Better By Tim Spann Growing Research Project Manager

The Architecture of Avocado Trees and Why it Matters

ll tree crop growers struggle to balance Mother Nature’s Anatural tendencies with their desire for high, consistent fruit pro- duction. To do this, they manipulate the trees by pruning, trellising, con- trolling fertilizer and water inputs, and using chemical growth regula- tors. The challenge to achieving high, consistent yield in avocados is argu- ably more difficult than in other tree crops that have been domesticated for thousands of years and have been bred so heavily that they bear little resemblance to their wild relatives. To manipulate an avocado tree hor- ticulturally, we need to understand how it grows because we ultimately have to work within Mother Nature’s bounds. In general, avocado trees have a round canopy with dense foli- age, but the form of avocado trees vary — Fuerte are characterized as spreading; Hass as rounded; Bacon as upright. The form of the avocado tree is also influenced by the pre- dominance of proleptic and sylleptic shoots and this, in turn, can influence Figure 1. A diagram showing the current season’s shoot growth arising from a avocado yields. terminal bud from the previous season. The current season’s growth occurred in In general, avocado trees grow the three flushes, with the formation of a terminal bud in between each flush. The in a rhythmic pattern with periodic remains of the terminal buds are visible later in the season as bud-scar rings be- — or seasonal — shoot growth. tween each flush. The diagram shows flushes with two types of lateral branches – Shoots grow, then stop; grow, then proleptic and sylleptic. The spring flush developed with no lateral branches (strong apical dominance), thus the axillary meristems became axillary buds (left). The stop. In California, shoot growth spring flush axillary buds grew at the same time that the summer flush elongated, usually consists of a spring flush (re- forming proleptic branches on the spring flush (note the bud-scar rings at the base productive), an early summer flush of these lateral branches; center). On the autumn flush, axillary meristems grew (vegetative) and an autumn flush at the same time the main axis was elongating (weak apical dominance), form- ing sylleptic shoots (note the absence of bud-scar rings on these lateral branches; (vegetative) (Fig. 1). This is impor- right). tant to recognize because this rhyth- mic pattern of dormancy and growth

12 / From the Grove / Spring 2014 — and do not have a bud-scar ring to the base of the shoot are released (Fig. 1). Because they do not arise from dormancy and do not become from dormant buds, sylleptic shoots dominant over the main shoot axis. do not have pre-formed nodes, and A tree with strong apical control (ba- can continue to grow if conditions sitonic growth) will have the central are favorable (“free” growth). Ide- axis remain dominant and thus will ally, avocado trees should have a mix have one clearly defined trunk. A tree of proleptic and sylleptic shoots for with weak apical control (acrotonic maximum productivity. growth) will have several large major The form of an avocado tree is limbs or scaffold-like branches. due to the interaction between api- Apical control takes place in cal dominance and apical control. the second and subsequent years of Together, these determine the domi- shoot growth when apical control nance of proleptic or sylleptic shoots either a) leaves the bud dormant or of a tree. b) allows the bud to reawaken and Apical dominance is the ex- elongate and thicken into a proleptic tent to which an actively growing branch. Buds that remain dormant shoot tip or terminal bud inhibits enter what is commonly referred to the growth of axillary buds (at the as the “bud bank” and can later form junction of the stem and leaf) further epicormic shoots, or water sprouts, down the shoot. In short, when an av- generally in response to major limb ocado tree forms a lateral meristem, breakage or severe pruning (e.g. apical dominance determines wheth- stumping). While auxin plays a role er the meristem forms a bud or a syl- in apical dominance — and thus the leptic branch. The growing shoot tip development of sylleptic branches — or terminal bud produces the plant its role in apical control is less clear. hormone auxin, which inhibits the Researchers are examining hormonal Figure 2. An illustration showing the loca- growth of lateral buds. (Growers im- and nutritional mechanisms to better tion of a terminal bud at the shoot tip and axillary (lateral) buds along a shoot where itate this response by applying Tre- understand their role in apical con- the leaves join the shoot. Hold, a synthetic auxin, to pruning trol. cuts to prevent shoot growth.) Api- When you put all of these fac- cal dominance is used to refer to the tors together — apical dominance, control the apical portion of a shoot apical control, acrotony, basitony plays a role in determining which has over lateral buds along that shoot — you can begin to understand why shoots develop when. during the year that the shoot first avocado trees take the form they do. Proleptic (or unbranched) grows. Because apical dominance Researchers examining shoot growth shoots develop from dormant ter- inhibits the growth of axillary buds and tree architecture discovered the minal or axillary (lateral) buds (Fig. when the shoot main axis is extend- following: 2) once the parent shoot has stopped ing, it involves the inhibition of syl- ‘Gwen’ and ‘Reed’: strong api- growing (Fig. 1). Proleptic shoots leptic — not proleptic —shoots. cal control, strong basitony, few ma- have a bud-scar ring at their base and Apical control refers to the in- jor limbs, short axillary shoots, few can be vegetative or reproductive. fluence apical portions of the crown proleptic shoots, weak apical domi- Because proleptic shoots originate have over the general tree form by nance and large numbers of sylleptic from dormant buds, their growth is releasing resting buds in subsequent shoots. Tends to be a more compact considered “fixed.” That is to say, the years of the tree’s life. How apical tree. number of nodes the shoot will have control is manifested determines tree ‘Sharwil’: weak apical control, when it grows is pre-formed (micro- form. Acrotony is a specific type of strong acrotony, many major limbs, scopically) within the dormant bud. apical control in which the buds clos- long and numerous proleptic shoots, On the other hand, sylleptic (or est to the terminal of the shoot are strong apical dominance and few syl- multi-branched shoots) develop from released from dormancy and become leptic shoots. Tends to be a taller axillary buds while the main shoot is dominant over the main shoot axis tree. still growing — without there being (Fig. 3). Basitony is a type of api- ‘Hass’: intermediate between an intervening bud dormancy period cal control in which the buds closer ‘Sharwil’ and ‘Reed.’ Tends to be of

14 / From the Grove / Spring 2014 intermediate size. Researchers have also found that the tendency to develop proleptic or sylleptic shoots is dependent upon the age and the location of the grove. They found that younger avocado orchards tended to have more syl- leptic shoot development, while pro- leptic shoot development increased as orchards aged. This is in keeping with research noting that apical con- trol (and thus proleptic shoot devel- opment) takes place in later-stage growth. Researchers continue to ex- amine the effect grove location plays on avocado shoot development. The concepts presented here describe characteristics of avocado tree growth in its natural state. In the next issue of From the Grove, we will examine how understand- ing these concepts allows us to make management decisions (e.g., pruning) Figure 3. A representation of two types of lateral branch development, acrotonic to manipulate tree growth to try to (A.) and basitonic (B.). improve yields and fruit quality.

Terminology

Apical control – the control of growth of proleptic branches

Apical dominance – the inhibition of growth of sylleptic branches leading to the formation of axillary buds

Epicormic shoot – rapidly growing, near vertical shoots that develop from buds that have been dormant for more than two years (commonly referred to as water sprouts)

Meristem – a region of undifferentiated cells in a plant that is found where growth can take place; meristems are found in areas such as shoot tips, lateral buds, and root tips

Proleptic shoot – lateral branches that develop from axillary buds after a period of dormancy

Sylleptic shoot – a lateral branch that develops from an axillary meristem without a period of dormancy while the main shoot axis is still

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