Willem J. Steyn

Regreening of involucral leaves of female Leucadendron (Proteaceae) after flowering

Involucral leaves of Leucadendron have the remarkable ability to turn yellow upon flowering and regreen naturally as the florets of the inflorescence wilt. This colour change results from degradation of chlorophyll and to a lesser degree carotenoids, resulting in the unmasking of yellow colour. Chlorophyll levels were restored upon regreening. Degreening coincided with the complete dismantling of the thylakoid system, while keeping the outer plastid envelope intact. Regreening resulted from the complete redifferentiation of these gerontoplast-like plastids into functional chloroplasts. The colour change was directly linked to the development of the inflorescence. Complete removal of the inflorescence before flowering prevented the colour change while removal at full bloom, when involucral leaves were yellow, resulted in significantly faster regreening. This designates the inflorescence or florets as the possible origin of the colour change trigger and suggests that the colour change is involved with attraction of pollinators. Degreening and regreening also took place in a growth chamber under continuous high light intensity. Therefore neither pollination nor the presence of roots is required for regreening. It appears that colour change in Leucadendron results from a well-regulated degradation and subsequent synthesis of photosynthetic pigments.

Growth regulator manipulation of apple bud dormancy progressions under conditions of inadequate winter chilling

Elgin (34°S, 19°E; 305 m.a.s.l.), typical of South African apple growing regions, accumulates 745 Utah Chill Units (CU) p.a. The chilling requirement of ‘Golden Delicious’ apple (Malus xdomesf/ca Borkh.) is c.a. 1100 CU. Consequently, the chilling requirement is not satisfied and delayed foliation is common. The aim of this study was to use plant growth regulators (PGR’s) to manipulate the progression of dormancy in order to reduce the chilling requirement of ‘Golden Delicious’ shoots in mature commercial orchards. A trial was conducted in a commercial orchard in Elgin during the winters of 2004, 2006 and 2007. To advance the onset of dormancy, 250 mg −1 abscisic acid (ABA) was sprayed several times during April and May of 2004 and 2006. To induce a shallower state of dormancy, cytokinins were applied during April and May of 2006 and 2007. Benzyl adenine (BA) was applied at concentrations between 250 and 1000 mg −1 and forchorfenuron (CPPU) at 15 mg −1. Progression of dormancy was assessed by harvesting shoots every 2–3 weeks from initial spray date until commercial rest breaking agents were applied in the orchard. The time interval for 50% of the shoots to exhibit budburst under controlled conditions was used as a parameter for depth of dormancy. Although shoots were sprayed on c.a. the same calendar dates each year and before any significant CU had accumulated, the physiological state of the buds at application varied from shallow to deep dormancy depending on the season. Therefore calendar dates were not a good criteria for spray applications and CU accumulation was not a prerequisite for the onset of dormancy. PGR’s altered the dormancy progression of ‘Golden Delicious’ shoots. However, their efficacy was dependant on the dormancy status of the buds at the time of application. Furthermore, the effect was not sustainable. The trees appeared to “normalize” after a short period of time and consequently the PGRs had no effect on the dormancy release or budburst the following spring.

Chemical thinning of ‘Barouni’ and ‘Manzanilla’ olives with naphthalene acetic acid under South African conditions

Table olives grown in the Mediterranean-type climate of Western Cape province of South Africa are prone to alternate bearing, as elsewhere in the world. To date, South African producers have tolerated the negative effects of alternate bearing, but due to increased olive production costs, profit margins are decreasing and producers have to reduce alternate bearing. Trials were conducted to evaluate the use of naphthalene acetic acid (NAA) on ‘Barouni’ and ‘Manzanilla’, the most important green table olive cultivars grown in South Africa. NAA is used elsewhere in the world, but local research is required since the efficacy of NAA is modulated by environmental conditions and genotype. NAA decreased the fruit number per tree, thereby improving fruit size in both cultivars. Thinning did not affect return bloom in ‘Barouni’, possibly due to the low level of thinning achieved, whereas adverse climatic conditions during flower development may be to blame for the lack of return bloom in ‘Manzanilla’. Although NAA application did not affect the income per hectare, profitability may increase as harvesting costs account for roughly 50% of the input costs. Based on these results, 200 mg L−1 NAA, applied 10–15 d after full bloom, is recommended for local conditions.

Differential dependence of apple (Malus domestica Borkh.) cultivars on the xanthophyll cycle for photoprotection

The dependence of fruit peel photosystems of ‘Granny Smith’, ‘Braeburn’, ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ apple (Malus domestica Borkh.) peel on the xanthophyll cycle for photoprotection was studied under laboratory conditions. Mature fruit peel was either treated with 1 mm dithiothreitol (DTT) to inhibit the xanthophyll cycle or not treated. Fruit peel was subsequently exposed to photosynthetic active radiation stress of 1 500 µmol m−2 s−1 combined with heat stress of 45 °C for 3 h. Fruit peel photodamage was assessed by measuring the change in the maximum light use efficiency of photosystem II (Fv/Fm). The change in the concentration of the xanthophyll-cycle carotenoids zeaxanthin, antheraxanthin and violaxanthin plus lutein and β-carotene were analysed. The Fv/Fm of heat- and light-stressed DTT-treated (+DTT) ‘Granny Smith’ and ‘Braeburn’ peel had a low recovery after stress compared with the recovery Fv/Fm of similarly stressed peel not treated with DTT (−DTT). However, there was no difference in the recovery Fv/Fm between +DTT and −DTT ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ peel. The photosystem of ‘Granny Smith’ and ‘Braeburn’ fruit therefore appear to have a higher dependency on the xanthophyll cycle for photoprotection than ‘Fuji’, ‘Golden Delicious’ and ‘Topred’ fruit.

The function of regreening in yellow female Leucadendron (Proteaceae)

What do female Leucadendron stand to gain by regreening their involucral leaves rather than discarding them after flowering? Regreening of involucral leaves could contribute to the carbon budget and/or protect the developing cone-shaped inflorescence during seed development. This was determined by measuring photosynthesis before anthesis (green involucral leaves), at anthesis (yellow) and after anthesis (regreened) along with photoinhibition (Fv/Fm) and temperature measurements of the floral bracts of exposed and non-exposed inflorescences. The degree of carbon loss or reallocation in response to leaf removal was also determined. Photosynthetic rates of green involucral leaves were only 50% of foliage leaves and removal of involucral leaves did not significantly alter the size or dry weight of the inflorescence. Therefore involucral leaves do not contribute significantly to the overall carbon budget. Although involucral leaves did protect developing cones from photoinhibition and temperature-related sunburn, there was no significant difference in the severity of final damage between control shoots and shoots where involucral leaves had been removed. Regreening solely for protective purposes is therefore not assumed. The answer to why Leucadendron regreen remains elusive and a more long-term approach is suggested. This work has, however, eliminated the most obvious reasons for the regreening phenomenon.

Elucidation of seasonal pigmentation patterns in the involucral leaves of Leucadendron ?Safari Sunset?

This study investigates seasonal pigmentation patterns found in the involucral leaves of Leucadendron ‘Safari Sunset’, with the long-term aim of colour manipulation. Treatments comprised deconed shoots and shoots in which the opening of the flower head was physically prevented. During the early reproductive phase anthocyanin, chlorophyll and carotenoid concentrations were high, resulting in an unmarketable muddy colouration of the involucral leaves of control shoots. Towards anthesis there was a gradual decrease in chlorophyll, carotenoid and anthocyanin concentrations in controls. At anthesis the flower heads had opened up, coinciding with a rapid drop in chlorophyll and carotenoid concentrations, with a concurrent peak in anthocyanins. In deconed shoots, the flower heads did not open and all pigment concentrations remained fairly constant. Shoots in which flower heads were physically kept closed flowered normally, with pigmentation patterns similar to control shoots, except that anthocyanin levels remained constant. The red colour change in ‘Safari Sunset’ is directly related to the exposure of previously unexposed involucral leaf surfaces to high irradiance as the flower head opens towards anthesis. The changes in chlorophyll and carotenoid concentrations are correlated to the phenological development of the inflorescence, as changes occur irrespective of flower head opening, as long as inflorescence development continues to anthesis.