Root Morphology, Plant Growth, Nitrate Accumulation and Nitrogen Metabolism of Temperate Lettuce Grown in the Tropics with Elevated Root-Zone CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; at Different Root-Zone Temperatures

Jie He,Lin Qin,Sing Kong Lee

doi:10.4236/ajps.2016.714169

Abstract

This paper investigated the effects of root-zone (RZ) CO2 concentration ([CO2]) on root morphology and growth, nitrate (NO3-) uptake and assimilation of lettuce plants at different root-zone temperatures (RZT). Elevated RZ [CO2] stimulated root development, root and shoot growth compared to ambient RZ [CO2]. The greatest increase in root growth was observed in plants grown under elevated RZ [CO2] of 50,000 ppm. However, RZ [CO2] of 10,000 ppm was sufficient to achieve the maximal leaf area and shoot productivity. Lettuce plants exhibited faster shoot and root growth at 20°C-RZT than at ambient (A)-RZT. However, under elevated RZ [CO2], the magnitude of increased growth was greater at A-RZT than at 20°C-RZT. Compared to RZ [CO2] of 360 ppm, elevated RZ [CO2] of 10,000 ppm increased NO3- accumulation and nitrate reductase activity (NRA) in both leaves and roots. NO3- concentrations of leaf and root were higher at 20°C-RZT than at A-RZT in all plants. NRA was higher in root than in leaf especially under A-RZT. The total reduced nitrogen (TRN) concentration was significantly higher in plants grown under elevated RZ [CO2] of 10,000 ppm than under ambient RZ [CO2] of 360 ppm with greater concentration in 20°C-RZT plants than in A-RZT plants. These results imply that elevated RZ [CO2] significantly affected root morphology, root and shoot growth and N metabolism of temperate lettuce with greater impacts at A-RZT than at 20°C-RZT. These findings have practical significance to vegetable production by growing the vegetable crops at cool-RZT with elevated RZ [CO2] to enhance its productivity.

Highlights

Aerial parts of plant physiology such as photosynthesis are closely associated with morphology and physiological activities of roots, especially under stress conditions [1]-[3]
It was shown that plants grown under elevated RZ [CO2] had longer total root length (Figure 1(A)), greater number of root tips (Figure 1(B)) and larger root surface area (Figure 1(C)) than under ambient RZ [CO2] at both root-zone temperatures (RZT) with greater values obtained from 20 ̊C-RZT plants than from A-RZT plants
We have previously reported that elevated RZ [CO2] stimulated photosynthesis of aeroponically grown lettuce plants and enhanced water use efficiency and plant growth, with greater increase at higher ambient temperature [8] and hot A-RZT compared to cool ambient temperature and cool-RZT [9]

Summary

Introduction

Aerial parts of plant physiology such as photosynthesis are closely associated with morphology and physiological activities of roots, especially under stress conditions [1]-[3]. High temperature limits growth and productivity of temperate crops grown in the tropics, which are mainly due to its poor root development [1]. Temperate lettuce can be grown in tropical Singapore at any time of the year by cooling their roots while their shoot are subjected to hot ambient temperature [2] [4] [5]. We have previously reported the effects of elevated RZ [CO2] on photosynthesis of aeroponically grown lettuce plants at different RZTs [6]-[10]

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