Development of Physiological Age 26

The development process of physiological ageing of a seed potato tuber has been reported to start from tuber initiation (in the mother plant) and progress until new tuber formation (Caldiz, 2009). According to this author, before dormancy breakage, PA is entirely reflected by the mother tuber. Moreover, Struik (2007) divided the development of PA in the seed potato into five stages: 1) dormancy (no sprout), 2) apical dominance (only one sprout), 3) normal sprouting (many sprouts per seed tuber which are often branched), 4) senility (excessive sprouting with very weak sprouts) and 5) incubation (little tuber formation). An overview of the sequence of physiological stages of a tuber, as well as the general effects on crop growth and development from them is described in the following sections.

2.12.2.1 Dormancy phase

At harvest, potato tubers are dormant and will not sprout even if placed in suitable environmental conditions (Reust, 1986; Van Ittersum, 1992; Struik and Wiersema, 1999). This dormant state can be affected by both the pre-harvest and post-harvest environment (Suttle, 2004). The duration of tuber dormancy also differs among varieties and is influenced by the date of harvest and the state of maturity of the tubers when they enter storage (Hay and Porter, 2006). During this phase, immediate morphological changes do not occur in the tuber albeit biochemical and physiological processes take place. However, this phase influences the number of sprouts produced after breaking of dormancy and their growth vigour, and directly affects the progress of physiological ageing (Struik et al., 2006).

While in storage, seed potatoes experience two forms of dormancy: instantly after harvest there is a phase of ‘innate dormancy’. This may last for several weeks during which the nodes (or buds) are impeded to induce growth regardless of any alteration in the environmental conditions. This can be followed by a state of ‘induced dormancy’. In this case sprouting is inhibited in the non-dormant tuber buds by suppressing factors like low temperature (Hartmans and Van Loon, 1987; Hay and Walker, 1989). In addition to that,

three dormancy types possible in potatoes seed have been described: 1endodormancy,

2_{ecodormancy and} 3_{paradormancy (Lang et al., 1987; Suttle, 2007; Vreugdenhil, 2007).}

Once both endodormancy and ecodormancy have been broken; sprouts > 1 mm (Celis-

Gamboa et al., 2003/4), 80% showed one sprout of 2 mm (Van Ittersum, 1992; Van

Ittersum and Scholte, 1992) or sprouts >3 mm in 80% of the seed potato tubers (O'Brien et al., 1983; Allen and O'Brien, 1986; Struik et al., 2006), the rate and pattern of sprout development are determined by the temperature and duration of storage (Coleman, 2000; Hay and Porter, 2006). The potato tuber then goes through a gradual breakdown of apical dominance (paradormancy) where progressively more sprouts are released from dormancy and begin to develop. This last process can also be influenced by different temperatures in certain cultivars. In some cultivars a high number of sprouts can be produced, in others only a few sprouts develop on the tuber (Struik and Wiersema, 1999).

2.12.2.2 Sprout phase

The loss of tuber dormancy (endodormancy or ecodormancy) and the onset of sprout growth are accompanied by biochemical changes (Suttle, 2004). This is because sprouting mobilizes starch and consumes a portion of the tubers biochemical reserves to grow sprout tissue. This leads to elevated sugar content, weight loss and wilting of the tubers (Daniels-

Lake and Prange, 2007). According to Espen et al. (1999), the increase in sucrose uptake

capability during dormancy may be linked to a reaction of the transport mechanisms. Therefore, the same authors speculated that an increase in transport activity was the triggering event in the dormancy release.

After the onset of sprouting the behaviour of the sprout is influenced by the age of the mother tuber (Struik and Wiersema, 1999). However, this influence may also be determined by storage conditions (Struik and Ewing, 1995; Struik and Wiersema, 1999)

1 _{endodormancy - the growth of the meristem is arrested due to factors within the}

structure itself;

2 _{ecodormancy - the meristem is arrested due to environmental factors, e.g., low}

temperatures prevent or delay sprouting of buds;

3_{paradormancy - the meristem is arrested due to external physiological factors, e.g., a}

and modified by conditions and treatments that directly interfere with the functioning of the sprout; (e.g. temperature, diffuse light or de-sprouting), (Caldiz et al., 2001).

Struik and Wiersema (1999) used a scale influenced by time (chronological age), and principally temperature, over the storage period to describe how PA progresses from the end of dormancy to total decay (when tubers no longer present vigour). Within this period,

both 4_{sprout capacity and sprout number increase up to their maximum followed by a}

decrease. The growth vigour of the seed potato reflects the combination of these two and may be manipulated to improve yield according to crop cycle at different physiological states. In agreement with this, Van Der Zaag and Van Loon (1987) have demonstrated that maximum growth vigour may be earlier, may last for a shorter period and may decrease when the physiological ageing process is hastened. The practical significance of this is that early potato growers could maximize their yield by planting physiologically old seed for the earliest harvests and progressively younger seed for later harvests (O'Brien et al., 1983). However, past a certain age the tubers may produce slender, weak sprouts, in the extreme termed “hairy sprout disorder” (Daniels-Lake and Prange, 2007), compromising crop establishment. Conversely, the use of physiologically young seed potato can be an advantage for long growing season (e.g. main crop potatoes). Struik (2007), for example, stated that generally, physiological younger potatoes provide higher yields when grown until full maturity. The reasoning is that younger seed potatoes emerge later, have fewer stems per seed tuber, show later tuberization but less secondary growth, have more foliage growth, more tubers per stem and a later maturity. Therefore, the challenge is to implement methods to manipulate sprouting to occur only when it is desirable, which will depend on the growing season and potato end-use (Daniels-Lake and Prange, 2007).

In reality, on some cultivars over 40 leaf primordia may be formed on the sprout during storage. In other cultivars, the apical differentiation (and thus the production of new leaf

primordial or nodes) is arrested after some time (Firman et al., 1991). The same authors

showed that initiation of the flower primordia may already start on the sprouts before planting. Physiologically this means that for some cultivars, and under certain storage

4_{Sprout capacity - defined as sprout growth (or re-growth after removal of the present sprouts),}

conditions, the size of the first level of the main stem is determined before planting. These two different types of behaviour during storage suggest that the development of leaf and flower primordia should be taken into account when explaining effects of PA on seed tubers.

To avoid any confusion the onset of sprout growth >3 mm in 80% of the seed potato tubers (O'Brien et al., 1983; Allen and O'Brien, 1986; Struik et al., 2006) will be defined as the end of dormancy in this thesis, since onset of sprout growth may occur following cold storage, long after natural dormancy has ended.