Publisher Summary This chapter discusses causes of monocarpic senescence and senescence in polycarpic plants and clones. Whole plant senescence is viewed primarily in terms of leaf senescence and it is measured mainly through chloroplastic parameters such as chlorophyll loss and decreased photosynthesis. In monocarpic plants where senescence and death closely follow reproductive development, the senescence (monocarpic senescence) is often controlled by the developing reproductive structures. In these cases, removal of the reproductive structures or prevention of their development usually prolongs the life of the plant. Monocarpic plants cease their vegetative growth fairly abruptly early in their reproductive phase. Conversely, the perennial polycarpic pattern requires continued vegetative growth. This prominent shift (diversion) in growth-related allocation of resources in monocarpy seems to be part of a reproductive strategy that optimizes reproductive output for the plants. This diversion/withdrawal is often quite prominent that leads to monocarpic senescence.

Whole plant senescence

In the glasshouse cultivation of a long-season tomato crop, maximum fruit production is obtained when there is a proper balance between the demand and the supply of assimilate, and an optimum proportion of vegetative growth throughout the season in order to sustain the crop photosynthetic capacity. These aspects of crop growth are mainly affected by the fruit load, defined as the assimilate demand of all fruits together. In practice fruit load is controlled by plant density, fruit thinning and temperature. These measures for crop control can be more precise and effective if their effects are known in quantitative terms. An explanatory dynamic growth model was developed that simulates assimilate demand and dry matter distribution in an indeterminate tomato crop. Number of growing organs was evaluated through prediction of initiation, abortion and harvest of individual organs. Assimilate demand was based on potential organ growth rates (growth at nonlimiting assimilate supply). Dry matter distribution in the model was in proportion to the potential growth rates of the organs.In total 11 glasshouse experiments were conducted, six of which included temperature treatments. Truss formation rate increased with temperature (17-27°C) and declined with plant age. Truss formation rate was found to depend on the genotype, while fruit load, plant density, season and electrical conductivity of the root environment (EC: 0.3-0.9 S m -1 ) had no effect. The number of fruit that develop per truss was positively correlated with the vegetative growth of the top of the plants. The duration of the fruit growth period (time between anthesis and start of colouring) was shortened with increasing temperature, young and old fruits being the most sensitive. At the same air temperature the fruit growth period in summer was shorter than in spring. Fruits of old plants had slightly longer growth period than fruits of young plants. Potential weight of the fruits at harvest was negatively correlated with temperature, mainly due to the shorter fruit growth period. Further, the potential size increased with ontogeny, which effect was more pronounced in early than in late spring. The course of potential weight in time was described by a Gompertz growth curve exhibiting the maximum growth rate at about 40% of the fruit growth period. When during fruit development a fruit changed from limiting to nonlimiting assimilate supply, it did not immediately reach the same growth rate as fruits grown constantly at nonlimiting assimilate supply., A mechanism is proposed that explains this phenomenon. The fraction of dry matter distributed to vegetative growth declined substantially with temperature. The (apparent) potential growth rate of a vegetative unit at 24°C was estimated to be as much as 50% lower than at 19°C. The dry matter-content of fruits was negatively correlated with temperature and EC of the root environment and was higher in summer than in spring and autumn.The model was tested with data from five commercial crops. Truss formation rate, fruit growth period and dry matter distribution were predicted reasonably well. The modelling of the number of fruits per truss requires more investigation. Simulated assimilate demand of a mature tomato crop reached values of 10 and 60g CH 2 O m -2 d -1 for maintenance respiration and growth respectively. The potential growth rate (as defined by the sinks) appeared to be about twice the actual growth rate.A simulation study indicated that maximum fruit production of tomato is probably obtained at a fairly low leaf area index (2-3 m 2 m -2 ). At supra-optimum leaf area index additional leaf area for extra light interception requires more assimilate than it would produce. Computations showed that in spring and early summer the optimum plant density is determined by the required number of fruits (sink capacity) whereas in summer a combination of high plant density and fruit thinning seems required for sufficient leaf area. The results are discussed with respect to the crop sink-source system and temperature control in the glasshouse. Prospects for practical applications of the model are presented.

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Development and dry matter distribution in glasshouse tomato : a quantitative approach

http://greenlab.cirad.fr/GLUVED/html/bibliography/H_Heuv_96.pdf

Dry Matter Partitioning in Tomato: Validation of a Dynamic Simulation Model

A Simulation Model for Dry Matter Partitioning in Cucumber

Growth, accumulation of sugars and starch, and the activity of enzymes involved in sucrose mobilization were determined throughout the development of sweet pepper fruits. Fruit development was roughly divided into three phases: (1) an initial phase with high relative growth rate and hexose accumulation, (2) a phase with declining growth rate and accumulation of sucrose and starch, and (3) a ripening phase with no further fresh weight increase and with accumulation of hexoses, while sucrose and starch were degraded. Acid and neutral invertase (EC 3.2.1.26) were closely correlated to relative growth rate until ripening and inversly correlated to the accumulation of sucrose. Acid invertase specifically increased during ripening, concurrently with the accumulation of hexoses. Sucrose synthase (EC 2.4.1.13) showed little correlation to fruit development, and in periods of rapid growth the activity of sucrose synthase was low compared to the invertases. However, during late fruit growth sucose synthase was more active than the invertases. We conclude that invertase activities determine the accumulation of assimilates in the very young fruits, and a reactivation of acid invertase is responsible for the accumulation of hexoses during ripening. During late fruit growth, before ripening, sucrose synthase is transiently responsible for the sucrose breakdown in the fruit tissue. Results also indicate that pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) and its activator fructose-2,6-bisphosphate (Fru2,6bisP) are involved in the regulation of the sink metabolism of the fruit tissue.

Carbohydrate metabolism during fruit development in sweet pepper (Capsicum annuum) plants

The shaded fruit of the pepper plant was the main sink for assimilates during much of the growth period of this organ. Up to 90 % of the plant daily dry weight increase was deposited in the fruit, and growth of all non-fruit organs was correlatively reduced. No net transfer of dry matter from other organs to the fruit could be detected. In continuously deflorated plants, the partitioning of dry matter among organs was more evenly balanced. Net assimilation rates of fruiting plants exceeded those of deflorated plants, and these differences appeared to be associated with alterations in assimilate demand. Net assimilation rates of fruiting plants remained constant for up to 40 days under conditions of very little leaf surface turnover, indicating that leaves of these plants retain their photosynthetic capacity long after full expansion. Foliar senescence and abscission in deflorated plants was marked, but these processes were inhibited during the growth of the fruit. Some possible causes of this differential response are considered.

AJ Hall

Papers

Assimilate Source-Sink Relationships in Capsicum annuum L. I. The Dynamics of Growth in Fruiting and Deflorated Plants