Abstract
Sub-optimal temperature extensively suppresses crop growth during cool-season greenhouse production. Root-zone (RZ) warming is considered an economical option to alleviate crop growth reduction. In this study we cultivated cucumber seedlings in nutrient solution under different air-RZ temperature treatments to investigate the effects of RZ warming on seedling growth- and photosynthesis-related parameters in leaves. The air-RZ temperature treatments included sub-optimal RZ temperature 13°C and sub-optimal air temperature 20/12°C (day/night) (S13), RZ warming at 19°C and sub-optimal air temperature (S19), and RZ warming at 19°C and optimal air temperature 26/18°C (day/night) (O19). In addition, for each air-RZ temperature treatment, half of the seedlings were also treated with 2% (m/m) polyethylene glycol (PEG) dissolved in nutrient solution to distinguish the effect of root-sourced water supply from RZ temperature. At the whole-plant level, S19 significantly increased the relative growth rate (RGR) by approximately 18% compared with S13, although the increase was less than in O19 (50%) due to delayed leaf emergence. S19 alleviated both diffusive and metabolic limitation of photosynthesis in mature leaves compared with S13, resulting in a photosynthetic rate similar to that in O19 leaves. In newly unfolded leaves, S19 significantly promoted leaf area expansion and alleviated stomatal limitation of photosynthesis compared with S13. PEG addition had a limited influence on RGR and leaf photosynthesis, but significantly suppressed new leaf expansion. Thus, our results indicate that under sub-optimal temperature conditions, RZ warming promotes cucumber seedling growth by differently benefiting mature and newly unfolded leaves. In addition, RZ warming enhanced root-sourced water supply, mainly promoting new leaf expansion, rather than photosynthesis.
Highlights
Low temperature is a prominent limiting factor for horticultural crop production in temperate regions, during cool-season cultivation
Biophysical parameters derived from Chl fluorescence and P700+ absorbance parameters
Biophysical parameters derived from transient Chl fluorescence parameters
Summary
Low temperature is a prominent limiting factor for horticultural crop production in temperate regions, during cool-season cultivation. To overcome the low-temperature imposed decrease of green-house plant production and to reduce energy consumption, environment control techniques that decrease heating costs, or raise crop resistance to unfavorable temperatures are required. Improved production by applying root-warming/cooling methods has been reported in some field experiments [3,4,12,13]. Most of the former studies were conducted under optimum air temperature conditions, and the specific effects of RZ temperature under unfavorable shoot temperature remain unclear
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