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Ravetta, D.A. and D.A. Palzkill. 1993. Variation and broad sense heritability of branching frequency of jojoba. p. 358-359. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Variation and Broad Sense Heritability of Branching Frequency of Jojoba

Damian A. Ravetta and David A. Palzkill


  1. METHODOLOGY
  2. RESULTS AND DISCUSSION
  3. REFERENCES
  4. Fig. 1

Flower buds (and later fruit) are typically produced at every other node on new growth near branch tips of jojoba, Simmondsia chinensis (Link) Schneid, Buxaceae (Gentry 1958). Because of this, the number of nodes produced puts an upper limit on the number of flowers, fruit, and seed that a plant can produce. Casual observation of many plants and clones suggested that, in general, highly branched plants have more flowers and are more productive, and indicated that this could be a useful trait to select for in a breeding program. To begin to test this, variability of this trait must be quanified to obtain an estimate of its heritability.

Since the variance of a non-segregating population (i.e. cloned plants) must be environmental, such populations have been used to estimate the environmental component of the total observed variance (Falconer 1990). Once the environmental component is known, the genetic variation can be calculated by subtracting environmental variance from the total phenotypic variance observed. The genetic variance calculated this way includes not only additive variance, but also dominance, interaction, and epistatic components. The relationship between genetic variance and total phenotypic variance is, then broad sense heritability. The objectives of this work were to document the natural variation in branching frequency within 58 jojoba clones and to calculate broad sense heritability using the observed variability for branching frequency.

METHODOLOGY

To evaluate the effects of branching frequency on flower bud production, a survey of 58 jojoba clones growing in a production field near Hyder, Arizona was conducted during the summer 1989. The number of nodes, number of branches (tips) and the number of flowers were measured on three branches per plant and three plants per clone. The length of each branch was fixed by marking on each major branch the tenth node from the tip. All the tips, nodes, and flowers above that mark were counted. In a sampling done on a wide range of jojoba clones with different branching habits, ten nodes was found to be the minimum length of a branch needed to give an accurate portrayal of branching frequency of a complete plant.

The same population of jojoba clones was used to calculate broad sense heritability (H) for branching related traits. Genetic variance for the number of tips per branch, the number of nodes per branch and the relationship tips to nodes was derived from the mean squares of clones (MSc) and error (MSE) in a regular analysis of variance by separating out the variance components (s2) according to the following formula

(MScr = mean square of clone by replicate; MSE = mean square of the error term; MSc = mean square of clones; nr = number of replicates; nobs = number of observations.)

RESULTS AND DISCUSSION

The results of the survey of 58 jojoba clones illustrate the relationship between branching frequency and flower bud production (Fig. 1). The number of flower buds along the main stem of 10 nodes was not only correlated with the number of nodes per branch but also with the amount of branching along the stem. The relationship between the number of nodes and flowers is illustrated in Fig. 1a, where line A illustrates the regression for all 58 clones (three plants of each clone and three branches/plant). As the number of nodes in a branch segment increases, there is a linear increase in the number of flowers. The slope of 0.49 reflects that on average there is one flower at every other node. Line B shows the regression for three selected clones that have an average of about one flower per node (a slope of 0.93). Thus, the number of flowers increases as the number of nodes increases.

A positive relationship also exists between the amount of branching (number of tips) along the stem segment and the number of nodes (Fig. 1b). Clones with more branching showed an increase in nodes. As a consequence of the relationship illustrated in Fig. 1a and 1b, the amount of branching (number of tips) was also correlated with the number of flowers per branch (Fig. 1c). We conclude that an increase in branching frequency (number of branch tips/number of nodes) would likely increase node production (everything else remaining constant), and if the ratio of flower buds to nodes remains constant, then the number of flower buds would be increased.

Calculated broad sense heritability was estimated to be 0.36 for the number of tips per branch, 0.19 for the number of nodes per branch, and 0.84 for the tip to node ratio.

The low heritability estimates for number of nodes produced by a particular branch indicates high influence of many conditions which affect growth. The number of tips per branch is less influenced by the environment, although it has been shown that the dormancy observed in lateral buds can be overcome with the external application of plant growth regulators (Ravetta and Palzkill 1990; Ravetta 1990). Increases in the number of tips per plant 17 months after treatment application of up to 120% have been observed with applications of 6-benzyladenine and Promalin (a combination of 6-benzyladenine and gibberellic acid4+7). The use of gibberellic acid produced an increase of more than 150% probably as a consequence of a change in the plant's architecture. The ratio of tips to number of nodes is little influenced by environmental factors and this is borne out by results of plant growth regulators experiments. In conclusion, selection for branching frequency, which is related to flower bud production and presumably yield, should be effective.

REFERENCES



Fig. 1. Relationship between number of nodes, number of tips, and number of flowers for 58 jojoba clones growing at Hyder, AZ. Counts were done on three branches of equal length per plant, and three plants per clone. Each branch had 10 nodes along the main stem axis. (a) Regression for number of nodes and number of flowers , line A for the 58 clones, line B for three selected clones with an average of 0.93 flowers/node; s = slope of regression line. (b) Regression for number of tips and number of nodes. (c) Regression for number of tips and number of flowers.
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