Lateral Bearing Cultivars: A step towards high potential Walnut production
Authors: RAFIQ AHMAD SHAH, PARSHANT BAKSHI, AND RUCKU GUPTA
Most of the walnut plantation is based on the selection of promising genotypes from the natural population which are mostly terminal bearing. Some rare walnut trees are lateral bearing, that is, a few buds behind the terminal on the new growth have female flowers too. Terminal bearing cultivars have long gestation period and low productivity. Nowadays more emphasis is given on the increase in production per unit area. Development of lateral bearing cultivars not only increases the production, but these cultivars are also resistant to climate change. Lateral bearing cultivars develop fruits on side small branches after three or four years of planting and bear heavily. In comparison to lateral bearing, tip bearing plants develop fruit on tips of one year old shoots after six or more years and produces yield smaller than lateral bearing cultivars. Germain, (1999) established that the lateral bearing cultivars have a higher productivity due to the larger number of fruit buds than the terminal and intermediate bearing cultivars.
Walnut can be successfully grown in climate ranging from warm temperate, Mediterranean semi arid to humid climate. Global warming raises concerns about insufficient chilling as winter temperatures rise throughout the growing region. If winter chilling requirements is not fulfilled, this would result in poor spread or delayed bud-break and lack of synchronized bud-break with a negative effect on pollination. Luedeling et al. (2009) concluded that the areas where safe winter chill exists for growing walnuts, pistachios, peaches, apricots, plums and cherries (>700 chilling hours) are likely to almost completely disappear by the end of the 21st century. For fruits that are cross pollinated such as walnuts and pistachios, insufficient chilling can reduce pollination leading to reduced crop yields (Gradziel et al., 2007). The greater rise in winter and spring (January to March) temperatures will lead to earlier flowering, which coincides with the time of spring frost resulting in a remaining risk of frost damage to walnut flowers. It takes 15 or more years to produce a new variety of walnut. An established orchard has a life span of 20 to 30 years or longer. As global temperatures rises, this will affect the flowering and fruiting of the walnut and this means the climate in which a grower plants a nut orchard may be very different from the climate the trees experience when they reach maturity. Either we need to develop new varieties that are tolerant of much higher temperature or we need to migrate these crops to places to higher altitudes which are cooler. The tip bearing walnut cultivars are more susceptible to early and late frost and climate change than lateral bearing cultivars. Late spring frosts at the time of final stage of leaf formation could kill trees. This is due to the fact that the terminal buds leaf out first in the spring ahead of the lateral buds, exposing them to injury from late spring frosts. The later leafing lateral buds can still produce a crop.
Terminal fruit-bearing walnuts do not tend to have a high crop but can regenerate very well. On the other hand, the lateral fruit bearing walnuts are more capable of flowering and possess a low ability to regenerate. The great generative potential of the lateral-fruiting walnuts was also reported by Germain, 1990. The poor vegetative potential of lateral walnuts is likely to be attributable to the length and number of shoots, their low growth potential, the small total number of 2-year-old and 1-year-old shoots, and the small number of vegetative buds on 1-year-old shoots. In addition, lateral-fruiting genotypes have thicker shoots than other types. Increasing thickness of shoots is associated with increasing hydraulic conductivity (Cochard, 1992) in various species. Thick shoots are therefore more likely to be able to support a heavy fruit load and to allow an appropriate nutrient and water supply to the fruit, thus favouring higher fruit quality. Lateral-fruiting walnuts are therefore expected to have large fruits with a better kernel quality than the fruits of other types. This conclusion differs from that of Lauri et al. (2001), who postulated that lateral bearing walnuts show a tendency to bear nuts of decreasing size from year to year. Also, the good generative potential of lateral bearing cultivars can be reflected in the short juvenile period. It is well known that lateral walnuts bear fruit in the fifth or sixth year after planting, but in other fruiting types the juvenile period can last for more than 7-12 years (Solar et al., 2001). The poor vegetative potential of lateral walnuts is likely to be attributable to the length and number of shoots, their low growth potential, the small total number of 2-year-old and 1-year-old shoots, and the small number of vegetative buds on 1-year-old shoots.
Thus it can be concluded that the lateral bearing cultivars not only produce 40% higher yield than the terminal bearing walnut cultivars, but these can be productive under present climate change scenario. Therefore we should develop and breed for walnut lateral bearing cultivars for precocity and early economic harvest instead conventional tip bearing cultivars which are more vigorous, take long time to first flowering and produce less yields.
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I am currently pursuing Ph. D in Horticulture (Fruit Science) at SKAUST-J
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