DEVELOPING STRATEGIES TO INCREASE GRAPEFRUIT YIELDS

Florida produces the most citrus in the United States. The number of grapefruit trees planted per acre in Florida has increased since 1990; however, total bearing acreage and total bearing grapefruit trees in the state have both declined by approximately 55% in the last decade (NASS 2011). Thus, in Florida, more grapefruit trees are being planted on less land. In the last decade, the total number of boxes of grapefruit produced in Florida has decreased from 46 million in 2000 to 20 million in 2010. Since 2000 (excluding the crop years of 2004-2006), the state average number of boxes of grapefruit yielded per acre has remained fairly steady, ranging from 405-497 boxes (NASS 2011). Thus, despite the trend of planting more grapefruit trees per acre, the average yield per acre over the last decade has not increased dramatically and total state production has decreased precipitously. Some plant diseases can significantly reduce healthy leaf area duration, aboveground biomass production, and yield (Serrago et al 2009). A particularly menacing disease of grapefruit is canker disease, caused by the bacterium Xanthomonas citri sbsp. citri. Grapefruit is very susceptible to canker, relative to other citrus species. Lesion development on fruit will negate the value of the crop if it is intended for the fresh market. Currently, canker remains a significant obstacle in major citrus-producing regions with a valuable fresh - fruit market, like Florida (Bock et al 2011; Francis et al 2009). Additionally, a relatively new disease of Florida citrus is huanglongbing (HLB), symptoms of which include chlorotic leaves. This disease is associated with a bacterium [Candidatus Liberibacter asiaticus (Las), in Florida]. A psyllid insect vector inoculates citrus hosts with Las, which is phloem - limited. This pathosystem is particularly pernicious for worldwide citrus production because there is currently no known cure for HLB (Bove 2006). Therefore, any strategies that aim to increase grapefruit yield should account for the potential of citrus canker and HLB to reduce plant productivity. Likewise, development of novel means to manage citrus canker and HLB outbreaks should not be phytotoxic and impinge on grapefruit yield potential. Processes that determine yield (i.e., radiation interception, radiation use efficiency, and cropping efficiency) in grapefruit, and Citrus spp. in general, need to be examined further. To that end, the proposed work will examine citrus yield processes in controlled field experiments. The aim of these experiments will be to develop durable strategies and practices for increasing radiation interception, radiation use efficiency, and / or cropping efficiency, including, but not limited to, grove design and planting density, training and pruning, rootstock and scion evaluations, irrigation and nutrition management, control of vegetative and root growth to increase fruit yield, and integrated citrus pest and disease management.

Crop yield has been defined as the product of three processes: cumulative radiation intercepted by the canopy (RI); radiation use efficiency of the canopy (RUE); and, the harvest index (HI; Sandana and Pinochet 2011; Serrago et al 2009; Sinclair and Muchow 1999). Processes that determine yield (i.e., RI, RUE, and HI) in grapefruit, and Citrus spp. in general, need to be examined further. To that end, the proposed work will examine citrus yield processes in controlled and replicated laboratory, greenhouse, and field experiments. The aim of these experiments will be to develop durable strategies and practices for increasing RI, RUE, and / or HI, including, but not limited to, grove design and planting density, rootstock and scion evaluations, irrigation and nutrition management, control of vegetative and root growth to increase fruit yield, and integrated citrus pest and disease management. Field - based experiments will assess the ability of various cultural and chemical control practices to prevent or delay disease progression of non - bearing and bearing grapefruit trees. In addition, production scenarios with the potential to significantly affect RI, RUE, and / or HI will be identified. Evaluations of field trees representing various scion and rootstock combinations will be planted at various densities to investigate their effects on disease progress and grapefruit tree health. Additionally, management options such as irrigation water, nutrient management, training or pruning, and utilizing different rootstocks to control relative canopy growth, root growth, and yield relating to RI, RUE, and HI will be assessed for their efficacy at preventing / delaying the progress of disease symptoms. Plots of trees will serve as experimental units. Response variables of field trees will include RI, RUE, and cropping efficiency. Measurements of incident and transmitted solar radiation will be used to estimate cumulative RI, with estimated individual measurements being resolved daily (Salvagiotti and Miralles 2008), RUE will be calculated using a "big - leaf" model approach to estimate aboveground cumulative gross primary productivity (GPP), and then aboveground cumulative GPP will be regressed on cumulative RI (reviewed in Medlyn et al 2003). Usually, HI is defined as the ratio of fruit mass to total plant mass (e.g. Grossman and DeJong 1998). To calculate aboveground HI for the proposed work would necessitate destructive sampling of entire trees which may prove prohibitively expensive. To that end, cropping efficiency (e.g. fruit mass per cubic meter of canopy volume or per square meter of trunk cross sectional area) will be used as a proxy measure to evaluate partitioning between aboveground vegetative and reproductive growth. Ancillary response variables needed to estimate RI, RUE, or cropping efficiency will include leaf area index, photosynthetic gas exchange rates, foliar disease symptom severity, and fruit yield. ANOVA and regression - based analyses will be employed, where appropriate, to evaluate treatment effects.