Almond Spurs: Photosynthetic Capacity of Their Leaves, Nitrogen Effects on Their Survival, and Their Use for the Prognosis of Nitrogen in Californian Almond Orchards (Prunus dulcis [Mill.] DA webb)

Spurs (compact shoot structures) serve as the fundamental bearing units in almond trees. Mature almond trees bear a high percentage of fruit on these short shoots, with only a small percentage of fruit produced laterally on long 1-year-old shoots. Leaves from non-fruiting almond spurs have typically been collected in mid-summer to monitor tree nutrient status as a standard practice in California almond orchards. Maintenance of the total number of living spurs per tree and ensuring their productivity is extremely important.;The first study aimed to elucidate how the presence of fruit impacts spur vitality through effects on the photosynthetic capacities of leaves in individual spurs. The results showed that the fruit has a negative influence on the photosynthetic capacity of the fruiting spur leaves. We concluded that this reduction in carbon assimilation in leaves from fruiting spurs was primarily due to a nitrogen competition between fruit and leaves at the spur level during the entire growing season. In the second study we attempted to improve spur vitality by applying targeted applications of nutrients to spur leaves of trees grown under three levels of soil nitrogen supply. The results showed that soil and foliar nitrogen treatments effectively increased spur leaf area, fruit, and leaf nitrogen concentration of the target spurs. However, none of these positive changes in leaf nitrogen or leaf area improved spur survival or return boom of any spur type. In the third study, we aimed to improve the current leaf sampling protocol by developing sampling strategies that can be applied earlier in the growing season than is currently practiced. The results show that leaf nitrogen concentration in mid-summer can be successfully predicted by collecting an early spring leaf sampling. The results of this study have been widely implemented by growers. The results presented in this dissertation provide basic knowledge, as well as practical solutions for the almond industry.