Light extinction coefficient and radiation use efficiency in different growth stages of tomato exposed to different irrigation regimes

A prerequisite to model crop growth is an appropriate quantification of crop canopy structure in response to management and environmental conditions. Under water stress, the light distributions over canopy depth are more complicated because water stress affects not only appearance and elongation of leaves, but also morphological aspects of leaf positioning, leaf angle and azimuth angle (Archontoulis et al., 2011). Water stress reduces RUE by preventing effective photosynthesis for growth due to lower intercepted PAR as a result of reduced leaf area and leaf rolling or wilting (Wilson and Jamieson, 1985; Xianshi et al., 1998; Ngugi et al., 2013).We devised a two-year field experiment under different irrigation regimes at the two stage growths tomato with the aims of quantifying and describing the response of canopy light extinction coefficient, radiation use efficiencies, leaf area index and yield to reduced water at vegetative and reproductive stages of tomato in order to obtaining the best yield.

A field experiment was conducted over two consecutive seasons (2016-2017) in the experimental field of Ferdowsi University of Mashhad located in Khorasan Razavi province, North East of Iran. The experiment was laid out in a split plot design with different irrigation regimes at the reproductive and at the vegetative stage as the main and subplot factors, replicated thrice. The following experimental factors were studied: three irrigation regimes (100= 100% of water requirement, 75= 75% of water requirement, 50= 50% of water requirement) and two crop growth stages (V= vegetative stage and R= Reproductive stage). The drip irrigation method was used for irrigation. The tomato water requirement was calculated using CROPWAT 8.0 software. The irrigation water was supplied based on total gross irrigation and obtained irrigation schedule of CROPWAT. In the both growing seasons, plant growth and physiological parameters were assessed in two weekly intervals on two plants per plot starting 45 days after transplanting (DAT) up to 145 DAT.

Leaf area index (LAI) of tomato varied between irrigation regimes. According to ANOVA, the treatments had a significant effect on maximum LAI in both years (Table 5).The fraction of PAR intercepted by tomato canopy, measured in two years of study, increased from a few days after transplanting (May) onwards, reaching its maximum value in mid-August (around 100 DAT) in all treatments and later on decreased with progress of season till final measurement except 75V-100R and 50V-100R where the fraction of intercepted PAR raised up to the end of growing season (Fig. 5).As shown in Fig. 4, with increasing LAI, the transmitted fraction of radiation through the canopy decreased exponentially in all treatments. The highest and lowest k value was recorded by full irrigation treatment with 0.69 and 50V-50R treatment with 0.41, respectively. k values were strongly variable among different irrigation regimes.RUE altered according to amount of water applied at different growth stages. In this research, RUE values ranged from 0.38 to 0.9 g MJ-1. However, applying irrigation during vegetative stages could accelerate increase in leaf area, light interception and photosynthesis (Comas et al., 2019) and, thus, improve RUE.Timing of water stress had a significant effect (p < 0.01) on total fresh and dry fruit yield in both years of the experiment (Table 6). In general, tomato yield increased as the amount of irrigation water increased, however it was severely affected by timing applied irrigation. In 2016 and 2017 total fresh fruit approached 99.81 and 101.01 t ha-1 under full irrigation (100V-100R), significantly greater than that produced under full deficit irrigation (50V-50R) with 22.2 and 15.66 t ha-1, respectively (Fig. 7).

The experiment results clearly indicate adverse effect of water shortage on tomato yield, so that the highest fresh yield was obtained with full irrigation. However, the results suggest that this adverse effect can be reduced by a proper irrigation management at different growth stages. We showed that the effect of water provided at sensitive growth stage on the productivity of tomato was largely due to the positive effect of water apply on RUE.