How to Reduce Yield Fluctuations in Sweet Pepper?

Flushing, i.e. cyclic fluctuations in fruit yield, is an important problem in sweet pepper production. These fluctuations result from cyclic variations in fruit set. Therefore, the relationship between fruit set and plant sink and source strength was investigated in several greenhouse experiments. A decrease in plant source strength by decreasing inter-plant distance or light intensity or by leaf removal led to a decrease in fruit set. For example, the percentage of fruit that set was 32, 20 and 16% at a plant density of 1.6, 3.1 and 4.6 plants m, respectively. Plant sink strength was varied by the number and position of competing fruits on the plant. Fruit set decreased with increased plant sink strength, e.g. in plants with 0, 1, 2 or 4 earlyformed fruits, fruit set of four later-formed flowers was 78, 56, 28 and 6%, respectively. Average rate of fruit set could be predicted from the ratio between plant source and sink strength. Removal of some flowers and/or young fruits in periods of abundant fruit set did reduce the cyclic fluctuations in fruit set and yield, but less than expected. When planting in late spring, as opposed to a winter planting, yield fluctuations at farm level could be substantially reduced by adopting two planting dates with four weeks in between. An increase in seed number, resulting from higher pollen load on the stigma of flowers, increased the inhibitory effect of a fruit on set and growth of later developing fruits. Variation in fruit set and yield was strongly reduced in plants with parthenocarpic fruit growth, obtained by preventing pollination through applying a lanolin-auxin paste onto the stigmas of all flowers on a plant. Hence, developing seeds in sweet pepper fruits are an important cause of the abortion of new flowers, and irregular fruit set and yield. INTRODUCTION Sweet pepper (1194 ha in 2001) is the most important greenhouse vegetable crop in The Netherlands. Its annual production value is equivalent to € 391 million (in 2001). Typically, sweet pepper crops are planted late November or early December, fruit set commences from the first weeks of January, first ripe fruits are harvested in March and final harvest is at the end of October. Yield levels of red sweet peppers are 25-30 kg per m per year (KWIN, 2002). An important problem in sweet pepper production is ‘flushing’: weeks with high yields are alternated by weeks with low yield (Fig. 1). This irregular harvest pattern makes it difficult for growers to meet regular weekly demands. As this cyclic production Proc. XXVI IHC – Protected Cultivation 2002 Ed. A.P. Papadopoulos Acta Hort. 633, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA) 350 pattern is more or less the same for all growers, it results in weeks with a high market supply and low prices alternating with weeks with a low market supply and high prices. Flushing also results in strong fluctuations in labour demand in the greenhouses. Cyclic fluctuations in yield result in variations in fruit load on the plants. In periods with low fruit load, high initial fruit growth rates may occur, resulting in blossom-end rot (BER), a physiological disorder, caused by local calcium deficiency during the initial stage of fruit development (Bangerth, 1979; Marcelis and Ho, 1999). BER causes high economic losses in sweet pepper production, as affected fruits have little value. Avoiding an irregular pattern of sweet pepper production is of great economic importance. Sweet pepper plants show irregular fruit set, which seems to be the main reason for yield fluctuations (Fig. 1). Furthermore, abscission of flower buds, flowers and fruits is an important yield-limiting factor (Wien et al., 1989). In each leaf axil typically one flower is formed. Several fruits set at more or less the same time. A number of axils above these fruits show severe abortion, followed by axils with good fruit set. This leads to the following hypothesis: the presence of developing fruits inhibits subsequent set and growth of new fruits both by competition for limited assimilates as well as by dominance due to the production of plant growth regulators (Marcelis and Baan Hofman-Eijer, 1997). Flowers initiated on the plant in this phase will generally abort. Only when the early formed fruits are almost full-grown and their sink strength (competitive ability to attract assimilates) is low, new fruit set can occur. Hence, on a sweet pepper stem, a group of nodes with fruits is followed by a group of nodes without fruits, explaining the periods of high and low yields. In this paper, support for the above-mentioned hypothesis, that the irregular fruit set in sweet pepper is positively correlated to fluctuations in plant source:sink ratio, is presented. Based on this hypothesis, practical solutions to overcome the problem of yield flushing are discussed, supported by experimental data. Data presented here are based on a large number of published and unpublished experiments conducted over the last 10 years. Here, a brief overview of the most important results is given. MATERIAL AND METHODS Unless stated otherwise, experiments were conducted with Capsicum annuum L. cv. Mazurka (red-fruited), grown in either aerated nutrient solution in climate chambers or on rockwool slabs in compartments of a multispan Venlo-type glasshouse in Wageningen (latitude 52°N). Plants show a dichotomic branching and were pruned to one or two main branches per plant with side shoots stopped at one leaf. Hence, for each node position on a stem there were two flowers, one on the main stem and one on the decapitated side shoot. INFLUENCE OF SOURCE AND SINK STRENGTH ON FRUIT SET Source Strength The influence of a decrease in source strength (assimilate supply) on fruit set was studied in three experiments by decreasing inter-plant distance or light intensity or by leaf removal. In all cases a decrease in source strength led to a decrease in fruit set. In a greenhouse experiment (planted in January and ended in July) the average proportion of fruit that set was 32, 20 and 16% at a plant density of 1.6, 3.1 and 4.6 plants m, respectively. In a greenhouse experiment, planted in August, plants were pruned to one main branch per plant. At anthesis of the first flower of the main branch leaf number was varied by removing 0, 20, 40, 60 or 80% of the leaves below the fourth node of the main branch while all flowers below that node were removed. Abortion of four flowers/fruit at the fourth and fifth node was studied. Above these flowers, all flowers except two at node position 6, were removed. Fruit set decreased with number of removed leaves and was 78, 69, 67, 50 and 50%, respectively. In a climate room experiment (one main branch per plant; 13 flowers retained per plant) fruit set was 25, 38, 54 and 92%, at light intensities of 1.1, 1.8, 2.8 and 4.8 MJ PAR m d, respectively. The lowest and the highest light