Cucumbers: the role of environmental and agronomic factors in carbon dioxide toxicity

Leaf damage resulting from very high CO2 concentrations (also known as CO2 toxicity) which is manifest as leaf bleaching, occurred in this study both in controlled environments and in the glasshouse, when CO2 concentrations of higher than 1000 vpm were employed but only when these levels of enrichment were supplied during bright, sunny weather (or high radiant loads in the controlled environment chambers).

Other morphological changes to the cucumber plants were observed at 2000 vpm CO2.  These included changes in the xylem diameter and leaf stomatal density and such changes may further stress the shoots and exacerbate the leaf bleaching symptoms observed with CO2 toxicity.

Our physiological investigations have shown that these symptoms result from a well-documented physiological condition in the leaves: this is known as oxidative stress. This condition arises when the leaf traps radiant energy from the sun and for a variety of reasons cannot use this energy, which then damages the leaf. The hypothesis here is that very high CO2 concentrations cause lesions which prevent the leaf from effectively using radiant energy. 

Most importantly, leaf damage was not apparent (even at the highest CO2 concentrations and the highest radiant loads) when the crop was grown in the glasshouse on the high-wire system.  This appears to be because only older leaves are susceptible to damage and with the high wire system of cultivation, these leaves are mostly less exposed to direct, intense sunlight. 

The project has also shown that leaf bleaching can be avoided at even the highest CO2 concentration tested (2000 vpm) if leaves are shaded during the brightest periods in the glasshouse in Spring (from January to April).  Mild shading (~35%) at 2000vpm CO2 prevented the development of leaf bleaching and also severe damage of the photosynthetic systems, as observed in the unshaded treatments.
A small scale controlled environment study demonstrated that doubling N levels in the liquid feed from 144 ppm N to 288 ppm N, prevented the onset of leaf bleaching (leaf damage) under 2000 vpm CO2 enrichment.

One glasshouse trial has shown that leaf bleaching can be associated with reductions in fruit production.  Preliminary glasshouse trials demonstrated that shading of the crop for an extended period of time avoids the development of leaf bleaching and sustains the rate of photosynthesis but this treatment did not improve fruit yield.  Further work is needed here to investigate the possibility that shading only during the brightest periods will allow us to avoid crop damage due to CO2 levels beyond 1000vpm and enhance yield further.

It has not been possible to test the effects of extra nitrogen fertiliser (288 ppm N) on fruit yield but controlled environment experiments have shown that leaf growth is promoted by this treatment and leaves remain very green and do not show the development of symptoms or biochemistry of oxidative stress under high CO2 enrichment.  Low N (144 ppm N) plants grown under the same conditions showed clear development of symptoms.  Further work is needed in this area.

Our biochemical results suggest that a simple diagnostic test could be developed as an early warning of CO2 induced oxidative stress and may allow growers to use higher CO2 concentrations without risk.

Downloads

• Annual report year 1
• Annual report year 2
• Final report

AIM:

This project was designed to try to understand the mechanistic basis of leaf damage due to high CO2 enrichment and where possible, develop methods to minimise the occurrence of the problem.  This might enable growers to use higher concentrations of CO2 without risk of crop damage and thereby enhance yield and profitability.