CO2 research reports

HH1302SPC (1994). 

To study the mechanisms controlling growth and development of greenhouse crops and their modification by the environment as a basis for improving product quality and the efficient use of energy and other inputs.  Leader - K.E. Cockshull.

HH1303SPC (1994).

To evaluate plant responses to the aerial environment leading to improved product quality and productivity for greenhouse crops. Leader – D. J. Hand – pepper.

HH1303SPC (1994).

To evaluate plant responses to the aerial environment leading to improved product quality and productivity for greenhouse crops. Leader - M. Harriman - tomato.

Four tomato cultivars (Calypso, Pronto, Liberto and gourmet) were grown in a 16 compartment multifactorial glasshouse at Stockbridge House. These were enriched with 425 ppm CO₂ in either the morning or the afternoon or all day in comparison to no enrichment. Yield was related to the daily average CO₂ concentration, there was no significant difference of adding CO₂ in the morning or the afternoon.

HH1304SPC (1994).

To develop energy efficient systems of greenhouse crop production. Leaders – M. Bradley and D.P. Wilson.

HH1306SPC (1998). 

Optimal control of carbon dioxide enrichment in glasshouse tomato crops. Leaders K.E. Cockshull and B.J. Bailey.

HH1307TPC(1997).

Regulation of tomato fruit growth. Leader - L.C Ho.

A three-year collaborative project between HRI and the University of Lancaster has investigated the mechanism and regulation of tomato fruit expansion, and has developed a conceptual model of fruit growth regulation. The model indicated new ways in which fruit growth might be artificially regulated.

The rate and the extent of fruit growth were found to be controlled almost entirely by the factors which determine mechanical properties of the skin. The model of fruit growth developed should also be applicable to comparable crops such as pepper and cucumber.

HH1311SPC (1997).

Optimisation of the aerial environment of protected crops.  Leader – K.E. Cockshull.

Edible crops.  One objective was to provide information in support of Project HH1306 on optimal control of carbon dioxide enrichment.  Most tomato growers now generate CO₂ by burning natural gas but must store the excess heat in summer.  The quantity of CO₂ available for enrichment is then limited by the capacity of the heat store.  So, how best to use this limited quantity of CO₂?  Analysis of data from an earlier experiment showed that it was as beneficial to enrich with CO₂ in the morning as in the afternoon in summer.  The models of leaf photosynthesis used in the control algorithm were checked by measuring tomato leaf photosynthesis over a range of CO₂ concentrations.  The results confirmed the accuracy of the model, showed that there was still an appreciable increase in photosynthetic rate between 1000 and 1500 vpm CO₂ at high irradiance and that the maximum rate was lower at a density of 4.28 than at 2.14 heads m^-2.  In a glasshouse experiment, fruit yield was increased and average fruit size decreased at higher crop densities but both yield and fruit size were increased by higher CO₂ concentrations.  However, there was no interaction between these factors.  A model for fruit growth generated an initial prediction for tomato fruit number and a timetable for their setting dates to enable the aggregate of their daily demands for photosynthate during growth to optimal size to be closely matched to the amount of photosynthate available.  A statistical analysis of variation in tomato fruit size found that the average fruit weight within a grade provided a useful practical method of describing overall fruit variabilty via an approximate standard deviation.  Another objective of the Project was to facilitate the optimisation of other environmental factors.  Under glass, there was a significant effect on yield of only 14 days at high humidity but the magnitude of the effect depended when treatment was given. High humidity also increased fruit softness at harvest and water loss during storage.  In a further glasshouse experiment, the tomato was shown to be capable of integrating night temperatures differing by 3oC above or below a control value as long as the average night temperature became the same as that of the control within a maximum of 16 days.  Within these limits, there were no significant effects on either flowering, yield, average fruit size or the occurrence of fruit disorders.

Ornamental crops.  One of the main objectives with ornamental crops was to establish whether CO₂ enrichment in summer could improve the weight of flower sprays of chrysanthemum.  The concentration of CO₂ within a commercial chrysanthemum crop was monitored in summer and found to be depleted during the day.  Chrysanthemum leaf photosynthesis was measured over a range of CO₂concentrations and the data confirmed that depletion reduced leaf net photosynthetic rate.  Three glasshouse experiments were conducted in which two cultivars of spray chrysanthemum were grown at two different CO₂ concentrations (350 and 450 vpm) and two different crop densities (64 and 72 plants m^-2).  Over the three experiments, the increase in crop density lowered spray weight by an average of 8%, while an increase in CO₂ concentration to 450vpm increased spray weight and, to a large extent, offset the loss in weight due to the increase in density.  An additional objective was to clarify which factors of the summer aerial environment cause loss of quality in pot plants and to devise means of mitigating the effects.  Flowering of a pot-plant chrysanthemum, ‘Charm’, was considerably delayed when high temperature (29^oC) was given either throughout the night or the day; it was almost unaffected by high temperature given only for the first three hours of the night and yet was greatly delayed when high temperature was given for the last three hours.  Similarly, high temperature treatment at the end of day delayed flowering more than at the middle of the day.  The results imply that a timing process is involved and that to maintain quality and scheduling in summer, high temperatures should be avoided at the ends of the night and of the day.

HH1318SPC (2000).

Optimising the aerial environments for protected ornamental crops.

Glasshouse compartments at Wellesbourne were used to study the relative effects of air temperature and direct solar irradiance on the growth and quality in summer of three pot plant species: chrysanthemum, dieffenbachia and New Guinea impatiens.

A single leaf, net photosynthesis model was constructed for chrysanthemum, and this indicated that CO₂ supplementation in summer ought to give significant increases in dry weight. However, CO₂ supplementation in summer can be prohibitively expensive because vents are frequently open. Introducing CO₂ only when vents were <10% open had little obvious benefit, and subsequent glasshouse trials were carried out to determine the effects of raising the vent temperature set-point during the first four hours of the day to increase the period during which vents were <10% open and during which CO₂ supplementation could be practiced. Raising the vent set-point from 21^oC to 27^oC (with CO₂ supplemented to a maximum of 1,000 vpm when vents <10% open in both cases) increased total plant fresh weight by 8.8% and dry weight by 15.7% (significant at P=0.001 in each case). Increases in air temperature which are a consequence of this strategy did, however, increase plant height and pedicel length, but did not delay time to harvesting stage. Increasing the vent temperature to 27^oC but without CO₂ supplementation gave little obvious benefit.

HH1319SPC (1997).

Current and likely future methods of delivering CO₂, heat/ cooling to glasshouses - desk study.

HH1326SPC(2000).

Optimisation of aerial environments for edible crops. Leader S.R. Adams.

Experiments were conducted in glasshouse and controlled environment facilities to investigate the effect of different fruit removal treatments on the pattern of tomato yields. While the removal of flowering trusses resulted in a yield loss about eight weeks later, there was little loss in cumulative yield due to the redistribution of assimilates to neighbouring trusses. Truss pruning (leaving five fruits on each truss) resulted in a significant loss of yield, although the pattern of yield was unaffected despite the stable fruit load. This indicated that fruit load was not the primary cause of cyclical fruit production.

The flushes in yield were found to be as a consequence of a hastening of fruit maturation. The changes in fruit development times and hence patterns of yield were primarily due to temperature. In controlled environment studies fruits ripened much earlier when the temperature was elevated, as did other developmental processes such as the rate of truss production. Thermal time proved to be a poor predictor of the time of fruit maturation as fruits became more sensitive to temperature as they approached maturity. This provided an explanation for the flush of ripe fruit following a period of elevated temperature seen in the glasshouse work.

The effects of the aerial environment on growth and development were quantified where possible and used to develop a tomato crop model in HIPPO. The rate of progress to ripening of each fruit is determined by its predicted temperature, and fruits become more sensitive to temperature as they approach maturity. The model has been used to predict crop yields based upon hourly environmental records. The model accurately predicted the mean fruit size and total yield.

An experiment was also conducted to investigate whether it is possible to modulate the time of yield production in a commercial crop by defined changes in greenhouse air temperature and to do so without unforeseen consequences. Elevating the air temperature of compartments for one week resulted in a flush of fruits for two weeks, followed by depressed yields in subsequent weeks. However, there was no significant yield loss as a result of this treatment indicating that there may be potential for growers to exert some control over the pattern of their tomato crop yields.

HH1328SPC(2001).

Natural ventilation of commercial greenhouses. Leader B.J. Bailey.

The primary objective of this work was to produce a scientific model capable of predicting reliably the natural wind-driven ventilation of large, commercial glasshouses.

The model was developed from the results of two programmes of physical testing. The first programme concerned tests on individual ventilators of different aspect (length-to-width) ratios with flaps opened to different angles from the plane of the opening, for flows both into and out of opening, and over a range of air speeds. The second programme was conducted using a specially constructed large-scale model of a multispan glasshouse in the brand new Atmospheric Flow Laboratory (AFL) at SRI. Extensive tests were conducted over a range of mean wind speeds from 1 to 4.5 m/s for a wide range of ventilator opening arrangements.

The model was applied to the test cases used to develop it and it accounted for 97% of the observed variations in ventilation rates. The model was validated against three independent sets of measurements made in three different full-scale Venlo glasshouses.

At time of reporting (2001) the model was amenable to implementation into horticultural environmental controllers, and discussions were being held with two leading companies and the HDC to this effect.

HH1329SPC (2003).

Optimisation of aerial environments for edible crops. Leader S.R. Adams.

This follow on work to HH1326SPC investigated whether it is possible to smooth out some of the fluctuations in yield by creating a more stable glasshouse temperature. The degree to which it is possible to reduce night temperatures following a warm sunny day and the effect this has on the pattern of yield were assessed. Another aim was to investigate in greater detail the change in sensitivity of fruits to temperature and incorporated this information into a model. The potential for using this model for commercial predictions was explored by validating it and testing the dependence of the output on the accuracy of weather forecasts.

A model was developed to predict crop yields. The fruit development model was used to predict the pattern of yield based up on when fruits were estimated to ripen for any given set of daily glasshouse temperatures. Yields were then estimated using recorded fruits loads, shoot densities and mean fruit sizes. The model was tested using two years of historical data. The model gave reasonable predictions of the pattern of yields and accounted for 75% and 90% of the variance in weekly yields. These simulations with historic data used the recorded mean diurnal temperatures. However, when the model is used to forecast weekly yields in real time, forecast temperatures will be needed. Simulations showed the predictions were highly sensitive to the temperature data. Therefore, to aid decision making the model predicts the weekly yields based upon the estimated glasshouse temperatures and also what might happen should the weather change and higher or lower temperatures occur.

If hot sunny weather is forecast it may be possible to raise temperature set-points in advance of the sunny weather so as to hasten ripening and increase yields before/during the sunny weather when demand for salads is high, this would also help to reduce the magnitude of the peak in yield that would naturally occur after the hot weather. However, such a strategy is limited by the need for accurate weather forecasting for around two weeks ahead.

HH1330SPC (2003).

Energy efficient production of high quality ornamental species. Leader F.A. Langton.

This was a large project and the relevant project objective was to model the photosynthetic responses of petunia and impatiens, relating net photosynthesis to temperature, light and CO₂, as a basis for predicting quality in contrasting temperature regimes.

Temperature is predicted to have relatively little effect on the relationship between net photosynthesis and light when CO₂ level is ambient. However, when CO₂ is at an enriched level (1000 ppm), increase in temperature is predicted to increase the level of net photosynthesis quite markedly. Increases in day temperature by solar gain as a consequence of energy saving regimes, should be accompanied by increases in CO₂ level to ensure maximum net photosynthesis and dry weight gain.