Seed potato physiological age and the components of yield 32

Potato yield differences have been strongly associated to PA of seed tubers. However, Van Der Zaag and Van Loon (1987) stated it is not always clear where the yield differences reported arise from. Size distribution responses to PA, for example, are usually reported using empirical correlations to total yield, stem number and date of planting. However, analyses of the main factors driving yield, expressed in Equation 1, are not often analysed together in a single study. As a consequence, PA has never been able to be incorporated into potato simulation models.

An alternative hypothesis in this thesis is that physiologically young potatoes are recommended for long growing seasons because seed potatoes emerge later, but produce larger canopies with longer duration, intercept more radiation and yield more than physiologically older seed potatoes. Conversely, physiologically older seed potatoes

emerge and start partitioning biomass to the tubers earlier than young seed potatoes but produce plants with smaller maximum leaf areas and have earlier canopy senescence.

2.12.6.1 Canopy formation and radiation interceptance

Seed potato PA has been suggested to affect canopy duration in potato crops (Firman et

al., 1995). Indeed Caldiz (2000) showed that PA altered the time of maximum radiation

interceptance and its duration. In this study the radiation interceptance increase was slower with young PA seed potato in the early season. In contrast, older PA seed potato advanced the time to maximum radiation interceptance in later plantings, but canopy duration varied with the cultivars used. In Uruguay, Sarries (2011) found that older PA seed reached maximum radiation interceptance faster compared with PA young ‘Chieftain’ potato for spring and autumn plantings. However, canopy duration averaged across season and years

was longer for the younger seed potato mainly because the crop from physiologically older potatoes senesced earlier.

O'Brien et al. (1983) showed that ‘Pentland’ emerged faster with increased PA (measured

as °Cd accumulated after sprouting; Tb=4 °C). In addition, for the early plantings LAI

increased with increasing PA up to 672 °Cd. Any further increase in PA caused LAI to decrease slightly.

These results contrast with the findings of Van Loon (1987). In this study, ‘Jaerla’ and ‘Désirée’ were stored at either 4 or 12 °C. ‘Jaerla’ stored at 12 °C showed incomplete emergence while ‘Désirée’ stored at this temperature emerged later, showed slower initial canopy development and, in one year, it had a later senescence than seed stored at 4 °C. In addition, the lateral stem growth (measure in mm) after the formation of the first flower (as an indication of foliage growth) did not seem to respond to different PA treatments in ‘Désirée’.

However, none of this research has provided a quantifiable value (e.g. accumulated °Cd) of the time of maximum radiation interceptance (or maximum canopy cover), duration of maximum radiation interceptance and rate of radiation interceptance decline (or canopy ground cover senescence) between different cultivars and for different potatoes of different PA. The quantification of these variables will be presented in this thesis. In addition, the amount of accumulated radiation intercepted will be compared between cultivars and PA treatments.

2.12.6.1.1 Individualleaf

Firman et al. (1995) tested the response of accumulated °Cd (Tb=4 °C) in storage over the

seed potato storage period and different planting dates on the rate of leaf appearance in six

cultivars in the UK. They found that phyllochron (measured as °Cd leaf-1_{; T}

b=0 °C)

differed among cultivars and ranged from 41.8 °Cd in young seed of ‘Home Guard’ to 27 °Cd in the old seed of ‘Maris Piper’. Old seed potato of cultivars ‘Home Guard’ (1128

°Cd) and ‘Estima’ (750 °Cd) had a shorter phyllochron (33 °Cd leaf-1) than young seed

potato (~40.5 °Cd day leaf-1) that had accumulated 0 °Cd during storage. However, in

another four cultivars tested, phyllochron was not different between old (750 °Cd) and young (0 °Cd in storage) seed potato and averaged 32.5 °Cd day leaf-1. In addition, the old

seed potato of all cultivars generally had a shorter phyllochron at later plantings (June and August) compared with earlier plantings (April and May). In addition, Firman et al. (1991) found that leaf number in the main stem was relatively stable among different cultivars for a range of PA seed potatoes, although older potatoes resulted in a higher number of leaves before the first inflorescence on the above-ground main stem.

2.12.6.2 Number of main stems and branches

According to Struik et al. (2006) the most noticeable manifestation of PA is in its effect on the number of stems emerged. However, the number of main stems produced per seed potato is highly cultivar-specific for a fixed PA (Struik and Wiersema, 1999).

Allen and O'Brien (1986) studied seed potato of cultivar ‘Pentland Javelin’ that had

accumulated between 0 and 840°C after sprout onset (Tb=4 °C) during storage. They found

that increasing the number of °Cd the number of main stems emerged decreased. These results were in agreement with those of Van Loon (1987), but contrasted with those of Sarries (2011) who showed an increase in the number of main stems emerged per plant from seed potato that had accumulated more thermal-time during the storage phase.

Moreover, Struik et al. (2006) showed that stem density usually increases when higher

temperatures are applied for a short period and then decreases after an optimum temperature has been accumulated during storage.

2.12.6.3 RUE

Of all the components of yield, RUE is the least investigated in PA studies. This seems reasonable because data on cumulative radiation intercepted is also scarce. For a study done in Uruguay using different PA treatments, RUE was shown to be conservative (Sarries, 2011). However, this study was done for an early crop and the same conclusion could not be extrapolated to a main crop.

2.12.6.4 Dry matter partitioning to tuber and HI

Van Loon (1987) found no difference between seed stored at 4 and 12 °C for the time of

ageing (chronologically and physiologically) anticipated the time of tuber initiation. In addition the number of below-ground nodes has been reported to increase with increasing PA (Firman et al., 1991).

Sarries (2011) found that physiologically older seed (~1000 °Cd accumulated during

storage; Tb=4 °C) started partitioning DM to tubers earlier compared with the young seed

(0 °Cd accumulated). This difference seemed consistent throughout many of the different seasons and years examined. In addition, the HI was found to be higher for older (0.78) than physiologically young (0.73) potatoes.