CO2 and ozone effects on canopy development of potato crops across Europe

This paper describes the effects of elevated CO(2) (550 and 680 mul 1(-1)) and O(3) (60 nl 1(-1) O(3) as an 8 h mean), alone or in combination, on canopy development and senescence in potato (Solanum tuberosum L. cv Bintje) across a range of European agro-climatic conditions. The assessments were made within the European CHIP project (CHanging climate and potential Impacts on Potato yield and quality) that was conducted for two growing seasons (1998 and 1999) in free air CO(2) enrichment systems (FACE) and open-top chamber facilities (OTCs) at seven European sites. A comparison of chambered and unchambered experimental plots was included to examine the effects of chamber enclosure. Phenological growth stages, plant height, leaf area index (LAI) and the number of green and yellow leaves were recorded non-destructively throughout the growing season and by a destructive intermediate harvest at maximum leaf area (MLA). In the dynamic growth analysis CO(2) and O(3) effects were studied over three developmental stages: canopy expansion, full canopy and canopy senescence. Chamber enclosures promoted potato crop development (taller plants, more leaves) during the initial growth stages and led to a faster decline of LAI and a higher number of yellow leaves. The growth in ambient plots varied between sites and seasons, as did the scale of the treatment responses. Despite the large background variation, some overall treatment effects could be detected across all sites. Both levels of increased CO(2) reduced final plant height in comparison to ambient concentrations, which indicates a premature ending of the active plant growth. At the stage of full canopy and crop senescence the average number of green leaves was significantly (P < 0.05) decreased by 680 mul 1(-1) CO(2) (OTC experiments) and LAI showed the same tendency (P = 0.07). As there was however no indication of a decreased leaf formation during initial growth and at full canopy, this must have been due to an earlier leaf fall. In the FACE experiments LAI had already began to decline at the stage of full canopy at 550 mul 1(-1) CO(2) but not in ambient CO(2) (DAE x CO(2), P < 0.05). These observations strongly indicated that elevated CO(2) induced a premature senescence during full canopy. O(3) did not have an overall detrimental effect on crop development during initial growth nor at full canopy, but did induce a faster reduction of LAI during crop senescence (DAE x O(3), P < 0.05). Final plant height was not affected by O(3). There were few CO(2) x O(3) interactions detected. There was a suggestion (P = 0.06) that O(3) counteracted the CO(2)-induced decrease of green leaves at full canopy, but on the other hand during crop senescence the decline of LAI due to elevated O(3) was faster at ambient, compared to elevated CO(2) (P < 0.05). These responses of canopy development to elevated CO(2) and O(3) help to explain the treatment responses of potato yield within the CHIP project at sites across Europe.