OPTIMIZING PHOTOSYNTHETICALLY ACTIVE RADIATION AND MOISTURE REGIMES CAN ENHANCE WATER USE EFFICIENCY IN CARROTS

Phenology of a plant is influenced by photosynthesis which is constantly under perturbation due to changing irradiance and moisture regimes and the limitation of one or both of these factors may have serious consequences on crop development and yield. Gas exchange measurements were made on 35 day-old ‘Red Core Chantenay’ carrots (Daucus carota), exposed to varied photosynthetically active radiation (PAR) and different moisture stress regimes. PAR ranged from 100 to 1000 µmol·m^-2·s^-1 and moisture stress regimes were set to -7, -30, -55 and -80 KPa. Moisture stress at -80 KPa reduced net photosynthesis (P_N) by 98% (0.11 µmol·m^-2·s^-1) and decreased transpiration (E) (0.23 mmol·m^-2·s^-1) and stomatal conductance (g_s) (0.00 mol·m^-2·s^-1) by 94% and 100%, respectively, compared with the unstressed control. No significant increases in P_N were observed beyond 700 µmol·m^-2·s^-1 irrespective of moisture regimes. The g_s did not differ with PAR levels. E increased as PAR increased with the highest (2.89 mmol·m^-2·s^-1) at a PAR level of 1000 µmol·m^-2·s^-1. The interaction between moisture regimes and PAR levels was not significant statistically. Increasing PAR to 600 µmol·m^-2·s^-1 and moisture stress levels up to -55 KPa resulted in the highest water use efficiency. Light response data for all moisture stress levels fitted to the Mitscherlich model. Photosaturated photosynthetic rate (A_max) was most sensitive and it declined with increasing moisture stress. The saturation point remained 600 to 700 µmol·m^-2·s^-1 until -55 KPa when compared with the unstressed control and was lower at -80 KPa. The quantum efficiencies (Aqe) were higher at -55 and -80 KPa when compared with the control. Light compensation point (LCP) (110 µmol·m^-2·s^-1) was the highest at extreme moisture deficit. Photosynthetic processes are vulnerable to moisture deficits and the benefits of increasing PAR can be lost under extreme moisture deficits; however, mild moisture deficits promote water use efficiency (WUE) under increasing PAR.