CASE STUDY 13.2  Heat therapy and CO2

Printer-friendly version

Paul Kriedemann


Figure 1  Potted grapevines in two naturally illuminated heat therapy cabinets (37-40 °C day and night), one at ambient CO2, the other held around three × ambient (CSIRO Horticulture Laboratory, Merbein) (Photograph courtesy E.A. Lawton)

Grapevine viruses are endemic to original habitats of certain Vitis species. They have spread worldwide via vegetative propagation with rootstocks a common source of scion infection (grapevine viruses are graft transmissable). Vine debilitation and reduced yields generally follow infection, though they are not always attributed to a causal agent because other visible symptoms can be diffuse.

Prior to the advent of molecular techniques during the 1980s virus detection was based on host plant symptomology plus visible reactions by indicator plants, and therapy of infected vines was based on prolonged exposure of entire potted vines to high temperature (37–40°C day and night) (Figure 1). Shoot tips generated during therapy were attenuated in virus ‘titre’, and if candidate plants lived long enough (100 d or more) virus-free tip cuttings could be produced.

According to practitioners of this method (Nyland and Goheen 1969), terminal meristems differentiate growing points ahead of vascular development, and during heat therapy those intensely meristematic growing points remain isolated from phloem-translocated virus. Put another way, sustained high temperature constrained the spread of virus to a greater extent than host plant cell division, and virus-free cells resulted. Harvesting those cells is technically demanding, and in former times well-nigh impossible, hence an early reliance on tip cuttings.

In principle, production of new meristems during heat therapy afforded good prospects of virus elimination, but host plants frequently died from heat stress, and especially if carbon resources had become depleted prior to treatment. Recognising that photorespiratory loss of carbon would be greatly enhanced at 40°C, and noting that ambient CO2 might be depleted in growth cabinets sealed for heat retention (cf. Figure 13.13), CO2 enrichment was implemented in the hope of restoring a positive carbon balance. Potted vines were held under high humidity in naturally illuminated cabinets at either ambient (c. 315 µmol CO2 mol–1) or enriched conditions (c. 1250 µmol CO2 mol–1) and responses measured over 14 d (further details in Kriedemann et al. 1976).

As anticipated, grapevine survival improved under CO2 enrichment. Net assimilation rate (NAR; Section 6.1) increased from 2.36 to 5.60 g m–2 d–1) and there was some evidence of leaf starch accumulation. Again, as theory predicted, transpiration decreased from around 1050 g m–2 d–1 at ambient CO2 to about 450 g m–2 d–1 under elevated CO2 and may have contributed to alleviation of high-temperature stress. There was also recorded an early example of photosynthetic acclimation where rates of CO2 assimilation in ambient air were down-regulated in grapevines heat treated under high CO2.



Figure 2 Seedlings of wong bok (Brassica pekinensis) show a spectacular growth response to elevated CO2 (three × ambient) when grown for 42 d under continuously warm conditions (32 °C day and night) intended to mimic a tropical environment (left side ambient, right side CO2 enriched). The same growth cabinets as shown in Figure 1 were used. Scale bar  = 10 cm. (Photograph courtesy P.E. Kriedemann)

At a time when heat therapy remained de rigueur for virus elimination, CO2 enrichment of heat therapy cabinets found wide application for vine improvement. That technology has since been replaced by molecular methods for detection and elimination, but plant growth response to CO2 at high temperature still holds relevance for protected cultivation in tropical environments where net carbon gain is compromised by photorespiratory loss. Leafy green vegetables such as wong bok (Brassica pekinensis) proved responsive in our heat therapy cabinets, and especially during early vegetative phases where faster assimilation plus gains in leaf area were quickly compounded during near-exponential growth (Figure 2; Section 6.2). Marketable mass is therefore reached more quickly than at ambient CO2, and this faster cropping cycle is significant commercially. A quest for genetic variation in this capacity for response to elevated CO2 at high temperature would be likely to pay practical dividends.


Kriedemann, P.E., Sward, R.J. and Downton, W.J.S. (1976). ‘Vine response to carbon dioxide enrichment during heat therapy’, Australian Journal of Plant Physiology, 3, 605-618.

Nyland, G. and Goheen, A.C. (1969). ‘Heat therapy of virus diseases of perennial plants’, Annual Review of Phytopathology,
7, 331–354.