 0.01% level of probability for all traits measured in the third growth period. Families interacted significantly with spacing for HGP2, HGP3, LEADER3, AGFWT3, SHNDWT3 and MSUL3. Interactions were due to rank changes and the interaction component of variance varied from 26 to 96% of the family variance. Estimates of hi2over wide and dense spacing ranged between 0.23 ± 0.11 (HGP2) and 0.51 ± 0.14 (NSU3). Spacing did not alter the variance structure, hi2for each trait under wide or dense spacing were similar. However, spacing affected the phytotron-field genetic correlations. The average of 80 genetic correlation estimates was 0.52 under wide spacing and 0.29 under dense spacing. Averaged over eight field traits, the efficiency of indirect juvenile selection for the field traits indirect-direct selection percentage ranged from -15% for NSU3 under dense spacing to 99% for HGP3 under wide spacing.

Phenotypic Differences Between Natural and Selected Populations of Picea Abies. II. Apical Mitotic Activity and Growth Related Parameters

Apical mitotic index (MI) and growth of non-juvenile Norway spruce [Picea abies (L.) Karst.] populations growing in northern Sweden (63°54' N) were monitored in 1996. Annual leader shoot lengths and shoot growth components for the period 1990-1997 were measured in 1997. In 1997 populations transferred more than approximately 3° in latitude were found to be shorter than local populations. MI levels were initially high in all populations in mid-April. In spring, populations originating north of 63°30' N showed higher MI levels, and started shoot growth earlier, than populations originating further south. In autumn, MI levels were higher in populations originating south of 63°30' N than in populations originating further north and higher in seed orchard populations than in natural populations of similar latitudinal origin. The number of stem-units (NSU) was correlated with tree height and leader shoot length. NSU and elongation of stem-units appeared to be primarily influenced by summer temperature. Prolonged MI activity in autumn did not result in high NSU but appeared to be associated with a higher risk of frost damage to buds. The results are discussed in relation to climatic factors, seed transfer, selection and inherent growth rhythms.

A retrospective early test of <i>Pinus sylvestris</i> seedlings grown at wide and dense spacing

Stored seeds from 30 open-pollinated families that had been field tested for 28 years provided a retrospective opportunity to assess the efficiency of early testing in Scots pine (Pinus sylvestris L.). Seedlings height at the end of first (HGP1), second (HGP2) and third growth period (HGP3), and in the third growth period, leader length (LEADER3), basal stem diameter (DIA3), number of stem units (NSU3), mean stem unit length (MSUL3), aboveground fresh weight (AGFWT3), stem dry weight (STDWT3), and shoot and needle dry weight (SHNDWT3) were studied under wide and dense spacing. Families differed at the 1 or 0.1% level of probability. Differences between spacing were significant at the 0.01% level of probability for all traits measured in the third growth period. Families interacted significantly with spacing for HGP2, HGP3, LEADER3, AGFWT3, SHNDWT3 and MSUL3. Interactions were due to rank changes and the interaction component of variance varied from 26 to 96% of the family variance. Estimates of hi2 over wide and dense spacing ranged between 0.23 ± 0.11 (HGP2) and 0.51 ± 0.14 (NSU3). Spacing did not alter the variance structure, hi2 for each trait under wide or dense spacing were similar. However, spacing affected the phytotron-field genetic correlations. The average of 80 genetic correlation estimates was 0.52 under wide spacing and 0.29 under dense spacing. Averaged over eight field traits, the efficiency of indirect juvenile selection for the field traits indirect-direct selection percentage ranged from -15% for NSU3 under dense spacing to 99% for HGP3 under wide spacing.

Apical mitotic activity and growth in clones of Norway spruce in relation to cold hardiness

Seasonal development of apical mitotic activity and growth in three clones of Norway spruce (Picea abies (L.) Karst.) of the same age and origin but with differences in accumulated height growth and cold hardiness were investigated. The clones showed no consistent difference in mitotic index (MI), either in period or in general levels. The response of MI to temperature differed in spring and fall. Differences in cold hardiness between the clones was not directly coupled to differences in MI. Diameter growth ended earlier in one clone than in the other two clones, and this clone also produced lower numbers of stem units in both lateral and leader shoots. Cessation of diameter growth showed no relation to the duration and level of apical MI. The tallest clone had, as a combined effect of both size and number of stem units, significantly longer leader shoots than the other two clones. The greater leader shoot growth of the tallest clone relative to the other two clones during 1987-1996 was also most prominent after years with sudden drops in fall minimum temperatures to below ca. -12°C (median; interval: -11 to -13°C), following several weeks with mean temperatures above 3°C.

Spring growth of Douglas-fir seedlings from two bud development regimes

The effect of two bud development treatments on post-dormancy shoot growth was investigated with 1-year-old container-grown Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. A difference in the date of budset and number of overwintering primordia was obtained by applying 8-hour or 17-hour photoperiods in growth rooms during summer months. Seedlings were then chilled for 14 weeks at 4 °C and finally held 2 months at 22 °C under either an 8-hour or 16-hour photoperiod. The new shoot growth of seedlings with early budset had greater length, stem unit numbers, average stem unit length, and dry weight. Stem unit number of the post-dormancy seedlings was unaffected by photoperiod during flushing. Significant free growth occurred only in late budset seedlings and accounted for 40% and 46% of the fixed growth under 16-hour and 8-hour flushing photoperiods, respectively.

Effect of nursery culture on morphological development of western hemlock seedlings during field establishment. II. Survival, shoot length components, and needle length

Western hemlock (Tsugaheterophylla (Raf.) Sarg.) seedlings grown in two different container cavities that received four different dormancy induction treatments, short (SD) or long days (LD) in combination with moisture stress (D) or no stress (W) in the greenhouse, and lifted and placed in cold storage (November, January, or March), were planted on two adjacent coastal reforestation sites in British Columbia and monitored for survival and morphological development. Survival was greatest in seedlings grown in the larger cavities in the greenhouse, in seedlings lifted in March, and in seedlings not treated to moisture stress. Seedling shoots had more stem units on the southeast than the northwest site, but the amount varied with nursery treatment. Seedlings from the LD treatments produced more stem units during free growth and lammas growth than those from the SD treatment. Nevertheless, most shoot growth was predetermined in the buds during nursery culture, accounting for a minimum of 67% of the final number of stem units. Stem unit length (SUL) was longer in seedlings on the southeast site than on the northwest site for those treated to LD in the nursery. Seedlings treated to short days showed the reverse pattern (SDW) to this, or were unaffected by site (SDD). Shoots and needles were shortest for seedlings from the SDD treatment and for those lifted in November. Shoot growth was greatest for seedlings lifted in March and for those treated with LD, mainly owing to their longer SUL. Lammas growth was most frequent in seedlings from the smaller cavities, and in those from the November and March lifts.

Genetic variation in first-year slash pine shoot components and their relationship to mature field performance

Seedlings of 64 open-pollinated slash pine (Pinuselliottii var. elliottii Engelm.) families were grown from seed to 24 weeks of age in raised outdoor boxes under two nitrogen treatments (5 and 50 ppm). Twenty-six shoot characteristics were measured, of which the most promising 12 were evaluated for use in a multitrait selection index to predict parental breeding values of 5- and 15-year field volume growth. Genetic parameters were estimated for each seedling trait and shoot components were analyzed for their contribution to total height increment. Number of stem units was the most important contributor to total height in both nitrogen treatments. Heritabilities were generally higher for all traits in the high-nitrogen regime (h2 = 0.13–0.66). All traits displayed moderate to high genetic stability across both treatments (type B genetic correlations = 0.64–1.32). Total number of stem units (low-nitrogen treatment), free growth stem unit number (low-nitrogen treatment), and caliper (high-nitrogen treatment) exhibited the strongest genophenotypic correlations with 15-year volume (rjm = 0.35–0.39). All possible combinations of two- and three-trait indices were calculated to derive correlations between predicted genetic values and true genetic values (corr (g,ĝ)). Cyclic growth length (high-nitrogen treatment), total height (low-nitrogen treatment), and free growth stem units (low-nitrogen treatment) combined to form the most precise three trait index for predicting 15-year volume growth (corr(g,ĝ) = 0.56). Total number of stem units, total flushes, and total mean stem unit length in the low-nitrogen treatment along with cyclic number of stem units and caliper in the high-nitrogen treatment were also determined to be of potential use in a multitrait selection index based on their heritabilities, juvenile–mature correlations, and performance in two- and three-trait indices.

Variation in shoot growth components among western larch families

Shoot growth components, including predetermined growth, free growth, number of stem units, and mean stem unit length, were evaluated over two growing seasons (8th and 9th) for their contribution to variation in lateral shoot length of 14 families of western larch (Larixoccidentalis Nutt.). All families produced some free growth stem units in both years, averaging 21 and 20% of the total number of stem units in years 8 and 9, respectively. Significant variation was detected among families in total number of stem units and predetermined growth stem units in both years, in mean stem unit length and terminal shoot length at age 8, and in lateral shoot length and tree height at age 9. Lateral shoot length and terminal shoot length were highly correlated with each other, as well as with the total number of stem units and predetermined growth stem units in the lateral shoot. Free growth stem units in the lateral shoot at ages 8 and 9 were not significantly correlated with 8- and 9-year terminal shoot growth or height at age 9. However, the total number of stem units in lateral shoots was well correlated with 8- and 9-year terminal shoot length and with 9-year height. The implications of these results for early selection for increased height growth are discussed.

Genetic variation in shoot growth components and their correlations in Psuedotsuga menziesii var menziesii seedlings

Stem‐unit measurements could be useful for early selection if these dimensions were highly heritable and strongly correlated with traits of commercial interest, such as height growth. Height increments and the number and length of stem segments were mesured in the first and second growth period in predetermined and free growth of Douglas‐fir seedlings from 40 families in each of 2 populations from southwestern Oregon. Populations, or families in populations, exhibited genetic variation in all traits except number of stem units in predetermined growth. Heritabilities for stem‐unit measurements were higher than those for height increment in the first growing season, but not in the second. Correlations among measurements of stem units and height increments were only moderate (generally <0.70). Stem units are not likely to be better measurements of height in early selection of Douglas‐fir than are measurements of either predetermined or free growth.

Gain efficiency in short-term testing: experimental results

Height growth of loblolly pine (Pinustaeda L.) was measured in trees subjected to one of five irrigation and fertilization regimes in a closely spaced genetic test for 3 years. Shoot components of 3rd-year annual height increment were measured over two contrasting treatments. Juvenile height and number of stem units in summer growth length in the fully irrigated and fertilized short-term test regime exhibited (i) the highest juvenile-mature correlations (family mean correlation = 0.41–0.68), (ii) high individual-tree heritabilities (0.38–0.44), which were two- to three-fold higher than older tree values in a conventional genetic test of the same families, (iii) high genetic stability across two extreme short-term test treatments (genetic correlation = 0.61–0.80), and (iv) an efficiency in genetic gain per generation of 81–87% relative to selection on height at age 8 years.

Seedling shoot growth of loblolly pine families under two nitrogen levels as related to 12-year height

Seedling shoot elongation and biomass were examined for 23 open-pollinated loblolly pine (Pinustaeda L.) families grown in a greenhouse under two nitrogen regimes. Significant genetic variation was detected among families for the number of growth cycles, total seedling height, cyclic-growth elongation, the number of stem units, and biomass of stem free growth, cyclic growth, total stem, needles, and total shoot. Heritability estimates were generally high for shoot characteristics except for mean stem-unit length. Shoot biomass measures were weakly or negatively correlated with 12-year-height performance. Elongation of cyclic growth, number of growth cycles, and number of stem units showed stronger correlations with field performance than did seedling height. Juvenile–mature correlations were higher for the low nitrogen treatment, suggesting that 12-year height of loblolly pine families can be more accurately predicted when the seedlings are grown under greenhouse conditions that mimic field nitrogen levels.

Growth and morphology of western red cedar seedlings as affected by photoperiod and moisture stress

One-year-old container-grown seedlings of western red cedar (Thujaplicata Donn) were subjected to short days (8 h) and moisture stress, applied separately and in combination, and compared with untreated seedlings. The treatments were applied at various times during the growing season and lasted 2 and 4 weeks. Short days slowed height growth only when applied during the most rapid period of shoot elongation in late June and early July. Short days combined with moisture stress always reduced the final height regardless of the time and duration of applied treatments. Moderate moisture stress applied alone in mid-August hastened root dry weight gain but did not affect other morphological characters. The applied treatments had no lasting effects upon mitotic activity of terminal shoot apices and resulted in no differences in the timing of fall cessation of mitotic activity. The study describes details of the 1st-year growth of containerized western red cedar seedlings that were not subjected to the above specified treatments. It reports on growth rates for several morphological characters, such as height, number of stem units, root collar diameter, and shoot and root dry weights. Also, apical transitions in leaf initiation, stomatal distribution on leaf and cotyledon surfaces, and leaf wax features are described.

Influence of dormancy induction treatments on western hemlock seedlings. II. Physiological and morphological response during the first growing season on a reforestation site

Western hemlock (Tsugaheterophylla (Raf.) Sarg.) seedlings from four dormancy induction treatments (DIT) (i.e., long-day dry, long-day wet, short-day dry, short-day wet) were planted on a coastal reforestation site in British Columbia and monitored for physiological response and morphological development over the first growing season. Seedlings in all DIT showed a similar net photosynthetic, needle conductance, and shoot water potential response to seasonal low temperature, non droughty, and late summer drought conditions. Pressure–volume analysis at the beginning of the growing season (late February) showed short-day wet seedlings to have lower saturated and turgor loss point osmotic potentials than other DIT, while at the end of the growing season (October) there was no osmotic potential difference between DIT. At the end of the growing season, short-day wet seedlings had a much lower maximum modulus of elasticity than other DIT, indicating their shoots were still elongating. Morphological assessment 1 month after field planting showed short-day DIT seedlings had less needle damage and greater root development in response to continuous low temperature exposure. Four months after planting, long-day wet seedlings had the greatest shoot (i.e., height, root collar diameter) and root development in response to mild temperatures and high soil moisture conditions. Eight months after planting, survival was between 95 and 97% for all DIT. Long-day DIT seedlings had the greatest height and all DIT had similar root collar diameter and root development. Short-day DIT seedlings had the best shoot/soil roots ratio. Short-day wet seedlings had the least number of stem units per centimetre of new shoot development throughout the growing season. Results are discussed in reference to stock quality assessment presented in the first paper of this series.

Variabilité génétique des composantes de la croissance en hauteur du Sapin de Céphalonie (Abiescephalonica)

The annual height increment of the shoot can be divided into two morphogenetic components: the number of stem units initiated in the bud prior to bud break and their mean length. Observations were made on the lateral shoots of 43 Abiescephalonica open-pollinated families from three provenances located in an experimental site in southern France. Genetic parameters of the two components were evaluated with an analysis of variance. Correlations both between components and with annual height increment were examined at the phenotypic, genotypic, and individual levels. Differences between provenances were the result of number of stem units and annual height increment. Number of stem units presented a very high heritability and was strongly correlated to annual height increment at all variance levels. Mean stem unit length appeared to be more dependent on environmental variations and was less closely correlated to annual height increment, especially when genotypic data were considered. This particular growth strategy could be the consequence of A. cephalonica's adaptation to a Mediterranean-type climate. Of the two components studied, number of stem units was a better genetic predictor of annual height increment. A negative correlation between number of stem units and mean stem unit length was made apparent at the phenotypic and individual levels and should be considered when using number of stem units as a selection criterium.

Height growth components in inter- and intra-provenance jack pine families

Duration (D), rate of shoot elongation (RSE), number of stem units (NSU), and mean stem unit lengths (MSUL) of current year's (1987) leaders were analyzed in 18 jack pine (Pinusbanksiana Lamb.) full-sib progenies from nine inter- and nine intra-provenance crosses. The parent material consisted of selected "plus" trees from Ontario, Quebec, Wisconsin, and Michigan provenances. The average within-family variance (phenotypic) of D was on the average 186 days or twice as large as the among-family variance of 90 days. Duration was positively correlated with the number of growing degree-days at the place of parental origin. Tree height and height growth of several crosses significantly exceeded a local check lot, and the provenance effects on heights were predominantly additive (but not the luxuriance of hybrids). In determining annual height growth, it was discovered that D and RSE were roughly equally important, but large differences in RSE were observed among families with almost equal growth duration. The superior height growth of two interprovenance families was attributed to higher growth rates rather than a longer growth period. Although NSU was more important than MSUL in determining final shoot length, a negative correlation between NSU and MSUL complicated the overall significance of NSU. It was concluded that jack pine had sufficient variation in the examined height growth components to facilitate genetic improvement in height growth without adverse effects on periodicity. The provenance hybrids produced sufficiently promising results and additional exploitable phenotypic variation to warrant continued hybridization efforts.

Control of shoot elongation in ponderosa pine: relative roles of apical and axillary meristems.

Buds of ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) from which the shoot apical meristems had been surgically removed elongated normally but did not form new terminal buds. Shoot, stem unit and needle growth were similar to those of controls. Removal of needle fascicles from buds starting to elongate inhibited growth, whether or not the shoot apical meristem had been removed. The length of completely defoliated shoots increased by 8.3%, the length of shoots defoliated on the upper half increased by 29.6%, and the length of those defoliated on the lower half increased by 44.9%. The length of controls increased by 137.4% during the same period. On partially defoliated shoots, defoliated stem units elongated less than undefoliated ones. Shoots developing from buds enclosed in light-proof, aluminum-foil bags elongated normally but apparently lacked chlorophyll. Increases in dry weight were significantly correlated with number of stem units (r(2) = 0.53). Results suggest that shoot elongation is controlled endogenously. This control may be mediated by IAA outputs from axillary meristems proportional to meristem number, and the importation of carbohydrates, proportional to the output of IAA.

Effects of temperature on shoot growth in northern provenances of Pinus sylvestris L.

Effects of temperature on shoot growth in northern provenances of Pinus sylvestris L. were studied under natural long-day conditions (Lat. 69 degrees 39' N) at controlled temperatures. The optimum constant temperature for formation of stem unit primordia in the terminal resting bud was 18 to 21 degrees C. Stem unit number decreased linearly with temperature between 15 and 9 degrees C. Temperature during bud formation had a significant effect on final stem unit length achieved in the following year. Final shoot length was significantly affected by temperature during shoot elongation. The optimum constant temperature for elongation growth was 18 to 21 degrees C. The rate of shoot elongation was positively correlated with the number of stem units.

Relationships between shoot length and shoot length components in Douglas-fir and blue spruce

Number of stem units (NSU) and mean stem unit length (MSUL) were examined for their influence on first-order lateral shoot length in a 14-year-old, range-wide provenance test of interior Douglas-fir and an 8-year-old, range-wide provenance–progeny test of blue spruce. The degree to which each component influenced variation in shoot length depended on (i) the population, (ii) the level of population structure (between and within provenances), and (iii) whether phenotypic, genotypic, or environmental data were considered. In Douglas-fir, northern and southern trees were consistently different in MSUL. However, in northern and southern Douglas-fir considered separately and in blue spruce, provenance differences in shoot length were primarily due to variation in NSU. Within provenances, phenotypic variation in shoot length was caused equally by MSUL and NSU in both Douglas-fir and blue spruce. However, in blue spruce, genetic variation was mostly due to the MSUL component, while environmental variation was caused primarily by the NSU component. As a result, MSUL had a higher heritability than NSU. MSUL was also well correlated with tree height (within provenances) while NSU was not. MSUL and NSU were negatively correlated among provenance means, within provenances, on a phenotypic, genetic, or environmental basis. This limits the usefulness of these components as selection criteria and emphasizes the need for better understanding of shoot growth physiology to identify independent sources of variation in growth rate.